\documentclass[journal]{rmaa}


\usepackage{paralist}
\usepackage{psfrag,color}
\usepackage[latin1]{inputenc}


\title{The nature and origin of Narrow Line AGN\\ activity in a sample of isolated SDSS galaxies}


\author{
R. Coziol,\altaffilmark{1}
J.~P. Torres-Papaqui,\altaffilmark{1}
I. Plauchu-Frayn,\altaffilmark{2}
J.~M. Islas-Islas,\altaffilmark{1}\\
R.~A. Ortega-Minakata,\altaffilmark{1}
D.~M. Neri-Larios,\altaffilmark{1}
and H. Andernach\altaffilmark{1}}


\altaffiltext{1}{Departamento de Astronom\'ia, Universidad de Guanajuato, Guanajuato, Mexico.}
\altaffiltext{2}{Instituto de Astrof\'isica de Andaluc\'ia (CSIC), Granada, Spain.}


\shortauthor{Coziol et al.}

\shorttitle{NLAGN in isolated galaxies}


\fulladdresses{
\item H. Andernach, R. Coziol, J.~M. Islas-Islas, D.~M. Neri-Larios, R.~A. Ortega-Minakata, and J.~P. Torres-Papaqui: Departamento de 
Astronom\'ia, Universidad de Guanajuato, Apdo. Postal 144, 36000 Guanajuato, Guanajuato, Mexico (heinz,~coziol,~jmislas,~daniel,~rene,~papaqui@astro.ugto.mx).
\item I. Plauchu-Frayn: Instituto de Astrof\'isica de Andaluc\'ia (CSIC), E-18008, Granada, Spain (ilse@iaa.ee).}


\listofauthors{R.~Coziol, J.~P.~Torres-Papaqui, I.~Plauchu-Frayn, J.~M.~Islas-Islas, R.~A.~Ortega-Minakata, D.~M.~Neri-Larios, \& H.~Andernach}

\indexauthor{Coziol, R.}
\indexauthor{Torres-Papaqui, J.~P.}
\indexauthor{Plauchu-Frayn, I.}
\indexauthor{Islas-Islas, J.~M.}
\indexauthor{Ortega-Minakata, R.~A.}
\indexauthor{Neri-Larios, D.~M.}
\indexauthor{Andernach, H.}


\SetYear{2011}
\ReceivedDate{2011 May 26}
\AcceptedDate{2011 August 1}


\abstract{We discuss the nature and origin of the nuclear activity observed in a sample of 292 SDSS narrow-emission-line galaxies, considered 
to have formed and evolved in isolation. The fraction of Narrow Line AGNs (NLAGNs) and Transition type Objects (TOs; a NLAGN with circumnuclear 
star formation) amounts to 64\% of the galaxies. We verify that the probability for a galaxy to show an AGN characteristic increases with the 
bulge mass of the galaxy \citep{TP11}, and find evidence that this trend is really a by-product of the morphology, suggesting that the AGN 
phenomenon is intimately connected with the formation process of the galaxies. The NLAGNs in our sample are consistent with a scaled-down or 
powered-down versions of quasars and Broad Line AGNs.\medskip}


\resumen{Discutimos la naturaleza y origen de la actividad nuclear observada en una muestra de 292 galaxias con l\'ineas de emisi\'on angostas 
del SDSS, cuya formaci\'on y evoluci\'on se considera que ha ocurrido en aislamiento. La fracci\'on de galaxias con un n\'ucleo activo (AGNs) 
o de objetos de transici\'on (TOs; un AGN con formaci\'on estelar circunnuclear) alcanza 64\% de la muestra. Verificamos que la probabilidad 
de que una galaxia muestre un n\'ucleo activo aumenta con la masa de su bulbo (Torres-Papaqui et al. 2011). Tambi\'en encontramos evidencia de 
que dicha tendencia es realmente un subproducto de la morfolog\'ia, y sugiere que el fen\'omeno AGN est\'a \'intimamente ligado al proceso de 
formaci\'on de galaxias. Los AGNs con l\'ineas de emisi\'on angostas en nuestra muestra son consistentes con una versi\'on a menor escala o 
menor energ\'ia de cu\'asares y AGNs con l\'ineas de emisi\'on anchas.\medskip }


\addkeyword{galaxies: active}
\addkeyword{galaxies: formation}



\begin{document}

\maketitle


\section{Introduction}
\label{sec:intro}

Spectroscopic surveys, like the Sloan Digital Sky Survey (SDSS) \citep{york00,stoughton02}, have revealed that a large fraction of galaxies in 
the nearby universe show emission lines consistent with some kind of nuclear activity. Using diagnostic diagrams that compare various emission 
line ratios, different classification criteria were proposed to classify these galaxies based on the possible sources of excitation of the gas 
\citep{baldwin81,veilleux87,kewley01,kauffmann03}. These classification criteria imply that there are two possible main sources: thermal sources, 
which are associated with star forming activity, and non-thermal sources, which are produced by the accretion of matter onto a Super-Massive Black 
Hole (SMBH) in the nucleus of the galaxies--the so called Active Galactic Nuclei (AGNs).

The majority of the emission line galaxies in the SDSS turn out to be star forming galaxies (SFGs). However, recent studies (e.g., Miller et 
al. 2003; Mart\'inez et al. 2010; Torres-Papaqui et al. 2011) inferred that the actual number of galaxies with a non-thermal ionization source 
in their nucleus could be much higher than previously believed, but the evidence was somewhat obscured by the large variation of characteristics 
presented by AGNs. For example, in the Seyfert~1, where we can distinguish broad emission line components akin to what is observed in quasars 
\citep{osterbrock89,weedman86,krolik99}, the presence of a SMBH seems indubitable. However, a significantly larger fraction of AGNs show only 
narrow emission lines. Also, many researchers have alluded to different characteristics for these Narrow Line AGNs (NLAGNs) by separating them 
into two main groups, the high ionization galaxies, generally called Seyfert~2 (Sy2), and the low ionization ones, called LINERs, which stands 
for Low Ionization Nuclear Emission-line Regions \citep{heckman80,coziol96,kewley06}. As a consequence, the source of ionization of NLAGNs, 
although clearly distinct from SFGs, is still an actively debated subject, and there is presently no consensus about their nature or evolutionary 
status.

It is usually admitted that both Sy2 and LINERs are phenomena that occur in early-type (Sa and Sb) spiral galaxies in the field, that is, galaxies 
forming in relatively low galactic density environments \citep{heckman80}. On the other hand, there is growing evidence that NLAGNs in clusters 
and compact groups of galaxies differ from those in the field by their earlier morphological types and intrinsic low luminosities 
\citep{phillips86,Coz98a,miller03,wake04,martinez08,martinez10,TP11}. These differences seem to point to distinct formation mechanisms for galaxies 
in different environments, which could have also affected the formation and evolution of their black holes. For example, the Low Luminosity AGNs 
(LLAGNs) found in compact groups and clusters of galaxies appear to prefer massive bulge spiral galaxies or elliptical galaxies--galaxies which 
are poor in gas--and it is this actual scarcity of matter that can be accreted that is assumed to explain the low luminosity of these objects 
\citep{martinez08,TP11}. This explanation is fully consistent with the starving quasar model \citep{richstone98,gavignaud08,martinez08,martinez10}, 
according to which a SMBH that had accreted at high rates in the past, producing a quasar-like activity, evolved as its reservoir of gas was depleted 
into a slowly accreting SMBH producing a LLAGN.

In order to test the idea of a variation of the AGN phenomenon in different environments we considered important to study galaxies for which the 
influence of their environment was minimal. These are galaxies that are relatively isolated--that is, they formed in low galactic density 
environments and evolved without major interactions with other galaxies.


\section{Description of the sample and data used for analysis}
\label{sec:sample}

Our sample of isolated galaxies was selected from the 2 Micron Isolated Galaxies catalogue (hereafter 2MIG), as compiled by \citet{karachentseva10}. 
The 2MIG catalogue contains 3227 galaxies, covering the entire sky. All the galaxies have a near-infrared magnitude brighter than $K_{s}=12$, 
and a projected image that extends across an angular diameter $a_{K}\ge 30''$. The isolation criterion used in this catalogue is the following: 
a galaxy is considered isolated when its nearest neighbor has a size within a factor 4 of the major-axis diameter of the target galaxy, and lies 
more than 20 diameters away from it. This criterion assures that a galaxy with a typical diameter of 20 kpc and peculiar velocity of the order 
of 150~km~s$^{-1}$ was not influenced by a similar type of galaxy during the last $\sim$3~Gyr \citep{turner79}.

To produce our sample, we have cross-correlated the positions of the galaxies in the 2MIG catalogue with spectroscopic targets in the Sloan 
Digital Sky Survey Data Release 7 (SDSS DR7) database \citep{abazajian09}. This resulted in 445 galaxies, which represents 88\% of the galaxies 
in the 2MIG catalogue covered by the SDSS DR7. The remaining 12\% are galaxies for which spectroscopic data were not obtained by SDSS, because 
they were too nearby or too bright.

Color images of the galaxies were obtained using the tool {\sc chart} of SDSS\footnote{\url{http://cas.sdss.org/dr7/en/tools/chart/chart.asp}.}. 
After visual inspection, we eliminated 28 galaxies with morphological peculiarities or possible faint companions, in apparent contradiction with 
the isolation assumption.

A preliminary examination of the spectra reveals that all these galaxies except one show emission lines. The only non-emission ``isolated'' 
galaxy is identified as SDSS J072635.39+431746.8. This is a nearby galaxy, $z=0.0105$, with a possible E or S0 morphology. Being the only 
galaxy of its kind it was excluded from our study.

Also excluded from our analysis were two extremely blue S0-like galaxies, with spectra typical of late-type spirals--showing a recent and 
extremely intense level of star formation. A careful examination of their images suggested that they are low mass irregular or morphologically 
peculiar galaxies. Their positions in the standard diagnostic diagram reveal that they are HII galaxies--small mass and low gas metallicity 
starburst galaxies (see Coziol 1996).


%%%%% FIGURA 1

\begin{figure*}[!t]
\centering
\includegraphics[width=10.8cm]{rcoziolfig1.ps}
\caption{Spectra of the broad line AGNs of our original sample of 2MIG galaxies. The 
flux scale is relative and was slightly displaced for clarity.} 
\label{broadem}
\vspace{0.3cm}
\end{figure*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


Only five galaxies in our sample were found to have broad emission line components. The spectra of these galaxies are presented in Figure~\ref{broadem}. 
These galaxies were also discarded from our analysis.

We classified the 409 remaining galaxies as isolated Narrow Emission Lines Galaxies (NELGs). The spectra of these galaxies were subsequently 
corrected for Galactic extinction \citep{schlegel98}, shifted to their rest frame, resampled to $\Delta \lambda$=1~\AA\ between 3400 and 8900~\AA, 
and processed using the spectral synthesis code {\sc starlight} \citep{cid05}, which produces a stellar population template that fits the 
continuum of each galaxy. We ran {\sc starlight} using a combination of $N_\star$ Simple Stellar Populations (SSPs) from the evolutionary 
synthesis models of \citet{bruzual03}. The models were computed with the {\sc miles} library \citep{vazdekis10}, following Padova's evolutionary 
tracks with the initial mass function of \citet{chabrier03}. An SSP consists of $N_\star=150$ elements, spanning six metallicities, $Z=0.005$, 
0.02, 0.2, 0.4, 1 and $2.5~Z_{\odot}$, and 25 stellar population ages in the range from 1~Myr to 18~Gyr.

In the template-subtracted spectra, we measured automatically different important attributes of the spectral emission lines, like their flux 
and full width at half maximum (FWHM). Other important features retrieved from the stellar population templates fitted by {\sc starlight} are 
the stellar velocity dispersions and the star formation history (SFH) of the host galaxies. The SFH map shows how the smoothed star formation 
rate (SFR) in a galaxy varies over a time period covering $\log(t)=5.7$~yr to $\log(t)=10.6$~yr \citep{asari07}.


%%%%% FIGURA 2

\begin{figure}[!b]
\centering
\includegraphics[clip,height=11cm,width=0.99\columnwidth]{rcoziolfig2.ps}
\caption{SFH for the isolated NELGs. In each graph we show the mean prototype (solid 
line) and its dispersion at two sigma (dotted line). In order to use the same scale 
for all morphological classes, we have applied a factor of 0.5 and 0.1 to the SFH curves 
of the Sd and Sm, respectively.} 
\label{ProtosClean}
%\vspace{0.3cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%%%%% TABLA 1

\begin{table*}[!t]
\vspace{0.1cm}
\centering
\setlength{\tabnotewidth}{0.8\textwidth}
\tablecols{7}
%\setlength{\tabcolsep}{0.94\tabcolsep}
\caption{Examples of spectroscopic characteristics of the isolated NELGs\tabnotemark{$\dagger$}} 
\label{TabBPT}
\small
\begin{tabular}{lcccccc}
\toprule
\multicolumn{1}{c}{Identification in SDSS} & $\log \left( \frac{{\rm O[III]}}{{\rm H}\beta} \right)$ 
& $\log \left( \frac{{\rm N[II]}}{{\rm H}\alpha} \right)$ & Activity & $L_\mathrm{H\alpha}$ & $\sigma_\star$ & $\log(\lambda~L_{5100})$ \\
      &   &   &  Type    & $L_\odot$     & km~s$^{-1}$   & erg~s$^{-1}$ \\
\midrule
SDSS J105809.84$-$004628.8 &  $-0.364$ &  $-0.351$ &  SFG  & 6.19 &    ~87  & 42.13    \\
SDSS J113903.33$-$001221.6 &  ~~0.228  &  ~~0.060  &  AGN  & 6.05 &    144  & 42.56    \\
SDSS J135807.05$-$002332.9 &  $-0.604$ &  $-0.521$ &  SFG  & 7.03 &    ~86  & 42.76    \\
SDSS J142223.76$-$002315.5 &  $-0.437$ &  $-0.494$ &  SFG  & 5.50 &    ~81  & 41.16    \\
SDSS J150654.85$+$001110.8 &  ~~0.253  &  ~~0.055  &  AGN  & 6.67 &    202  & 43.11    \\
SDSS J113423.32$-$023145.5 &  ~~0.363  &  ~~0.238  &  AGN  & 6.23 &    136  & 43.00    \\
SDSS J115425.04$-$021910.3 &  $-0.275$ &  $-0.453$ &  SFG  & 5.36 &    ~78  & 41.29    \\
SDSS J122353.98$-$032634.4 &  $-0.218$ &  $-0.148$ &  TO   & 5.39 &    113  & 41.64    \\
SDSS J124428.12$-$030018.8 &  $-0.193$ &  $-0.300$ &  TO   & 6.28 &    115  & 42.69    \\
SDSS J170128.21$+$634128.0 &  $-0.671$ &  $-0.375$ &  SFG  & 7.20 &    ~99  & 42.52    \\
\bottomrule 
\tabnotetext{$\dagger$}{Full table available in electronic form at \url{http://www.astroscu.unam.mx/rmaa/RMxAA..47-2/RMxAA..47-2_rcoziol-tables.zip/}.}
\end{tabular}
\vspace{0.3cm}
\end{table*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\subsection{Morphologies and star formation history}
\label{sec:morphologySFH}

To determine the morphologies of the galaxies we used a composite method combining an eye estimate with the SFH. First, two of us (IP-F and RC) 
determined by eye the morphologies of the 409 isolated NELGs following the standard Hubble classification. Then, using only the galaxies with 
concordant morphologies--meaning the two observers gave exactly the same morphology--an average SFH prototype for each morphology class was 
created. Using these prototypes, the residuals of the SFH for each galaxy were calculated separately in three distinct time periods: Recent, 
$5.7\le \log(t/{\rm yr}) \le 7.9$, Intermediate, $7.9 < \log(t/{\rm yr}) \le 9.0$, and Old, $9.0 < \log(t/{\rm yr}) \le 10.6$. A correction 
was then made to the eye morphology classification by choosing a morphology that minimized the variance of the residuals in each time period. 
For example, by eye we could have given the type S0 or Sa to a galaxy, but the residuals being minimal for Sa we adopted this last classification.

Once the morphologies were correctly adjusted to the SFH, we recalculated the prototypes by calculating the mean SFH in each morphological bin. 
Since we found the Sb and Sbc types to have SFH prototypes that are extremely similar and almost impossible to distinguish, we merged these 
two morphological bins together, keeping only the Sb identification. The final prototypes, including 327 (80\%) of the 409 isolated NELGs, are 
presented in Figure~\ref{ProtosClean}. The prototypes suggest that S0, S0a, and Sa galaxies formed most of their stars in the past, $\log(t)>9.0$~yr, 
while the Sab had a near constant star formation activity over the whole time lapse covered by {\sc starlight}. This is also true for the Sb 
galaxies, but with a slightly larger dispersion. In the Sc galaxies, recent star formation activity becomes more important than in the past, but 
the dispersion also increases significantly the later the type. In Sd and Sm galaxies most of the stars appear to have formed recently, 
$\log(t)<8.0$~yr. These results are in good agreement with similar studies based on integrated spectroscopy (e.g., Kennicutt 1992).


%%%%% FIGURA 3

\begin{figure*}[!t]
\centering
\includegraphics[width=10cm]{rcoziolfig3bw.ps}
\caption{Diagnostic diagram for the activity classification of the isolated NELGs. The 
separation between SFGs and TOs was suggested by \citet{kauffmann03} and the separation 
between TOs and AGNs by \citet{kewley06}. Mean uncertainties on the line ratios are 
indicated as a cross on the lower right of diagram (not a datum). The color figure can 
be viewed online.}
\label{bptdiagram}
\vspace{0.3cm}
\end{figure*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


The remaining 20\% galaxies were considered peculiar. Among these galaxies we count 36 late-type spirals (Sc or Sd) with a SFH typical of an Sa. 
These galaxies turned out to be cases where the fiber was covering only the innermost part of the galaxies. Also considered peculiar were 22 
galaxies with a burst of star formation $\sim$100~Myr in the past. No external cause for the bursts could be determined, the galaxies being 
confirmed as isolated. Another 24 E or S0 galaxies were found to show a trace (not intense) of very recent star formation. All these peculiar 
galaxies were excluded from our analysis.


\vspace{0.15cm}

\subsection{Activity classification}

For our spectral activity analysis we further reduced our sample to 292 NELGs with a S/N~$\ge 3$ in the four most important emission 
lines: H$\beta$, [OIII]$\lambda$5007, H$\alpha$ and [NII]$\lambda6584$. In Table~\ref{TabBPT} we compile the spectroscopic characteristics 
of these galaxies as measured in the template subtracted spectra. Column~1 gives the SDSS identification of the galaxies. In Columns~2 and 
3 we list the values of the two line ratios used to determine the activity type of the galaxies. The adopted classification is given in 
Column~4. In Column~5 we list the H$\alpha$ emission line luminosity. In Column~6 we give the velocity dispersion of the stars, $\sigma_\star$, 
as deduced from the {\sc starlight} template and corrected for the SDSS spectral resolution. Considering the size of the fiber used in 
SDSS ($3''$), this value can be taken as the stellar velocity dispersion of the bulge of the galaxy. Finally, in Column~7, we give  the 
luminosity of the continuum at $\lambda$5100~\AA\ as measured in the spectra before template subtraction.


%%%%% TABLA 2

\begin{table*}[!t]
\vspace{0.1cm}
\centering
\caption{Examples of basic physical properties of the isolated NELGs}
\label{Tabbasic}
\setlength{\tabnotewidth}{0.99\textwidth}
\tablecols{9}
\setlength{\tabcolsep}{0.94\tabcolsep}
\small
\begin{tabular}{lcccccccc}
\toprule
\multicolumn{1}{c}{Identification in SDSS} & RA (J2000) & DEC (J2000) & $z$ & Morph. &  $R_{90\%}$  & CI & $\log$($L_K$) & Mean $(t_*)$  \\
                       & (degree)   & (degree)    &     &        &  kpc        &    & $L_\odot$      & yr         \\
\midrule
SDSS J105809.84$-$004628.8 &  164.54102 &  $-0.77468$ & 0.0215 &  Sc   & ~6.5 &  2.18 &  10.43 &  8.79  \\
SDSS J113903.33$-$001221.6 &  174.76388 &  $-0.20600$ & 0.0181 &  Sab  & ~8.6 &  3.06 &  10.77 &  9.82  \\
SDSS J135807.05$-$002332.9 &  209.52940 &  $-0.39248$ & 0.0296 &  Sc   & ~9.8 &  2.11 &  10.80 &  8.61  \\
SDSS J142223.76$-$002315.5 &  215.59903 &  $-0.38766$ & 0.0055 &  Sd   & ~4.6 &  1.90 &  10.01 &  7.81  \\
SDSS J150654.85$+$001110.8 &  226.72857 &  ~~0.18634  & 0.0351 &  Sa   & 12.1 &  2.72 &  11.19 &  9.60  \\
SDSS J113423.32$-$023145.5 &  173.59719 &  $-2.52933$ & 0.0395 &  Sb   & 13.9 &  2.09 &  11.06 &  9.79  \\
SDSS J115425.04$-$021910.3 &  178.60434 &  $-2.31955$ & 0.0080 &  Sd   & ~4.5 &  2.12 &  ~9.78 &  8.05  \\
SDSS J122353.98$-$032634.4 &  185.97495 &  $-3.44292$ & 0.0068 &  Sab  & ~5.5 &  2.79 &  10.58 &  8.92  \\
SDSS J124428.12$-$030018.8 &  191.11718 &  $-3.00525$ & 0.0239 &  Sc   & ~8.6 &  2.65 &  10.61 &  9.17  \\
SDSS J170128.21$+$634128.0 &  255.36758 &  ~63.69112  & 0.0163 &  Sc   & ~5.5 &  1.76 &  10.42 &  7.89  \\
\bottomrule 
\tabnotetext{$\dagger$}{Full table available in electronic form at \url{http://www.astroscu.unam.mx/rmaa/RMxAA..47-2/RMxAA..47-2_rcoziol-tables.zip/}.}
\end{tabular}
\vspace{0.3cm}
\end{table*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


We have identified the activity type of the 292 isolated NELGs in Figure~\ref{bptdiagram}, where we compare the two line ratios 
[OIII]$\lambda$5007/H$\beta$ and [NII]$\lambda6584$/H$\alpha$. The typical low uncertainty levels on these ratios (cross in Figure~\ref{bptdiagram}) 
have practically no effect on our classification. We distinguish 105 SFGs (36.0\%), 83 TOs (28.4\%), and 104 AGNs (35.6\%).


%%%%% FIGURA 4

\begin{figure*}[!t]
\centering
\includegraphics[clip,width=12cm]{rcoziolfig4bw.ps}
\caption{Comparison of absolute $K$ magnitude vs. heliocentric velocity for the 2MIG 
catalog and our subsample of 292 isolated NELGs. The color figure can be viewed online.} 
\label{velK}
\vspace{0.3cm}
\end{figure*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\subsection{Physical characteristics of the isolated NELGS}

We present the basic physical properties of the 292 isolated NELGs in Table~\ref{Tabbasic}. After the SDSS identification in Column~1, 
we list in Columns~2, 3 and 4 the right ascension, declination and redshift of the galaxies as listed in SDSS DR7. The results of our 
morphological classification are given in Column~5. Also from the SDSS DR7 database, we give in Columns~6 and 7 the Petrosian radii 
(in kpc) at 90\% of the light distribution and the concentration index, CI, which is the ratio of the two Petrosian radii 
(CI~$=R_{90\%}/R_{50\%}$). In Column~8 we list the luminosity in the $K$ band as determined from 2MASS (K20 from 2MASS). In Column~9, we 
give the mean stellar age of the stellar populations consistent with the {\sc starlight} template
\citep{asari07}.

The K20 magnitudes from 2MASS were extracted from the extended sources catalog \citep{jarrett00} by choosing the closest galaxy 
within 5 arcsecs of the best match. They were corrected for Galactic extinction at the position of the galaxies using the dust maps 
published in  Schlegel et al. (1998). We also applied a $k$-correction using the method described in \citet{kochanek01}. This 
method is reported to be valid for $ z <0.25$ and to be independent of the galaxy morphological type. The $K$ band absolute magnitude 
was calculated using the standard relation:
\begin{equation}
 M_K  = m_K - 25 - 5 \log(D_L) - A_K - k(z) ,
\end{equation}
where $m_K$ is the total apparent magnitude in $K$, $A_K$ is the Galactic extinction, $D_L$  is the luminosity distance in Mpc, 
calculated assuming H$_{0}=75$~km~s$^{-1}$~Mpc$^{-1}$, and $k(z)=-6.0\log(1+z)$ is the $k$-correction. From the magnitudes we 
deduce the $K$ luminosities applying the relation:
\begin{equation}
 \log (L_K/L_\odot)=0.4(M_{\sun K}-M_K) ,
\end{equation}
where $M_{\sun K}=3.28$ is the absolute magnitude of the Sun in the $K$ band \citep{binney98}. From the uncertainties in the fluxes 
\citep{jarrett00}, a mean uncertainty of 6\% is estimated for our $K$ luminosities.


%%%%% FIGURA 5

\begin{figure*}[!t]
\centering
\includegraphics[width=10.5cm]{rcoziolfig5bw.ps}
\caption{Diagnostic diagrams for isolated NELGs with different morphologies. The 
separations and colors are as explained in Figure~\ref{bptdiagram}. The color 
figure can be viewed online.} 
\label{actmorph}
\vspace{0.351cm}
\end{figure*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


In Figure~\ref{velK} we compare the distributions of the 2MASS absolute magnitudes in $K$ and heliocentric radial velocities, as found 
in our subsample of 292 isolated NELGs and in the entire 2MIG catalogue. The distribution for the 2MIG catalogue is typical of a flux 
limited survey. The galaxies of our subsample are randomly distributed, suggesting they form a statistically fair sample of all the isolated 
galaxies in the 2MIG catalogue.


%%%%% TABLA 3

\begin{table}[!t]
\centering
\caption{Distribution of morphologies\\ of 292 isolated NELGs}
\label{TabMorphFraction}
\tablecols{3}
\setlength{\tabcolsep}{1.2\tabcolsep}
\begin{tabular}{ccc}
\toprule
Morphology  &  Number &  Fraction \\
type        &         &  \%       \\
\midrule
S0          & ~9      & ~3.1 \\
S0a         & ~4      & ~1.4 \\
Sa          & 55      & 18.8 \\
Sab         & 68      & 23.3 \\
Sb          & 83      & 28.4 \\
Sc          & 53      & 18.2 \\
Sd          & 17      & ~5.8 \\
Sm          & ~3      & ~1.0 \\
\bottomrule
\end{tabular}
\vspace{0.5cm}
\end{table}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\section{Analysis}

\vspace{0.1cm}

\subsection{Activity type vs. morphology}

We present the fractions of morphology types in our sample of 292 isolated NELGs in Table~\ref{TabMorphFraction}. The majority 
(70.5\%) of these galaxies are classified as intermediate spirals (Sa-Sab-Sb). In Figure~\ref{actmorph} we show how the distribution 
of activity type varies in the different morphology classes: AGNs and TOs are mostly found in early-type galaxies (S0-Sa), while 
the proportion of SFGs gradually increases in later types (Sab-Sm). This result is not new, but confirms the common view about 
AGNs in the field: they are mostly found in early-type spiral galaxies (e.g., Melnick, Terlevich, \& Moles 1986; Osterbrock 1989; 
Blandford, Netzer, \& Woltjer 1990).


%%%%% TABLA 4

\begin{table}[!t]
\vspace{0.1cm}
\centering
\caption{Examples of masses of\\ isolated NELGs\tabnotemark{$\dagger$}}
\label{MASS}
\setlength{\tabnotewidth}{0.99\columnwidth}
\tablecols{4}
\setlength{\tabcolsep}{0.965\tabcolsep}
\small
\begin{tabular}{lccc}
\toprule
\multicolumn{1}{c}{Identification in SDSS} & \multicolumn{3}{c}{$\log\,(M)(M_\odot)$} \\
      & $M_\mathrm{Bulge}$   & $M_\mathrm{BH}$  &  $M_B$  \\
\midrule
SDSS J105809.84$-$004628.8  &   9.04    &           & 10.71 \\
SDSS J113903.33$-$001221.6  &   9.40    &   6.41    & 10.55 \\
SDSS J135807.05$-$002332.9  &   9.17    &           & 11.04 \\
SDSS J142223.76$-$002315.5  &   8.39    &           & 10.17 \\
SDSS J150654.85$+$001110.8  &   9.99    &   7.06    & 11.12 \\
SDSS J113423.32$-$023145.5  &   9.69    &   6.74    & 11.22 \\
SDSS J115425.04$-$021910.3  &   8.52    &           & ~9.86 \\
SDSS J122353.98$-$032634.4  &   8.77    &   5.70    & 10.28 \\
SDSS J124428.12$-$030018.8  &   9.33    &   6.33    & 10.96 \\
SDSS J170128.21$+$634128.0  &   9.03    &           & 10.77 \\
\bottomrule
\tabnotetext{$\dagger$}{Full table available in electronic form at \url{http://www.astroscu.unam.mx/rmaa/RMxAA..47-2/RMxAA..47-2_rcoziol-tables.zip/}.}
\vspace{0.1cm}
\end{tabular}
\end{table}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%%%%% TABLA 5

\begin{table}[!t]
\vspace{0.1cm}
\centering
\caption{Median masses and\\ concentration indices}
\label{Median_Mass}
\tablecols{5}
\setlength{\tabcolsep}{1.2\tabcolsep}
\begin{tabular}{lcccc}
\toprule
          &  \multicolumn{3}{c}{$\log \,(M)(M_\odot)$} & CI \\
          &  $M_\mathrm{Bulge}$ & $M_K$ & $M_B$  &    \\
\midrule
AGN       &  9.64 & 10.90 & 10.92 & 2.41 \\
TO        &  9.41 & 10.85 & 10.78 & 2.21 \\
SFG       &  9.10 & 10.66 & 10.56 & 2.11 \\
S0/S0a    &  9.73 & 10.98 & 10.94 & 3.21 \\
Sa        &  9.72 & 10.91 & 11.00 & 2.87 \\
Sab       &  9.46 & 10.82 & 10.78 & 2.62 \\
Sb        &  9.39 & 10.94 & 10.77 & 2.34 \\
Sc        &  9.10 & 10.66 & 10.58 & 2.12 \\
Sd        &  8.44 & ~9.86 & ~9.66 & 2.07 \\
\bottomrule
\end{tabular}
\vspace{0.4cm}
\end{table}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


In Table~\ref{MASS} we give our estimated masses for the bulges, $M_\mathrm{Bulge}$, as determined using the velocity dispersions, 
$\sigma_\star$, and applying the virial theorem. The box-whisker plots for the bulge masses of the galaxies separated by activity 
type are presented in Figure~\ref{BWbulges}a. The median values are reported in Table~\ref{Median_Mass}. AGNs and TOs possess more 
massive bulges than SFGs. The trends observed for the AGNs and TOs are confirmed using a non-parametric statistical test (Kruskall-Wallis 
with Dunn's multiple comparison tests). The results of these tests are presented in Table~A1 of the Appendix. The tests find the 
medians in the three samples to be significantly different at a level of confidence of 99\%.

In Figure~\ref{BWbulges}b we compare the bulge mass of galaxies with different morphologies. We also observe a significant difference, 
the bulge mass being larger in earlier morphological types. This trend is also found to be statistically significant. The results for 
the tests, presented in Table~A2 of the Appendix, show that the farther apart the morphological class, the more significant the difference 
in bulge mass. For example, the statistical tests reveal no difference between Sa and Sab, but a significant one between Sa and Sb. 
Similarly, no difference appears between Sab and Sb, but a significant one is found between Sab and Sc.


%%%%% FIGURA 6

\begin{figure}[!t]
\centering
\includegraphics[clip,width=0.99\columnwidth]{rcoziolf6.eps}
\caption{Box-whisker plots for the bulge masses of galaxies as function of (a) activity 
type, (b) morphology. The middle line is the median, the box is limited by the percentiles 
and the whiskers indicate the full range of values.} 
\label{BWbulges}
\vspace{0.45cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%%%%% FIGURA 7

\begin{figure}[!t]
\centering
\includegraphics[clip,width=0.99\columnwidth]{rcoziolf7.eps}
\caption{Box-whisker plots for the bulge masses as found (a) in group 1 (Sa and Sab galaxies), 
and (b) in group 2 (Sab and Sb galaxies). Box-whiskers are defined as in Figure~\ref{BWbulges}.}
\label{BWbulgesvsM}
\vspace{0.3cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


The above results lead to the question of whether the most important parameter is the mass of the bulge or the morphology of the 
galaxy. As a test, we compare the bulge masses in two statistical groups: Group~1 which is composed of Sa and Sab galaxies, and 
Group~2 which is composed of Sab and Sb. The box-whisker plots are drawn in Figure~\ref{BWbulgesvsM}a for Group~1 and 
Figure~\ref{BWbulgesvsM}b for Group~2. We find no significant difference in bulge mass between the different activity types in 
Group~1, while in Group 2 there is a significant difference only between the SFGs and AGNs. This is confirmed by the statistical 
tests in Table~A1 of the Appendix. We conclude that the AGN activity is related to the bulge mass, but mostly because of the strong 
connection of this parameter with the morphology: the earlier the morphology, the larger the bulge mass, and the higher the 
probability to see an AGN. This is also consistent with the diagnostic diagrams, which show a larger frequency of AGNs in earlier 
morphological types. In Group 1, the numbers of AGNs, SFGs and TOs are 67, 21 and 35, respectively, while in Group 2 these numbers 
change to 47, 53 and 51 respectively. For an increase by a factor of 2 in bulge mass, from Sa to Sb (the bulge mass increases by 
factor of 3.5 from AGN to SFG in Table~5), the number of AGNs slightly falls and the number of TOs and SFGs doubles.

The bulge mass being an important morphological parameter, the strong correlation found with AGN activity suggests this phenomenon 
is intimately connected with the formation process of the galaxies.


\subsection{Activity type vs. galaxy mass}

We have estimated the masses of our galaxies using two different methods. The first method is based on 2MASS $K$ band luminosities 
(as compiled in Table~2), while the second is based on the absolute $B$ magnitudes, which were obtained using the Johnson-$B$ band 
magnitude synthesized from the SDSS magnitudes \citep{fukugita96}. The absolute magnitudes in $B$ are compiled in Tables~\ref{TableOHSFG}, 
\ref{TableOHAGN} and \ref{TableOHTO}, for the SFGs, AGNs, and TOs, respectively. The uncertainty is of the order of 0.05~mag. Being 
insensitive to dust extinction and to the morphological type of the galaxies the near-infrared emission is a better tracer of the 
stellar mass than the $B$ magnitudes, for which we need to apply a correction depending on the morphology.


%%%%% FIGURA 8

\begin{figure*}[!t]
\centering
\includegraphics[clip,width=12cm]{rcoziolf8.eps}
\caption {Comparison of the two mass estimates. The dotted line is the one-to-one 
relation and the continuous line is a linear fit.}
\label{MASScomparison}
\vspace{0.4cm}
\end{figure*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


The $K$ luminosities were transformed into masses using the mass-to-light ratio $M/L_K=0.95$, as estimated by \citet{bell03}. To 
transform the $B$ absolute magnitudes into masses, we first corrected for galactic extinction and {\it k}-correction using the 
code developed by \citet{BR07}. Then we applied the different mass-to-light ratios for galaxies as published by \citet{FG79}. 
These ratios were adjusted for our adopted value of the Hubble constant. The $B$ mass estimates appear in Column~4 of Table~\ref{MASS}.

In Figure~\ref{MASScomparison} we compare the two mass estimates. The linear relation fitted has a correlation parameter $r^2=0.72$, 
implying that 72\% of the total variance of the $B$ mass estimates is explained by the variation of the $K$ mass estimates (and vice 
versa). The median values for the masses in the $B$ and $K$ bands are given in Table~\ref{Median_Mass}. They are in excellent agreement 
with those reported by \citet{roberts94} for galaxies having similar morphological types.

In Table~\ref{Median_Mass} we see that both median masses, $M_B$ and $M_K$, tend to increase from the SFGs to the AGNs. The general 
trend observed in the box-whisker plots presented in Figure~\ref{BW_ATvsMass} looks slightly more obvious using $M_K$ than $M_B$. 
In Table~A1 of the Appendix we see that using $M_B$ the statistical tests detect a slightly more significant difference between the 
AGNs and SFGs than between the TOs and SFGs, while both differences are similarly statistically significant using $M_K$. No significant 
difference is observed between the AGNs and TOs.


%%%%% FIGURA 9

\begin{figure}[!t]
\includegraphics[clip,width=0.94\columnwidth]{rcoziolf9bca.eps}
\caption{Box-whisker plots for the total masses in galaxies showing different 
activity types; (a) $M_B$ and (b) $M_K$.} 
\label{BW_ATvsMass}
\vspace{0.3cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%%%%%% FIGURA 10

\begin{figure}[!t]
\centering
\includegraphics[clip,width=0.99\columnwidth]{rcoziolf10bca.eps}
\caption{Box-whisker plots for the total masses in galaxies having different 
morphologies; (a) $M_B$ and (b) $M_K$.} 
\label{BW_MvsMass}
\vspace{0.4cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


In Figure~\ref{BW_MvsMass} we compare the masses of the galaxies having different morphological types. Although we found a significant 
variation of the bulge mass with the morphology, the total mass $M_B$ and $M_K$ do not seem to vary as much between galaxies having 
different morphologies. In Table~A2 of the Appendix, we find that the observed differences start to be statistically significant only 
when the comparison is done with the latest types, Sc for $M_B$, and even later, Sd, for $M_K$. We conclude that galaxies with different 
morphologies show only marginal differences in their total masses.

Based on our analysis, the trends observed between the AGN activity, the bulge mass and morphology does not appear to be ``quantitative'' 
(based on a difference in total mass), but more ``qualitative'', the AGN activity appearing more frequently when a higher fraction of 
the  mass of the galaxy is in the form of a bulge. This seems to favor a mechanism based on different astration rates--defined as the 
efficiency with which a galaxy transforms its gas into stars \citep{Sandage86}. The higher the astration rate, the more massive the bulge 
and the higher the probability to observe an AGN.


%%%%% FIGURA 11

\begin{figure}[!t]
\centering
\includegraphics[width=0.99\columnwidth]{rcoziolfig11bw.ps}
\caption{Distributions of mean ages of stellar populations for galaxies with 
different activity types. The color figure can be viewed online.} 
\label{medianage01}
\vspace{0.3cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\subsection{Relation with stellar populations}

In Figure~\ref{medianage01} we compare the distributions of the mean ages of the stellar populations in galaxies showing different 
activity types. The stellar populations of the SFGs are dominated by young stars, with mean ages ranging from 0.32 to 1.0~Gyr. 
These  values correspond to the 25 and 75 percentiles, respectively. The mean age of the stellar populations is observed to go up 
in the TOs, with values ranging from 0.5 to 3.2~Gyr, and to culminate in the AGNs with values ranging from 1.6 to 6.3~Gyr. Comparable 
results were encountered before by \citet{Boisson00}.

A strong connection is also found with the morphologies. In Figure~\ref{medianage02} the morphologies were regrouped into three 
broad classes: Late (Sc, Sd, and Sm), Intermediate (Sa, Sab, and Sb), and Early (S0 and S0a). The SFGs show a mixture of Late and 
Intermediate morphologies, while the TOs have only Intermediate morphologies. The AGNs on the other hand have mostly Early morphologies.


%%%%% FIGURA 12

\begin{figure}[!t]
\centering
\includegraphics[width=0.99\columnwidth]{rcoziolfig12bw.ps}
\caption{Distributions of mean ages of stellar populations for galaxies with 
different activity types and different morphologies. The color figure can be 
viewed online.}
\label{medianage02}
\vspace{0.45cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


The correlations between morphology, activity type and mean stellar ages of the stellar populations support a strong connection between 
the AGN activity and the formation process of the galaxies. In particular, the AGN phenomenon appears like a normal by-product of the 
formation process of galaxies that produces more massive bulges.

There are many structural similarities between the bulges of spiral galaxies and elliptical galaxies, suggesting similar formation 
mechanisms (e.g., Jablonka, Martin, \& Arimoto 1996). In particular, elliptical galaxies are known to have endured higher astration 
rates than spiral galaxies when they formed \citep{Sandage86}, transforming almost all their gas into stars in a very short period 
of time. Assuming that galaxies form by a succession of star forming episodes--an assumption necessary to produce the mass-metallicity 
relation--galaxies with high astration rates would thus be expected to have formed most of their stars in the past because, the 
reservoir of gas being limited, the galaxy would have consumed it rapidly and stopped forming stars relatively early. This would 
explain the differences in mean ages for the stellar populations observed in our sample of isolated NELGs.

\subsection{Relation with gas metallicity}

After correcting for dust absorption, the metallicities of the gas in the SFGs were estimated using the empirical correlation 
found between $\log (\mathrm{O/H})$ and the emission-line ratio $R_3=([\mathrm{OIII}]\lambda4959+[\mathrm{OIII}]\lambda5007)$/H$\beta$ 
\citep{EP84,VC92}. The correlation originates from the cooling effect of oxygen: as the metallicity of the gas increases, the gas is 
cooled more efficiently, the temperature drops and the line ratio $R_3$ decreases \citep{McCall85,EvansDopita85}.


%%%%% TABLA 6

\begin{table}[!t]
\vspace{0.1cm}
\centering
\caption{Examples of gas metallicities and absolute $B$ magnitudes of SFGs\tabnotemark{$\dagger$}}
\label{TableOHSFG}
\setlength{\tabnotewidth}{0.99\columnwidth}
\tablecols{3}
%\setlength{\tabcolsep}{0.99\tabcolsep}
\begin{tabular}{lcc}
\toprule
\multicolumn{1}{c}{Identification in SDSS} & [O/H] & $M_{B}$ \\
\midrule
SDSS J105809.84$-$004628.8 & 0.26 & $-18.46$  \\
SDSS J135807.05$-$002332.9 & 0.43 & $-19.68$  \\
SDSS J142223.76$-$002315.5 & 0.31 & $-17.11$  \\
SDSS J115425.04$-$021910.3 & 0.20 & $-16.35$  \\
SDSS J170128.21$+$634128.0 & 0.47 & $-18.65$  \\
SDSS J214907.29$+$002650.3 & 0.51 & $-18.28$  \\
SDSS J235106.25$+$010324.1 & 0.48 & $-17.67$  \\
SDSS J021859.64$+$001948.0 & 0.44 & $-18.69$  \\
SDSS J025154.58$+$003953.3 & 0.29 & $-19.06$  \\
SDSS J003823.71$+$150222.4 & 0.38 & $-18.69$  \\
\bottomrule
\tabnotetext{$\dagger$}{Full table available in electronic form at \url{http://www.astroscu.unam.mx/rmaa/RMxAA..47-2/RMxAA..47-2_rcoziol-tables.zip/}.}
\end{tabular}
\vspace{0.3cm}
\end{table}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%%%%% FIGURA 13

\begin{figure*}[!t]
\centering
\includegraphics[width=10cm]{rcoziolfig13bw.ps}
\caption{Gas metallicities vs. absolute $B$ magnitudes for the SFGs (filled dots). The 
giant spirals sample is taken from Zaritsky et al. (1994) and the SBNGs from \citep{Coz98b}. 
The continuous straight line is the Brodie-Huchra (1991) relation for elliptical galaxies. 
Also shown are two models of multiple mergers by \citet{TL79} and \citet{Struck81}. The 
numbers indicate the approximate number of mergers necessary in the models to reproduce 
the metallicities. The color figure can be viewed online.}
\label{MBvsMetal1}
\vspace{0.4cm}
\end{figure*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


The gas metallicities in the SFGs are compiled in Table~\ref{TableOHSFG}, together with their absolute $B$ magnitudes. We assume 
the solar metallicity is $12+ \log(\mathrm{O/H})=8.66 \pm 0.05$ \citep{Asplund04}. The uncertainty in the metallicities is of the 
order of $\pm0.2$~dex (Edmunds \& Pagel 1984). In Figure~\ref{MBvsMetal1} we plot the gas metallicities in the SFGs against their 
absolute $B$ magnitudes. The results are in excellent agreement with what is expected based on their late-type morphologies 
\citep{ZKH94}. The results are also in good agreement with what was observed in Starburst Nucleus Galaxies (SBNGs): the gas in the 
late-type SBNGs is more metal rich than in the early-type SBNGs, due to their different formation processes \citep{Coz98b}.

The usual way to determine the metallicity of AGNs is to compare the observed line ratios with the outputs obtained using an 
ionization code like {\sc cloudy} \citep{Ferland98}. However, the results are usually ambiguous, with more than one possible solution 
\citep{Nagao06}. There maybe also other physical mechanisms, like shocks or special ionizing structures, that can complicate the 
interpretation based solely on photoionization model results \citep{Viegas92,Cooke00}.


%%%%% FIGURA 14

\begin{figure*}[!t]
\centering
\includegraphics[clip,width=12.5cm]{rcoziolf14bca.eps}
\caption {Diagnostic diagram with three curves obtained by Bennert et al. (2006) using 
{\sc cloudy}: $U$, varying the ionizing parameter at constant solar metallicity; $O$, 
varying the metallicity keeping $U$ constant; $N$, varying the nitrogen abundance. Also 
shown is our calibration using $R_3$. The bold circle on this curve corresponds to 
1.0~$Z_\odot$. The color figure can be viewed online.} 
\label{BPTcal0}
\vspace{0.5cm}
\end{figure*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


An excellent study was presented recently by \citet{Bennert06}. With the permission of the authors, we reproduce some of their results 
in Figure~\ref{BPTcal0}. The models discussed in their study apply exactly to our NELGs. From their study we can see that the line ratio 
[NII]/H$\alpha$ is extremely sensitive to the abundance of nitrogen. One possible cause for the increase of nitrogen emission in the 
AGNs could be an excess of nitrogen \citep{osterbrock70,storchi89,storchi91,hamann93}. In the center of galaxies with massive bulges and 
old stellar populations, we do not expect the nitrogen abundance to follow the normal secondary relation (Zaritsky et al. 1994; Thurston, 
Edmunds, \& Henry 1996; van Zee et al. 1998; Coziol et al. 1999).

In Figure~\ref{BPTcal0} we also observe that, for any value in excess of nitrogen abundance, increasing the ionizing parameter, $U$, 
while keeping the metallicity constant causes the ratio [OIII]/H$\beta$ to rise. But, if we increase $U$ too much the ratio [NII]/H$\alpha$ 
begins to decrease. In the same figure we observe that when we lower the metallicity while keeping $U$ constant, both line ratios increase, 
which is consistent with the cooling effect of oxygen. However, if we decrease the metallicity too much, both line ratios eventually 
decrease. This behavior is consistent with a coupling effect between $U$ and the metallicity \citep{EvansDopita85}: as the metallicity 
decreases, $U$ increases and vice versa. Therefore, one can arbitrarily change one parameter or the other with almost 
the same effect.

In Figure~\ref{BPTcal0} the TOs and AGNs trace a continuous sequence where the ratios [NII]/H$\alpha$ and [OIII]/H$\beta$ increase 
together. According to the {\sc cloudy} models of Bennert et al. one way to explain this sequence would be to decrease the metallicity 
gradually while increasing the overabundance of nitrogen. Consistent with this model, we fitted on the TOs and AGNs distributions 
an empirical relation between the ratio [OIII]/H$\beta$ and [NII]/H$\alpha$. We then searched for a calibration that would yield the 
same range in metallicities for the TOs as obtained using the model of \citet{Bennert06}, from 2.5 to 1~$Z_\odot$. As a first approximation 
we found that the $R_3$ calibration for the SFGs nearly reproduces these values. Extrapolating this calibration over the AGNs 
region suggests a decrease in metallicities for these objects from 1~$Z_\odot$ to 0.3~$Z_\odot$. Based on the models of \citet{Bennert06}, 
the ionizing parameter would increase from $10^{-3.5}$ to $10^{-2.5}$, which seems in good agreement with the values found by \citet{baskin05}, 
and the abundance of nitrogen would not exceed 2 times the solar value, which is a lower excess than suggested originally by 
\citet{osterbrock70}, but still fully consistent with what is observed in the bulges of normal galaxies (Thurston et al. 1996; van Zee 
et al. 1998; Coziol et al. 1999).


%%%%% TABLA 7

\begin{table}[!t]
\vspace{0.1cm}
\centering
\caption{Gas metallicities of AGNs from the literature, compared to those obtained using our calibration}
\label{TableSy2Z}
\setlength{\tabnotewidth}{0.85\columnwidth}
\tablecols{4}
\setlength{\tabcolsep}{1.2\tabcolsep}
\begin{tabular}{lccc}
\toprule
\multicolumn{1}{c}{ID} & $Z/Z_\odot$ & $Z/Z_\odot$ & Literature \\
                       &  (lit.)    & (ours)     & sources \\
\midrule
Mrk 78   &  0.30     &   0.30  & (1)(2)  \\
NGC 3393 &  0.10     &   0.21  & (3)     \\
NGC 1068 &  0.30     &   0.16  & (4)     \\
NGC 4507 &  0.50     &   0.27  & (4)     \\
NGC 5135 &  0.2-0.5  &   0.43  & (4)     \\
NGC 5506 &  0.50     &   0.23  & (4)     \\
Mrk 1388 &  0.70     &   0.23  & (4)     \\
\bottomrule 
\tabnotetext{}{(1)Ramos Almeida et al. 2006, (2) Ulrich 1971, (3) Cooke et al. 2000, (4) Nagao et al. 2006.}
\end{tabular}
\vspace{0.3cm}
\end{table}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%




Our model suggests that there is an inversion in the metallicity-nitrogen abundance relation in the AGNs compared to that in the SFGs: 
in the SFGs, the nitrogen abundance increases with the metallicity, while in the TOs and AGNs the nitrogen abundance increases as the 
metallicity decreases (consistent with an excess in nitrogen abundance). For the TOs and AGNs the relation is:
\begin{equation}
\textrm{[O/H]} = -0.52 + (\log(\textrm{[NII]/H}\alpha)-0.6)^2\,.
\end{equation}
For the SFGs the relation is:
\begin{equation}
\textrm{[O/H]} = - [0.99 + 0.61 / (\log(\textrm{[NII]/H}\alpha)-0.05)]\,.
\end{equation}


%%%%% FIGURA 15

\begin{figure*}[!t]
\centering
\includegraphics[clip,width=12.2cm]{rcoziolf15bca.eps}
\caption {Metallicity calibrated diagnostic diagram. The two empirical relations (equations~3 and 
4) are also traced over the data. Also shown are Sy2 galaxies from the literature for comparison. 
The color figure can be viewed online.} 
\label{BPTcal1}
\vspace{0.3cm}
\end{figure*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


To test our calibration, we searched the literature for NLAGNs for which the metallicity of the gas was previously estimated using 
{\sc cloudy}, and could also be evaluated by our method (with reported spectrophotometry in the optical). We found very few examples, 
and all are Sy2 located in the upper part of our diagram in Figure~\ref{BPTcal1}, where we expect our calibration to yield the most 
discrepant results. The values obtained are compiled in Table~\ref{TableSy2Z}. Only the low metallicity solutions reported by Nagao et 
al. (2006) for their objects are consistent with our model. Surprisingly, the differences observed are not systematic, suggesting that 
other physical parameters (e.g., shocks, as we mentioned before, or special geometries, like an ionization cone or an obscuring torus) could 
also be important in these galaxies.

The above comparison suggests that the uncertainty in our metallicities is of the order of 0.3~dex, increasing to as much as 0.5~dex in 
some Sy2 like Mrk 1388. However, this galaxy appears to be an extreme case, for which we do not expect our calibration to apply. In 
Figure~\ref{BPTcal1}, most of the AGNs and TOs in our sample fall in a different regime than the AGNs from the literature used for 
the test (that is, the AGN in our sample fall in a different part of the diagnostic diagram, and they are clearly tracing a continuous 
relation). A better estimate of the uncertainty in the gas metallicities obtained with our method may be $\sim$0.2~dex, which is 
consistent with the variance of our fitted relation.


%%%%% TABLA 8

\begin{table}[!t]
\vspace{0.1cm}
\centering
\caption{Examples of gas metallicities and absolute $B$ magnitudes of AGNs\tabnotemark{$\dagger$}}
\label{TableOHAGN}
\setlength{\tabnotewidth}{0.99\columnwidth}
\tablecols{3}
%\setlength{\tabcolsep}{0.99\tabcolsep}
\begin{tabular}{lcc}
\toprule
Identification in SDSS     & [O/H]   & $M_{B}$   \\
\midrule
SDSS J113903.33$-$001221.6 & $-0.15$ & $-18.06$  \\
SDSS J150654.85$+$001110.8 & $-0.16$ & $-19.52$  \\
SDSS J113423.32$-$023145.5 & $-0.24$ & $-19.75$  \\
SDSS J172613.73$+$620858.1 & $-0.19$ & $-18.85$  \\
SDSS J173044.85$+$562107.1 & $-0.35$ & $-19.25$  \\
SDSS J025017.75$-$083548.5 & $-0.20$ & $-17.84$  \\
SDSS J032501.68$-$054444.8 & $-0.06$ & $-18.25$  \\
SDSS J142757.71$+$625609.3 & $-0.22$ & $-19.46$  \\
SDSS J114743.68$+$014934.3 & $-0.66$ & $-19.12$  \\
SDSS J133548.24$+$025956.1 & $-0.36$ & $-19.28$  \\
\bottomrule 
\tabnotetext{$\dagger$}{Full table available in electronic form at \url{http://www.astroscu.unam.mx/rmaa/RMxAA..47-2/RMxAA..47-2_rcoziol-tables.zip/}.}
\end{tabular}
\vspace{0.2cm}
\end{table}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%%%%% FIGURA 16

\begin{figure}[!t]
\centering
\includegraphics[clip,width=0.96\columnwidth]{rcoziolf16bca.eps}
\caption{Box-whisker plots for the gas metallicity variation in galaxies having (a) different 
activity types, and (b) different morphologies.} 
\label{BW_ZvsT}
\vspace{0.3cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


The gas metallicities as deduced from our calibration together with the $B$ luminosities are presented in Table~\ref{TableOHAGN} 
for the AGNs, and in Table~\ref{TableOHTO} for the TOs. In Figure~\ref{BW_ZvsT} we show the box-whisker plots for the gas 
metallicities in galaxies having different activity types and different morphologies. The AGNs and TOs have lower gas metallicities 
than the SFGs and we find an excellent correlation with the morphological type of the galaxies, the metallicities increasing in the 
later types. The statistical tests (Table~A1 and Table~A2 in the Appendix) confirm the differences observed at a level of confidence 
of 99\%.

In Figure~\ref{MBvsMetal2} we compare the gas metallicities of the TOs and AGNs with those of the SFGs. The AGNs seem to follow 
the Brodie-Huchra (1991) mass-metallicity relation for elliptical and bulge-dominated galaxies (Zaritsky et al. 1994; Coziol et al. 
1998b). The TOs, on the other hand, with their intermediate morphologies, show also metallicities which are intermediate between 
those of the SFGs and the AGNs. The differences in metallicities observed between the SFGs, TOs and AGNs are in excellent agreement 
with the differences in mean ages of the stellar populations and bulge masses. All these parameters are consistent with higher 
astration rates for the AGNs as compared to the SFGs \citep{Sandage86}.


%%%%%% TABLA 9

\begin{table}[!t]
\vspace{0.1cm}
\centering
\caption{Examples of gas metallicities and\\ absolute $B$ magnitudes of TOs\tabnotemark{$\dagger$}}
\label{TableOHTO}
\setlength{\tabnotewidth}{0.99\columnwidth}
\tablecols{3}
%\setlength{\tabcolsep}{0.99\tabcolsep}
\begin{tabular}{lcc}
\toprule
Identification in SDSS     & [O/H]      & $M_{B}$   \\
\midrule
SDSS J122353.98$-032634.4$ & $\ \ 0.16$ & $-17.40$  \\
SDSS J124428.12$-030018.8$ & $\ \ 0.14$ & $-19.10$  \\
SDSS J224424.36$-000943.5$ & $\ \ 0.24$ & $-19.44$  \\
SDSS J020540.31$-004141.4$ & $\ \ 0.21$ & $-19.37$  \\
SDSS J031347.83$+004139.7$ & $\ \ 0.10$ & $-19.27$  \\
SDSS J032406.50$-010328.2$ & $\ \ 0.03$ & $-18.49$  \\
SDSS J012853.25$+134737.6$ & $\ \ 0.02$ & $-18.95$  \\
SDSS J030848.32$-070226.1$ & $\ \ 0.28$ & $-19.53$  \\
SDSS J033358.81$-070826.6$ & $\ \ 0.10$ & $-18.82$  \\
SDSS J090513.20$-002947.8$ & $-0.01$    & $-17.82$  \\
\bottomrule 
\tabnotetext{$\dagger$}{Full table available in electronic form at \url{http://www.astroscu.unam.mx/rmaa/RMxAA..47-2/RMxAA..47-2_rcoziol-tables.zip/}.}
\end{tabular}
\vspace{0.5cm}
\end{table}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%%%%%% FIGURA 17

\begin{figure*}[!t]
\centering
\includegraphics[width=10cm]{rcoziolfig17bw.ps} 
\caption{Gas metallicities vs. absolute $B$ magnitudes for all the isolated NELGs. The 
multiple merger model of \citet{TL79} is identified as the dry merger scenario and the 
\citet{Struck81} model is denoted as the wet merger scenario (see explanations in our 
discussion section). The color figure can be viewed online.} 
\label{MBvsMetal2}
\vspace{0.5cm}
\end{figure*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\section{Discussion}

Our study suggests that the formation of a SMBH in the center of a galaxy is tightly connected with the formation process of its 
bulge \citep{haring04,peterson05,Gultekin09}. Some authors have even suggested that the formation of the bulge is a self-regulated 
process with strong feedback from the SMBH growing in its center (see Younger et al. 2008 and references therein). Our observations 
may support such interpretations.


%%%%% FIGURA 18

\begin{figure*}[!t]
\centering
\includegraphics[clip,width=13cm]{rcoziolfig18bw.eps}
\caption{Gravitational potential energy per baryon of the galaxies in our sample. The 
color figure can be viewed online.} 
\label{EGgal}
%\vspace{0.3cm}
\end{figure*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


According to \citet{Aller07} the gravitational binding energy is one key factor explaining the relation between the SMBH and the 
bulge mass. The gravitational binding energy is the negative of the gravitational potential energy. For a system of $N$ particles, 
with an approximate mass $M=N m_p$, where $m_p$ is the mass of a proton, the binding energy per baryon is equal to:
\begin{equation}
 U/N= G M m_p/R\,,
\end{equation}
where $G$ is the gravitational constant and $R$ is the radius of the object. By definition, a SMBH represent a huge mass, which 
represents maybe only 1\% to 0.1\% of the bulge mass, but which is concentrated in an extremely small region of space at the 
center of mass of the galaxy. The formation of such a highly gravitationally bound object implies a significant increase in binding 
energy of the host galaxy itself.

The above description gives us a new test for the NLAGNs in our sample. If these galaxies host a SMBH in their center, we would 
expect them to show relatively high gravitational binding energies compared to, for example, the SFGs. To verify this assumption, we 
have used the masses as determined from the $K$ magnitudes, $M_K$, and the Petrosian radii, $R_{90\%}$, reported in Table~2, to calculate 
the gravitational binding energy per baryon of all the galaxies in our sample. As expected, we observe in Figure~\ref{EGgal} that the 
AGNs and TOs have higher gravitational binding energies than the SFGs. This trend is better seen in Figure~\ref{BWEGgal}a, which shows 
the box-whisker plots for the gravitational binding energies. The trend is also confirmed to be statistically significant in Table~A1 
of the Appendix.


%%%%%% FIGURA 19

\begin{figure}[!t]
\centering
\includegraphics[clip,width=0.99\columnwidth]{rcoziolf19bca.eps}
\caption{Box-whisker plot of gravitational potential energy per baryon of the isolated 
NELGs, (a) with different activity types, (b) with different morphologies.} 
\label{BWEGgal}
\vspace{0.5cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


The variation of gravitational binding energies with galaxy morphologies is shown in Figure~\ref{BWEGgal}b, and the trend is 
statistically tested in Table~A2 of the Appendix. Galaxies with massive bulges, S0, S0a, and Sa, have comparable gravitational 
binding energies, which are significantly higher than those of galaxies having less massive bulges, Sab and later. These results 
are in good agreement with the hypothesis of a SMBH in the NLAGNs (and possibly also in the TOs).


\subsection{Masses of SMBHs vs. accretion rates}

Our analysis of the NLAGNs in our sample is consistent with the standard AGN interpretation: this phenomenon is the product of the 
accretion of matter onto a SMBH that developed at the same time as the massive bulge of the galaxy. According to this interpretation, 
the nearby NLAGNs in our sample are possibly scaled-down (and/or powered-down) versions of quasars and broad-line AGNs (BLAGNs). One 
good example is our own galaxy, with an intermediate Sb or SBb morphology \citep{binney98}, where evidence was found in its center 
for both a SBMH, with a mass of the order of 3 or $\sim$4$\times10^{6}~M_{\odot}$  \citep{GT87,GMB00,SME09}, and of recent star formation 
episodes \citep{Figer04}, which suggests it could be classified as a LINER or a TO by outside observers.

In \citet{haring04} a strong correlation was found between bulge mass and black hole mass (see also G\"ultekin et al. 2009, and 
references therein). In Shemmer et al. (2004) and Matsuoka et al. (2011), a strong correlation was also found between the black 
hole mass and the gas metallicity. These two correlations are fully consistent with our observations: while the formation of the 
SMBH follows (or self-regulates) the formation of the bulge, the gas metallicity, being a product of the evolution of the stars, 
also depends on the bulge formation through typically high astration rates \citep{Sandage86}. Based on these observations, we may 
therefore expect the NLAGNs to follow the same correlations as the quasars and BLAGNs.



%%%%%% FIGURA 20

\begin{figure}[!t]
\centering
\includegraphics[width=0.99\columnwidth]{rcoziolfig20bw.ps}
\caption{Metallicities vs. black hole mass. The dashed line is our linear fitted correlation 
(excluding the NLSy1). The metallicity for the Milky Way is that of the Sun, which is now in 
better agreement with those of stars in its immediate neighborhood (Asplund et al. 2004). The 
color figure can be viewed online.} 
\label{OHvsMBH}
\vspace{0.3cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%%%%% FIGURA 21

\begin{figure}[!t]
\centering
\includegraphics[width=0.99\columnwidth]{rcoziolfig21bw.ps}
\caption{Relation between ionization luminosity (H$\alpha$) and continuum luminosity (at 
4800~\AA). The different linear relations were determined by \citet{TP11} using  a sample 
of 318486 SDSS galaxies. The SFGs follow a linear relation $L_{\mathrm{H}\alpha}\propto L^{1.09\pm0.03}_{4800}$, 
while the NLAGNs follow a steeper power law relation $L_{\mathrm{H}\alpha}\propto L^{1.45\pm0.03}_{4800}$. 
The color figure can be viewed online.}
\label{powerlaw}
\vspace{0.4cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


To calculate the black hole masses, $M_\mathrm{BH}$, as reported in Column~3 of Table~\ref{MASS}, we used the relation between the bulge 
mass and black hole mass as determined by \citet{haring04}. For the isolated NLAGNs we find a median value of $4.8\times10^{6}~M_{\odot}$, 
which is two to three orders below the value found in BLAGNs, but in good agreement with the black hole mass found in the center of our 
galaxy (Genzel \& Townes 1987; Ghez et al. 2000; Sch\"odel et al. 2009).

In Figure~\ref{OHvsMBH} we compare the gas metallicities and black hole masses for the NLAGNs of our sample with those measured in 
quasars and BLAGNs \citep{shemmer04}. The metallicities measured by \citet{shemmer04} were transformed to the units used in our study 
by \citet{Neri-Larios11}. The NLAGNs seem to continue the linear correlation found for the quasars and BLAGNs in the lower metallicity 
regime. A linear fit with a correlation coefficient of $r_\mathrm{Pearson}=0.77$ and $r_\mathrm{Spearman}=0.83$, both with chance 
probability $P(r_\mathrm{Pearson})$ and $P(r_\mathrm{Spearman})$ practically equal to zero, suggests the metallicity increases with 
the black hole mass as:
\begin{equation}
 \log(\textrm{[O/H]}) = -2.56 + 0.3590\times \log(M_\mathrm{BH})\,.
\end{equation}

In \citet{TP11} it was shown that all the different kinds of NLAGNs (Seyfert 2, LINERS and LLAGNs) follow the same power law between 
the luminosity in H$\alpha$ and luminosity at 4800~\AA\ \citep{osterbrock89}, while the SFGs follow a different, less steep, linear 
relation. In Figure~\ref{powerlaw} we verified that this also applies to the isolated NLAGNs of our sample. One can note also the 
intermediate position of the TOs, consistent with their being a mixture of SFGs and AGNs. This result suggests that we can use the black 
hole masses in conjunction with the luminosities at 5100~\AA\ to estimate the accretion rates, as was done by \citet{peterson05}.

In Figure~\ref{accretion}, we follow the method of \citet{peterson05} to estimate the accretion rates of the NLAGNs of our sample. 
The SMBHs in the NLAGNs seem to accrete matter at relatively high rates of 0.1 times the Eddington limit. The isolated NLAGNs are 
consistent with scaled-down versions of quasars and BLAGNs, not powered-down versions: the lower luminosity is a result of smaller-mass 
black holes, not of lower accretion rates.

In Figure~\ref{OHvsAccretion} we compare the gas metallicities and accretion rates of the NLAGNs of our sample with those found in 
quasars and BLAGNs, as determined in \citet{Neri-Larios11} for the sample studied by \citet{shemmer04}. Again, the NLAGNs seem to 
continue the linear correlation found for the quasars and BLAGNs in the lower metallicity regime. A linear fit with a correlation 
coefficient $r_\mathrm{Pearson}=r_\mathrm{Spearman}= 0.66$ with both chance probabilities practically equal to zero, suggests the metallicity 
increases with the accretion rate as:
\begin{equation}
\log(\textrm{[O/H]}) = 0.77 + 0.66\times \log(L_\mathrm{Bol}/L_\mathrm{Edd})\,.
\end{equation}


%%%%%% FIGURA 22

\begin{figure}[!t]
\centering
\includegraphics[width=0.99\columnwidth]{rcoziolfig22bw.ps}
\caption{Black hole mass as a function of continuum luminosity at 5100~\AA\ for the NLAGNs 
compared to the BLAGNs studies in Peterson et al. (2005). The typical uncertainty for the 
NLAGNs is indicated by the red dot with error bars. The color figure can be viewed online.} 
\label{accretion}
\vspace{0.3cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


Although our results are fully consistent with the standard interpretation of NLAGNs as scaled-down versions of quasars and BLAGNs, 
we must state that there is an unfortunate uncertainty in the black hole masses we have determined. The fact is that there is no 
consensus in the literature about what is the correct relation to use. For practical reasons, we have used the relation proposed by 
\citet{haring04} between the bulge mass and the black hole mass. However, in  \citet{Gultekin09} the relation proposed between black 
hole mass and velocity dispersion of the bulge yields masses which are ten times higher. The problem lies with what \citet{haring04} 
called the ``black hole sphere of influence''. These authors argued that the black hole representing only a small fraction of the 
bulge mass (1\% to 0.1\%) cannot be responsible for the full value of the velocity dispersion, which explains why they apply a 
correction and obtain ten times smaller black hole mass values. In \citet{Gultekin09}, this correction is simply rejected as non valid, 
without further arguments.


%%%%% FIGURA 23

\begin{figure}[!t]
\centering
\includegraphics[width=0.98\columnwidth]{rcoziolfig23bw.ps}
\caption{Relation between metallicity and accretion rate. The NLAGNs are compared 
to the quasars and BLAGNs as studied by \citep{shemmer04}. The linear relation was 
fitted for all the galaxies (including the NLSy1). The color figure can be viewed online.} 
\label{OHvsAccretion}
\vspace{0.15cm}
\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


If we adopt the relation of \citet{Gultekin09}, then the SMBH in the NLAGNs would be ten times more massive. In Figure~\ref{OHvsMBH}, 
the relation between metallicity and black hole mass would be slightly steeper and the NLAGNs would differ significantly from the 
Milky Way. In Figure~\ref{accretion} the NLAGNs would not be scaled-down versions of BLAGNs but rather powered-down versions, that is, 
the SMBH of the NLAGNs would have masses comparable to those found in the BLAGNs but they would now be accreting at only 0.01 times the Eddington 
limit. Finally in Figure~\ref{OHvsAccretion} the NLAGNs would fall farther to the left, but would still be in good agreement with the 
metallicity vs. accretion rate relation suggested by \citet{shemmer04}.


%\vspace{0.2cm}

\section{Summary and conclusion}

We have constructed a sample of galaxies that have formed in low galactic density environments and have evolved in relative isolation. 
All these galaxies show a spiral disk and some kind of nuclear activity. Using a standard diagnostic diagram we have established that 
as many as 64\% of these galaxies are classified as NLAGNs or TOs.

We have established a strong connection between the AGN phenomenon and the mass of the bulge, but have also found a strong correlation with 
the morphological type of the galaxies. This suggests that the AGN phenomenon is a ``normal'' occurrence related to the formation process 
of massive bulges in early-type spiral galaxies.

Consistent with this interpretation, our analysis suggests the AGNs and TOs have experienced during their formation higher astration rates 
than the SFGs--transforming their gas into stars more efficiently \citep{Sandage86}. Evidences favoring higher astration rates in AGNs 
than in SFGs are: (1) their older stellar populations; (2) their lower oxygen abundance and a possible excess of nitrogen. We also found 
the NLAGNs to have higher binding energies than the SFGs, suggesting they host a SMBH in their center.

Our results are in good agreement with the standard interpretation of NLAGNs, either as scaled-down (lower BH mass but same accretion 
rates) or powered-down (same BH mass but lower accretion rates) versions of quasars and BLAGNs. The NLAGNs also seem to follow the 
same relation between the BH mass, the accretion rate and the gas metallicity as the quasars and BLAGNs.


\renewcommand{\thetable}{A\arabic{table}}
\setcounter{table}{0}


%%%%% TABLA A1

\begin{table*}[!t]
\vspace{0.1cm}
\centering
\setlength{\tabcolsep}{0.82\tabcolsep}
\caption{Dunn's post tests for activity types vs. bulge mass, total mass,\\ metallicity and gravitational binding energy}
\label{A1}
\small
\begin{tabular}{lccccccccccccccccccccc}
\toprule
 &  & \multicolumn{2}{c}{$M_\mathrm{Bulge}$}  &  & \multicolumn{2}{c}{Group 1}  &  & \multicolumn{2}{c}{Group 2}  &  & \multicolumn{2}{c}{$M_B$} 
 &  & \multicolumn{2}{c}{$M_K$}              &  & \multicolumn{2}{c}{[O/H]}    &  & \multicolumn{2}{c}{U/N} \\
 &  & AGN  & TO   &  & AGN  & TO   &  & AGN & TO   &  & AGN  & TO   &  & AGN  & TO   &  & AGN  & TO   &  & AGN  & TO \\
\cmidrule{1-1}
\cmidrule{3-4}
\cmidrule{6-7}
\cmidrule{9-10}
\cmidrule{12-13}
\cmidrule{15-16}
\cmidrule{18-19}
\cmidrule{21-22}
AGN & &      &       & &      &    & &      &    & &      &     & &      &       & &      &        & &      &    \\
TO  & & ***  &       & &  ns  &    & &  ns  &    & & ns   &     & & ns   &       & & ***  &        & & ***  &    \\
SFG & & ***  &  ***  & &  ns  & ns & &  *** & ns & & ***  & **  & & ***  &  ***  & & ***  &  ***   & & ***  & ** \\
\bottomrule
\end{tabular}
\vspace{0.4cm}
\end{table*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%



\acknowledgments

We dedicate this article to the memory of Gabriel Garc\'ia Ruiz, night assistant and friend at the Observatorio Astron\'omico Nacional, 
San Pedro M\'artir, Mexico, who died tragically while on duty at the end of 2010. The authors would also like to thank N. Bennert and 
S. Komossa for making their results of CLOUDY models available to us, and O. Shemmer for furnishing the data for the quasars and BLAGNs. 
T-P acknowledges PROMEP for support grant 103.5-10-4684 and I. P.-F. acknowledges a postdoctoral fellowship from Conacyt, Mexico No. 
145727. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of 
Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and 
Space Administration and the National Science Foundation. The SDSS is managed by the Astrophysical Research Consortium (ARC) for the 
Participating Institutions. The Participating Institutions are: the American Museum of Natural History, Astrophysical Institute Potsdam, 
University of Basel, University of Cambridge (Cambridge University), Case Western Reserve University, the University of Chicago, the 
Fermi National Accelerator Laboratory (Fermilab), the Institute for Advanced Study, the Japan Participation Group, the Johns Hopkins 
University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist 
Group, the Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), 
the New Mexico State University, the Ohio State University, the University of Pittsburgh, University of Portsmouth, Princeton University, 
the United States Naval Observatory, and the University of Washington.


%%%%% TABLA A2

\begin{table*}[!t]
\vspace{0.1cm}
\centering
\setlength{\tabcolsep}{0.98\tabcolsep}
\caption{Dunn's post tests for morphology vs. bulge mass, total mass,\\ metallicity and gravitational binding energy}
\label{A2}
\small
\begin{tabular}{lcccccccccccccccccc}
\toprule  
 &  & \multicolumn{5}{c}{$M_\mathrm{Bulge}$}  &  & \multicolumn{5}{c}{[O/H]}    &  & \multicolumn{5}{c}{U/N} \\
 &  & S0/S0a & Sa & Sab & Sb & Sc            &  & S0/S0a & Sa & Sab & Sb &     &  & S0/S0a & Sa  & Sab & Sb & Sc  \\
\cmidrule{1-1}
\cmidrule{3-7}
\cmidrule{9-13}
\cmidrule{15-19}
Sa   &  & ns   &     &     &     &     &  & ns   &     &     &     &    &  & ns  &     &     &     &     \\
Sab  &  & ns   & ns  &     &     &     &  & ns   & **  &     &     &    &  & ns  & *** &     &     &     \\
Sb   &  & **   & *** & ns  &     &     &  & **   & *** & ns  &     &    &  & ns  & *** & ns  &     &     \\
Sc   &  & ***  & *** & *** & **  &     &  & ***  & *** & *** & ns  &    &  & *** & *** & **  & ns  &     \\
Sd   &  & ***  & *** & *** & *** & ns  &  &      &     &     &     &    &  & *** & *** & *** & *** & ns  \\
\cmidrule{1-19}
 &  & \multicolumn{5}{c}{$M_B$}              &  & \multicolumn{5}{c}{$M_K$}     &  &  &  &  &  &  \\
 &  & S0/S0a & Sa & Sab & Sb & Sc            &  & S0/S0a & Sa & Sab & Sb & Sc   &  &  &  &  &  &  \\
\cmidrule{1-1}
\cmidrule{3-7}
\cmidrule{9-13}
Sa   &  & ns   &     &     &     &     &  & ns   &      &     &     &    &  &  &  &  &   \\
Sab  &  & ns   & ns  &     &     &     &  & ns   & ns   &     &     &    &  &  &  &  &   \\
Sb   &  & ns   & ns  & ns  &     &     &  & ns   & **   & ns  &     &    &  &  &  &  &   \\
Sc   &  & **   & **  & ns  & *** &     &  & ns   & ***  & ns  & ns  &    &  &  &  &  &   \\
Sd   &  & ***  & *** & *** & *** & **  &  & ***  & ***  & *** & *** & ** &  &  &  &  &   \\
\bottomrule
\end{tabular}
\vspace{0.45cm}
\end{table*}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%5


\begin{appendix}

\section*{APPENDIX -- RESULTS OF STATISTICAL TEST}

The statistical tests used in this study are the non-parametric Kruskal-Wallis test (KW), together with the Dunn's post tests. The 
KW test compares the medians in three or more unmatched groups of data and the post tests do multiple one-to-one comparisons. The 
$P$ values reported in Tables~A1 and A2  are codified in the following way: non significant difference, ns, significant, ** for 
$P<0.01$, highly significant, *** for $P<0.001$.

In Table~A1, comparing the bulge mass in galaxies having different activity types, the post tests indicate that all three samples have 
different medians. In Group~1, no difference is found while in Group~2 only the AGNs differ from the SFGs.

In Table~A2 we compare the bulge mass in galaxies having different morphologies; the post tests find highly significant differences 
between the most separated morphological classes. No differences are observed between the S0, S0a, Sa and Sab, the Sab and Sb, and the 
Sc and Sd. These tests suggest that the relation between bulge mass and activity type is a by-product of the morphology, consistent 
with the results for the diagnostic diagrams.

In Table~A1 we verify that the total mass does not vary as strongly as the bulge mass in galaxies having different activity types. The 
same is true for the morphologies in Table~A2. Therefore, the trend observed for the AGN activity is more qualitative than quantitative: 
it is not the mass of the galaxy that counts, but the fraction of mass that is in the form of a bulge.

In Table~A1 we find significant differences in metallicities, the AGNs being less metal rich than the SFGs, and the TOs being intermediate. 
We also find a significant difference in metallicity between the early and late-type galaxies in Table~A2, early-type galaxies being metal 
poor compared to late-type ones. These results are in good agreement with the difference in mean stellar population ages.

In Table~A1 we find a strong relation between the gravitational binding energy and the AGN activity. This applies for galaxies having more 
massive bulges, Table~A2. Since we find no such relation for the masses, these results are consistent with a strong dependence of the AGN 
phenomenon with the gravitational binding energy: the larger the gravitational binding energy, the larger the bulge mass and the higher 
the probability to find an AGN.

All these results favor an explanation in terms of astration rates: high astration rates produce massive bulges which increases the 
probability of finding an AGN.


\end{appendix}


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