\documentclass[journal]{rmaa} 


\title{Missing Seyfert Galaxies}


\author{
Hrant M. Tovmassian\altaffilmark{1} and
O. Yam\altaffilmark{2}}

\altaffiltext{1}{Instituto Nacional de Astrof\'{i}sica \'Optica y Electr\'onica, Puebla, Mexico.}
\altaffiltext{2}{Depto. de Ciencias, Universidad de Quintana Roo, Mexico.}


\fulladdresses{
\item Hrant M. Tovmassian: Instituto Nacional de Astrof\'isica \'Optica y Electr\'onica, Apdo. Postal 51 y 216, C.P. 72000, Puebla, Puebla, 
Mexico (hrant@inaoep.mx).
\item O.Yam: Depto. de Ciencias, Universidad de Quintana Roo,  Blvd. Bah\'ia, 77019 Chetumal, Q.~Roo, Mexico (oyam@uqroo.mx).}


\shortauthor{Tovmassian \& Yam}

\shorttitle{Missing Seyferts} 


\ReceivedDate{2011 May 11} 
\AcceptedDate{2011 July  1} 
\SetYear{2011}


\listofauthors{H.~M. Tovmassian \& O. Yam}

\indexauthor{Tovmassian, H.~M.}
\indexauthor{Yam, O.}


\resumen{El objetivo de este trabajo es la estimaci\'on del n\'umero de galaxias Seyfert \emph{faltantes} debido a la ocultaci\'on 
por el polvo del disco de las galaxias espirales. Comparamos la distribuci\'on de las inclinaciones de las galaxias anfitrionas de 
las Sy1s y Sy2s con la de la muestra de control de las galaxias espirales. Encontramos que el n\'umero relativo de galaxias Seyfert 
es mayor para las Seyfert vistas de frente, en las galaxias con $ i<30\arcdeg$, y es menor en las galaxias muy inclinadas con 
$i>61\arcdeg$ que en las galaxias espirales sin n\'ucleos activos. Concluimos que la diferencia observada se debe a la absorci\'on 
de los n\'ucleos Seyfert en el disco de polvo de las galaxias inclinadas. Estimamos que las Seyfert faltantes son alrededor del 100\% 
para las Sy1y y del 50\% para las Sy2s.}


\abstract{The aim of this work is to estimate the number of Seyfert nuclei missing in catalogs due to obscuration by the dust disc in 
host spiral galaxies. We compared the distribution of inclinations of host galaxies of Sy1s and Sy2s with that of in control sample 
of spiral galaxies, and found that the relative number of Seyferts is higher in almost face-on galaxies with $i<30\arcdeg$ and smaller 
in highly inclined, $i>61\arcdeg$, spiral galaxies without active nuclei. We conclude that the difference found is 
due to absorption of the Seyfert nuclei by the dust-disc of inclined galaxies. We estimate that about 100\% and 50\% of the observed 
Sy1s and Sy2s respectively are missing.}


\addkeyword{galaxies: Seyfert}


\begin{document}


\maketitle


\section{Introduction}

According to the Unified scheme (Miller \& Goodrich 1990; Antonucci 1993; Urry \& Padovani 1995), the difference between Sy1 and Sy2 
galaxies is caused by the orientation of the inner dust torus in relation to the observer. If the opening angle of the torus is pointing 
towards the observer, and the active galactic nuclei (AGN) with the broad line region (BLR) is directly seen, the galaxy is classified 
as Sy1. If the orientation of the torus is such that the BLR is hidden, and only the narrow line region is observable the galaxy is 
classified as Sy2. The ratio of the numbers of Sy1 and Sy2 galaxies determines the opening angle of the putative dust torus. 
In order to understand the nature of Seyferts it is also important to know the frequency of their occurrence among galaxies. 

Numerous efforts have been made in compiling new a large list of Seyfert galaxies (e.g., Huchra 1977; V\'{e}ron-Cetty \& Veron 2010, 
and references therein). Some Seyfert nuclei, however, may not be revealed because they are hidden behind the galactic dust-disc. Indeed, 
Keel (1980), Kirhakos \& Steiner (1990), McLeod \& Rieke (1995) found, that Seyfert nuclei avoid edge-on galaxies. Malkan, Gorjian, \& 
Tam (1998) also found dust lanes and patches in the central regions of Seyferts. Driver et al. (2008) showed that the role of absorption 
of starlight by dust grains in galactic discs is substantial. Therefore, the emission of the AGN may be completely hidden by the dust-disc 
in some highly inclined spiral galaxies, and a certain number of Seyferts may be missing. The real ratio of the numbers of Sy2 and Sy1 galaxies 
may be altered, and consequently the  value of the opening angle of the dust-torus in the Unified scheme may be in error. 


%%%%%% TABLA 1

\begin{table*}[!t]
\vspace{0.1cm}
\centering
\setlength{\tabcolsep}{1.3\tabcolsep}
\caption{The number of spiral galaxies galaxies in three bins of inclination \lowercase{$i$}}
\begin{tabular}{ccccccccc}
\toprule
  &  & $i\leq30\arcdeg$  & $31\arcdeg<i<60\arcdeg$  & $i>61\arcdeg$  &  & $i\leq30\arcdeg$  & $31\arcdeg<i<60\arcdeg$  & $i>61\arcdeg$ \\ 
\cmidrule{3-5}
\cmidrule{7-9}
Type &               & \multicolumn{3}{c}{V-CV}  &   & \multicolumn{3}{c}{SSRS2} \\
\cmidrule{1-1}
\cmidrule{3-5}
\cmidrule{7-9}
$n$  &               &  56	& 231	& 157   &  & 163   & 703   & 557   \\
$n/n_\mathrm{total}$  &   & 0.13	& 0.52  & 0.35  &  & 0.12  & 0.49  & 0.39  \\
\bottomrule
\end{tabular}
\label{tab:1}
\vspace{0.5cm}
\end{table*}

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


It has been suggested that the activity of the galactic nucleus may be triggered by dense environments. However, contradictory results 
have also been obtained. Petrosian (1982), Dahari (1985), Laurikainen  \& Salo (1995), Dultzin-Hacyan et al. (1999), Storchi-Bergmann et al. 
(2001), Chatzichristou (2002), S\'{a}nchez \& Gonzal\'{e}z-Serrano (2003), Kouluoridis et al. (2006) showed that Seyferts are found in 
more dense environments. Meanwhile, Viriani, De Robertis, \& van Dalfsen  (2000), Schmitt (2001, and references therein) did not find a 
relation between nuclear activity and environment. The discrepancy may be explained by the inclusion of galaxies with hidden Seyfert nuclei 
in the control samples.

The first attempt to estimate the number of missing Seyferts was made by Tovmassian (2001). In this paper we reconsider the problem, and 
estimate the number of missing Seyferts by the study of the distribution of inclinations of spiral galaxies in which Seyfert nuclei mainly 
reside. We take into account that, as Kinney et al. (2000) showed, the distribution of the angle $\beta$ between the direction of the radio 
jet (or the dust torus axis) and the galaxy rotation axis is homogenous in the $0\arcdeg-90\arcdeg$ range. This means that Seyferts should be 
observed in host galaxies with any inclination angle, if there are no biases in the catalogs. Kirhakos \& Steiner (1990) suggested detecting 
the missing edge-on Seyferts on the basis of the IR and X-ray emission of galaxies. The presence of the broad line region (BLR) emission in 
some galaxies was detected by observation of the polarized emission scattered by particles located far from the nucleus of a galaxy. Such 
galaxies are called hidden Sy1s (Sy1h). 


%%%%% TABLA 2

\begin{table*}[!t]
\vspace{0.1cm}
\centering
\caption{The numbers and relative numbers of Sy1s and Sy2s galaxies\\ in three bins of inclination \lowercase{$i$}}
\small
\begin{tabular}{ccccccc}
\toprule
Type   & \multicolumn{6}{c}{VC-V}      \\
\cmidrule{1-7}
$i$  & $N(<30\arcdeg)$ & $N(31\arcdeg-60\arcdeg)$ & $N(>61\arcdeg)$ & $N(<30\arcdeg)/N_t$ & $N(31\arcdeg-60\arcdeg)/N_t$ & $N(>61\arcdeg)/N_t$ \\
\cmidrule{1-7}
Sy1  & 20  & ~49  & 10  & 0.25 & 0.62 & 0.13 \\  
Sy2  & 35  & 103  & 50  & 0.19 & 0.55 & 0.26 \\
\bottomrule
\end{tabular}
\label{tab:2}
\vspace{0.3cm}
\end{table*}

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



\section{Results and Discussion}

\subsection{The data}

For our study we used the Sy1 and Sy2 galaxies from the 13-th Edition of the Catalogue of Quasars and Active Nuclei (V\'{e}ron-Cetty \& 
V\'{e}ron 2010, hereafter VC-V) with magnitudes $V<16^m$) and $z<0.035$. Seyferts hosted in spiral galaxies of morphological types S0/a 
and later were included in the study. Morphological types of host galaxies of Seyferts were taken from the NED (The NASA/IPAC Extragalactic 
Database). In accordance with Meurs \& Wilson (1984), Osterbrock \& Shaw (1988), we joined galaxies with Sy1.2 and Sy1.5 types into the Sy1 
category, and Sy1.8s and Sy1.9s into the Sy2 category. The sample of Sy1s includes galaxies of Sy1n type, and the sample of Sy2s includes 
galaxies classified as Sy1h (hidden Sy1). By cross-checking with the data in the NED we excluded from the studied sample those galaxies 
whose classification in the NED differed from those in the VC-V catalog. The compiled sample contains 79 Sy1s and 188 Sy2s. 

For comparison we compiled two lists of spiral galaxies using the Revised Shapley-Ames catalog of bright galaxies (Sandage \& Tammann 1981) 
and the Southern Sky Redshift Survey (SSRS2) (da Costa et al. 1998). From the latter catalog we used galaxies with radial velocities 
$V_r<9000$~km~s$^{-1}$ and magnitudes $V<15.5^m$. From both catalogs we selected spiral galaxies of morphological types from S0/a to Scd. 
The morphological types were taken from the NED. Galaxies with active galactic nuclei, such as Seyferts, LINERs or those having powerful radio 
emission were excluded. We did not include in the sample galaxies with any peculiar morphology or members of dense doubles.The compiled list 
from the Revised Shapley-Ames catalog contains 444 galaxies with measured $a$ and $b$ axes given in the NED. The corresponding list from the 
SSRS2 catalog consists of 1423 galaxies. 


\subsection{Analysis and Results}

We calculated the inclinations $i$ of the galaxies using their observed axial ratio $b/a$, and binned them in three intervals of inclination 
$i\leq30\arcdeg$, $31\arcdeg<i<60\arcdeg$ and $i>61\arcdeg$. In Table~1 we present the numbers of spiral galaxies of the compiled lists from 
the Revised Shapley-Ames (1st row) and the SSRS2 (3rd row) catalogs  in these intervals of inclination. The relative numbers for both catalogs 
in corresponding bins are given in Rows~2 and 4 of Table~1. Table~1 shows that the distribution of inclinations is not uniform which is evidence  
of the rather prolate space configuration of spiral galaxies (Vincent \& Ryden 2005).

In the absence of biases one would expect  the distribution of inclinations of the Seyferts to be the same as that of the normal galaxies, 
and they should also have about the same relative numbers in the three intervals of inclinations considered. However, we found that the 
distribution of inclinations of Seyfert galaxies differs from that of spiral galaxies not containing active nuclei (Table~2). A comparison 
of Table~1 and Table~2 shows that the relative number of both types of Seyferts in almost face-on galaxies ($i\leq30\arcdeg$) is higher than 
in the whole sample of spiral galaxies. The relative number of Seyferts in the second bin is about the same, as for spiral galaxies. At the 
same time, the relative number of galaxies in the third bin of highly inclined Seyfert host galaxies ($i\geq61\arcdeg$) is smaller than that 
of spirals.  




\subsection{The number of missing Seyfert galaxies}

We suggest that the higher abundance of Seyferts seen in almost face-on host spiral galaxies and their paucity in galaxies with very high 
inclination (third bin) is due to absorption of the emission of the Seyfert nuclei by the dust disc of their host galaxies. In order to 
estimate the number of missing Seyferts we assume that the relative number of their host galaxies with $i\leq30\arcdeg$ ($b/a<0.86$) must 
be equal to the corresponding value of spiral galaxies, i.e. about 0.125. Dividing the numbers of Sy1s and Sy2s with inclinations 
$i\leq30\arcdeg$ to the deduced relative number $n_{i}\leq30\arcdeg$ of spiral galaxies with inclinations in the same bin, we estimate the 
expected total number $n_t$ of Sy1s and Sy2s correspondingly. Subtracting from the estimated total number $n_t$ of Seyferts the corresponding 
observed number $n_o$, we find the number $n_m$ of the missing Seyferts. The results of our accounting are presented in Table~3. In the sixth 
column the relative number $n_m/n_t$ is given.


%%%%% TABLA 3

\begin{table}[!t]
\vspace{0.1cm}
\centering
\setlength{\tabcolsep}{1.2\tabcolsep}
\caption{Number of Sy1s and Sy2s with inclinations of the host spiral galaxies \lowercase{$i\leq30\arcdeg$}, 
their estimated total number, number of missing\\ ones and relative number\\ of missing Seyferts}
\begin{tabular}{cccccc}
\toprule
 & $n_{i\leq30\arcdeg}$ & $n_t$ & $n_o$ & $n_m$ & $n_m$/$n_o$ \\
\midrule
Sy1 & 20 & 160 & ~79 & 81 & 1.02 \\
Sy2 & 35 & 280 & 188 & 92 & 0.49 \\	
\bottomrule
\end{tabular}
\label{tab:3}
\vspace{0.3cm}
\end{table}

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


Table~3 shows that the number of missing Sy1s is about the same as the number of the known ones. The number of the missing Sy2s 
is about half of the known ones. 


\section{Conclusions}

We compared the distribution of inclinations of the host galaxies of Sy1 and Sy2 galaxies from the VC-V catalog with that of the control 
sample of spiral galaxies not containing AGN from the Revised Shapley-Ames and the SSRS2 catalogs, and estimated the number of Seyferts 
in which the AGN is hidden by the absorbing dust in the disc of inclined galaxies. We find that about 100\% of Sy1s and 50\% of Sy2s are 
missing. The relatively smaller number of missing Sy2s might be expected, since the NLR of some of them are observed at large distances 
from the galactic plane, where absorption of the dust disc is smaller. The number of missing Seyferts is large and may influence the 
conclusions on the nature of these galaxies and on the mechanisms of their formation. The relative number of Seyferts among galaxies is thus 
higher than previously estimated. Taking into account the missing Seyferts increases the opening angle of the dust torus from $90\arcdeg$ 
to $100\arcdeg$. When considering the influence of the environment in activating the Seyfert nuclei, it is necessary to be careful in compiling 
a control sample: the inclusion of large numbers of highly inclined galaxies in the latter, strongly affected by the disc absorption, may 
result in erroneous conclusions.

\vspace{0.1cm}

\acknowledgements

We are grateful to the referee for a careful reading of the paper and valuable suggestions for its improvement. HMT thanks the IPAC (Pasadena, 
CA, USA) for hospitality. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion 
Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

\vspace{0.22cm}

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\end{thebibliography}



\end{document}


