Setting the normalcy level of HI properties in isolated galaxies
D. Espada$^{1,2}$
L. Verdes-Montenegro$^{1}$
E. Athanassoula$^{3}$
A. Bosma$^{3}$
W. K. Huchtmeier$^{4}$
S. Leon$^{5}$
U. Lisenfeld$^{6}$
J. Sabater$^{1}$
J. Sulentic$^{1}$
S. Verley$^{6}$ and M. Yun$^{7}$
%%% Abstract to run on from here. Studying the atomic gas (HI) properties of the most isolated galaxies is essential to quantify the effect that the environment exerts on this sensitive component of the interstellar medium. We observed and compiled HI data for a well defined sample of $\sim$ 800 galaxies in the Catalog of Isolated Galaxies, as part of the AMIGA project (Analysis of the ISM in Isolated GAlaxies, \verb!http://amiga.iaa.es!), which enlarges considerably previous samples used to quantify the HI deficiency in galaxies located in denser environments. By studying the shape of 182 HI profiles, we revisited the usually accepted result that, independently of the environment, more than half of the galaxies present a perturbed HI disk. In isolated galaxies this would certainly be a striking result if these are supposed to be the most relaxed systems, and has implications in the relaxation time scales of HI disks and the nature of the most frequent perturbing mechanisms in galaxies. Our sample likely exhibits the lowest HI asymmetry level in the local Universe. We found that field samples present an excess of $\sim$ 20\% more asymmetric HI profiles than that in CIG. Still a small percentage of galaxies in our sample present large asymmetries. Follow-up high resolution VLA maps give insight into the origin of such asymmetries.
despada-AMIGAHI.tex
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\begin{document}
\title{Setting the normalcy level of HI properties in isolated galaxies}   %%% Fill in title
\author{D. Espada$^{1,2}$,
           L. Verdes-Montenegro$^{1}$,
	    E. Athanassoula$^{3}$,
           A. Bosma$^{3}$,
           W. K. Huchtmeier$^{4}$,
           S. Leon$^{5}$, 
           U. Lisenfeld$^{6}$, 
           J. Sabater$^{1}$,
           J. Sulentic$^{1}$,
           S. Verley$^{6}$ and
           M. Yun$^{7}$ }   %%% Fill in author names
\affil{$^{1}$ Instituto de Astrof\'{i}sica de Andaluc\'{i}a (IAA-CSIC) (daniel@iaa.es); $^{2}$
 Harvard-Smithsonian Center for Astrophysics (CfA); $^{3}$ Laboratoire d'Astrophysique de Marseille; $^{4}$ Max-Planck-Institut fuer Radioastronomie (MPIfR); $^{5}$ Instituto de Radioastronom\'{i}a Milim\'{e}trica (IRAM); $^{6}$ Universidad de Granada; $^{7}$ UMass-Amherst}    %%% Fill in author affiliations

\begin{abstract} %%% Abstract to run on from here.
Studying the atomic gas (HI) properties of the most isolated galaxies is essential  to quantify the effect that the environment exerts on this sensitive  component of the interstellar medium.  We observed and compiled HI data for a  well defined sample of $\sim$ 800 galaxies in the Catalog of Isolated Galaxies, as part of the AMIGA project (Analysis of the ISM in Isolated GAlaxies, \verb!http://amiga.iaa.es!), which enlarges considerably previous samples used to quantify the HI deficiency in galaxies located in denser environments. By studying the shape of 182 HI profiles, we revisited the usually accepted result that, independently of the environment, more than half of the galaxies present a perturbed HI disk. In isolated galaxies this would certainly be a striking result if these are supposed to be the most relaxed systems, and has implications in the relaxation time scales of HI disks and the nature of the most frequent perturbing mechanisms in galaxies.  Our sample likely exhibits the lowest HI asymmetry level in the local Universe. We found that field samples present an excess of $\sim$ 20\% more asymmetric HI profiles than that in CIG. Still a small percentage of galaxies in our sample present large asymmetries. Follow-up high resolution VLA maps give insight into the origin of such asymmetries.

\end{abstract}


\vspace{-0.8cm}

\section{Introduction}

Atomic gas (\hi ) is presumably the most extended cold
component  of the interstellar medium (ISM) and is very sensitive to any kind of perturbation. 
For instance, the \hi\ component in spiral galaxies has long been known to show both  geometric and kinematic
asymmetries \citep[e.g.][]{1969Natur.221..531B,1980MNRAS.193..313B}, 
and the HI content is found to be deficient in galaxies in denser environments \citep[e.g.][]{1973MNRAS.165..231D}.
%Asymmetries in the stellar disk are also a common feature as traced by their optical and 
%near infrared light (the latter less affected by dust extinction). Observations
%show that 30\% of galaxies are significantly lopsided at near infrared wavelengths \citep{1994A&A...288..365B,1995ApJ...447...82R,1997ApJ...477..118Z,2005A&A...438..507B}.  
%Measures of \hi\ properties in galaxies are thus arguably 
%the most sensitive interaction diagnostics at our disposal. 
%First, the HI content has been since long seen to be HI deficient in galaxies in denser environments. 
Although asymmetries have also been seen in stellar disks \citep[e.g.][]{1997ApJ...477..118Z}, the fact that HI is usually twice more extended than the optical component makes it arguably a better tracer of interaction. 
The study of \hi\ global velocity profiles of galaxies has proven to be very useful for both 
the quantification of the HI content as well as the asymmetry level.  In the context of the AMIGA project, we use \hi\ global velocity profiles of a large sample of strictly isolated galaxies in order to: $i)$ revise the reference to study the HI content normalcy level  in galaxies, $ii)$ quantify the rate of \hi\ asymmetries, and $iii)$ study the relevance of different mechanisms producing asymmetries in galaxies, with the additional help of aperture synthesis maps.
% Finally only aperture synthesis maps can provide 
%full details about the \hi\  distribution and kinematics, the one-dimensional profiles provide a 
%valuable diagnostic at a small fraction of the cost in observing time. We study the origin of 
%asymmetries for a subsample of galaxies with higher resolution VLA observations.

%We report here on a study of \hi\ profile asymmetries in some of the most isolated 
%galaxies in the local Universe, in the context of the AMIGA project (Analysis of the Interstellar Medium of Isolated GAlaxies). %This project provides a statistically-significant and complete sample of the most isolated galaxies in the northern sky. 
%This presents us with an excellent opportunity to assess the range of intrinsic \hi\
%asymmetries (i.e. internally generated) thus providing a 
%``zero-point'' calibration for evaluating such measures 
%in less isolated samples.
\vspace{-0.3cm}
\section{HI content}

We present \hi\ data  for $N$ = 834 CIG galaxies (Catalog of Isolated Galaxies, \citealt{1973AISAO...8....3K}) and study the correlations between the \hi\ content and different parameters associated with the stellar component: optical luminosities (see Figure~1, left), diameters and morphological type. We calculated \hi\ fluxes, mean velocities, line-widths and \hi\ masses for galaxies we observed at the Arecibo, Effelsberg, Nan\c{c}ay and Green Bank (GBT) radio-telescopes, or compiled  from different sources in the literature  \citep{2009E}. Survival analysis was applied in order to use the information given by upper limits.
  This sample is about 3 times larger than that of \citet{1984AJ.....89..758H}, the prevalent reference of HI data for isolated galaxies to date, increasing considerably the population per morphological bin. This is important especially for the less populated bins, such as E and Sd/Irr, where our sample has a factor 5--10 more galaxies.
Our improved sample is thus essential as a control sample for studies involving galaxies in denser environments.
We note that to compare the HI content of two different samples, one needs to homogeneously adopt the same constants and corrections, since otherwise the differences may be comparable to the observed levels of deficiency. 

\vspace{-0.3cm}
\section{Global measure of HI asymmetry}

 The frequency and amplitude of gaseous disk asymmetries have been studied by several authors, with the main conclusion that at least half of the galaxies are asymmetric even in the field \citep[e.g.][]{1994A&A...290L...9R}. 
%We use global \hi\ velocity profiles with $S/N>10$ for isolated galaxies in AMIGA. 
We restricted our study to a subsample of $N$ = 182 galaxies for which we have  global \hi\ velocity profiles with $S/N$ $>$ 10 and minimized undesired artificial lopsidedness. We calculated a flux ratio asymmetry parameter to quantify the asymmetry of the profiles, defined as $A_{flux~ratio}= A_{l/h}$, if $A_{l/h}$ $>$ 1, and
$1/A_{l/h}$  otherwise,  where $A_{l/h}$ is the ratio of the areas under the profile at velocities lower and higher than
the central velocity (e.g. \citealt{1998AJ....115...62H}). We minimized artificial asymmetries by avoiding large pointing offsets and poor determinations of the central velocity which resulted in possible errors of $A_{flux~ratio}$ larger than 10\%.

%{\bf characterization HI asymmetry in the distribution of isolated galaxies}
The resulting $A_{flux~ratio}$ distribution was characterized by a half-Gaussian profile with a FWHM/2 of 0.13 (Figure~\ref{fig1}, right). 
This width is very likely smaller than 0.13, since remaining systematic errors were estimated to be $\sim$ 7\%, mostly produced by pointing offsets and an inaccurate determination of the mean velocity.
%We have found that the small observed shift and broadening of the peak
%can be explained by random errors in the pointing. 
For large values of the parameters, $A_{flux~ratio}$ $>$ 1.3, we found a total of 16 galaxies. This corresponds to an excess of 12 more than what would correspond to the fitted half-Gaussian profile, or a  6\% of the subsample. This is very likely a residual of interacting systems.
 %We have estimated the average of those errors to be $\sim$ 7\%, and thus the true width of the Gaussian distribution would be 0.11.%, either from flux loss because of
%known pointing errors or an inaccurate determination of the mean velocity. 

%According to our study if the expected errors due to random pointing offsets 
%could be removed, the underlying distribution of the asymmetry parameter
%would peak at 1 and would have a much smaller dispersion of the order of 0.06. 
%This supports the idea that when
%all errors are taken into account we would be in front of a sample with minimal intrinsic deviations from HI
%asymmetry in their gaseous discs, except for an excess of extreme values from
%Aflux ratio > 1.3, 

Among our sample of  isolated galaxies we find a weak trend for larger \hi\ asymmetries to be related to less isolated systems.  We plot in Figure~\ref{fig2} (left) the comparison of the distribution of
 the tidal strength ($Q_{Kar}$) and galaxy surface number density ($\eta_K$) parameters (\citealt{2007A&A...470..505V}$a$,$b$) for the most symmetric  ($A_{flux~ratio}$ $<$ 1.13) and the most asymmetric profiles ($A_{flux~ratio}$ $>$ 1.13). Smaller $\eta_K$ and $Q_{Kar}$ are found for galaxies with symmetric \hi\ profiles. %The mean of $\eta_K$  for the symmetric profiles is 1.4 $\pm$ 0.5 (median 1.36) while for the asymmetric profiles it is 1.5 $\pm$ 0.6 (median 1.48). As for $Q_{Kar}$, the mean  for the symmetric profiles  is  -3.2 $\pm$ 0.6 (median -3.33) while for the asymmetric profiles it is -3.0 $\pm$ 0.7 (median -3.10). 
  
%Only 38\% of the galaxies in this sample have \hi\ profiles with an asymmetry parameter outside the intrinsic width. Usually in these cases the shape is characterized by different heights on the two horns. 
%This rate is larger than 50\% in samples of field or not obvious interacting galaxies. This difference suggest that this difference could be 
%%A larger difference between this found for a 0.20 width, where 19\% outside a respective 33 - 43\% in other samples of galaxies.
%%This asymmetry rate should be 
%even higher in strongly interacting galaxies. 

%While 32\% of the galaxies are expected to show an asymmetry parameter beyond 1.13, we find 38\%, which show a small excess above the expected value. 

In  Figure~\ref{fig2} (right) we show the $A_{flux~ratio}$ cumulative probability distribution of previous samples of field/isolated galaxies and compare it with that in our sample. We found a $\sim$ 20\% excess of  asymmetric profiles in samples of field galaxies or not obviously interacting galaxies. The only similar distribution to ours is that of Haynes et al. (1998), whose sample comprises some isolated galaxies, although not members of CIG. 
The rate of asymmetric profiles is expected to be even higher in strongly interacting galaxies.

\begin{figure}
\begin{center}
%\includegraphics[width=10cm]{MHI-LB_CIG_survivalfits.eps}
\includegraphics[width=8cm]{despada_fig1a.eps}
\includegraphics[width=5cm]{despada_fig1b.eps}
\caption{$(Left):$ \mhi\ vs $L_B$ plot for the whole sample of $N$ = 837 galaxies. Detections (cross signs) as well as upper limits (arrow signs) are included. The fit presented by Haynes \& Giovanelli (1984) is shown as a red line, and that of our sample in blue.
$(Right):$ $A_{flux~ratio}$ distribution, half-Gaussian fit and the residuals. 
\label{fig1}}
\end{center}
\end{figure}





 %Finally, we do not find any strong correlation between the \hi\ asymmetry and the morphological type, optical and FIR luminosities or  signature of interaction in the optical. A signature of perturbed optical emission is not a necessary condition for the \hi\  profile to be asymmetric, and viceversa. 
 %A trend is found for larger \hi\ asymmetries in galaxies that are more luminous in FIR, which are likely interacting systems. 
%We also find evidence that galaxies with high spiral arm strength have less \hi\ asymmetries.

%We find no strong correlations 
%between \hi\ asymmetry and optical morphology or
%luminosity. On the other hand, there is a  tendency for isolated galaxies with  larger 
%\hi\ asymmetries to show higher IR luminosities and smaller spiral arm
%strength. 



\begin{figure}
\begin{center}
%\includegraphics[width=5cm]{asim-histo-paper}
\hspace{-2cm}
\includegraphics[height=5.3cm,width=6.5cm]{despada_fig2a.eps}
\includegraphics[width=8.5cm]{despada_fig2b.eps}
\hspace{-2cm}
\caption{$(Left):$ Isolation parameter distribution ($Q_{Kar}$ and $\eta_K$) for symmetric ($A_{flux~ratio}$ $<$ 1.13, solid-line histogram) and asymmetric ($A_{flux~ratio}$ $>$ 1.13, dashed-line histogram) HI profiles.  
   The mean, standard deviation and median are shown as  circle, bar and triangle signs, respectively.
 $(Right):$  Comparison of  the cumulative  $A_{flux~ratio}$ distribution between our sample (solid line) and other samples (dashed line). 
 %Note that the subtraction of their cumulative distribution from that of our sample indicate the asymmetry rate difference for a given $A_{flux~ratio}$.
 }
\label{fig2}
\end{center}
\end{figure}


\vspace{-0.4cm}

\section{Origin of HI asymmetries}

%We are studying the origin of the asymmetries in the small fraction of strongly asymmetric \hi\ profiles in our sample of isolated galaxies. 
We carried out a follow-up study of the origin of the \hi\ asymmetries for eight galaxies with the most asymmetric profiles as well as four galaxies with the most symmetric \hi\ profiles for comparison, using VLA aperture synthesis \hi\ observations.
One of the isolated galaxies with an asymmetric profile in this subsample is CIG~96 (NGC~864), whose \hi\ synthesis imaging from the VLA has been studied in detail in \citet{2005A&A...442..455E}. The asymmetry in its \hi\ profile is associated with a strong kinematical perturbation in the gaseous envelope of the galaxy, where on one side the decay of the rotation curve is faster than Keplerian. Although a companion is detected, no tidal tail is found, and it is probably not massive enough to cause such a perturbation. Probably we are witnessing the capture of a gaseous companion.
Aperture synthesis maps for the rest of the sources showed more symmetric structures, both in geometry and kinematics. Neither tidal tails nor relevant HI rich companions were found close to any of the galaxies. The asymmetry was usually found in the kinematics, in the sense that the rotation curve on one side of the galaxy was steeper.
These results are similar to that in other galaxies \citep{1999MNRAS.304..330S}, not necessarily isolated. 
%Given the $\sim$ 3~Gyr time scale without a major interaction of galaxies in our sample \citep{2005A&A...436..443V},  these asymmetries might be the remnants of a previous major interaction or minor interaction (short-lived) that occured $<$ 1~Gyr ago.
  Given that in average galaxies in our sample did not likely undergo a major
   interaction since less than 3~Gyr ago \citep{2005A&A...436..443V},
   these asymmetries may either be the remnants of a major
   interaction longer ago, or of a more recent short-lived mechanism such as a minor merger.


{\bf Acknowledgments.} This work has been partially supported by Spanish DGI grant  AYA2008-06181-C02, as well as Junta de Andalucia (Spain) grants P08-FQM-4205-PEX and TIC-114 16-C02-01.
DE is supported by the Marie Curie International fellowship MOIF-CT-2006-40298.%, within the 6$\rm ^{th}$ European Community Framework Programme.

\vspace{-0.3cm}
\begin{thebibliography}{}
%\expandafter\ifx\csname natexlab\endcsname\relax\def\natexlab#1{#1}\fi
\bibitem[Baldwin et~al.(1980)]{1980MNRAS.193..313B}
{Baldwin}, J.~E., {Lynden-Bell}, D., \& {Sancisi}, R. 1980, \mnras, 193, 313
\bibitem[Beale \& Davies(1969)]{1969Natur.221..531B}
{Beale}, J.~S., \& {Davies}, R.~D. 1969, \nat, 221, 531
%\bibitem[Block et~al.(1994)]{1994A&A...288..365B}
%{Block}, D.~L., {Bertin}, G., {Stockton}, A., {Grosbol}, P., {Moorwood},
%  A.~F.~M., \& {Peletier}, R.~F. 1994, \aap, 288, 365
  \bibitem[Zaritsky 
\& Rix(1997)]{1997ApJ...477..118Z} Zaritsky, D., \& Rix, H.-W.\ 1997, \apj, 477, 118 
\bibitem[Bournaud et~al.(2005)]{2005A&A...438..507B}
{Bournaud}, F., {Combes}, F., {Jog}, C.~J., \& {Puerari}, I. 2005, \aap, 438,
  507
\bibitem[Davies \& Lewis(1973)]{1973MNRAS.165..231D}
{Davies}, R.~D., \& {Lewis}, B.~M. 1973, \mnras, 165, 231
\bibitem[Espada et~al.(2005)]{2005A&A...442..455E}
{Espada}, D., {Bosma}, A., {Verdes-Montenegro}, L., {Athanassoula}, E., {Leon},
  S., {Sulentic}, J., \& {Yun}, M.~S. 2005, \aap, 442, 455
\bibitem[Espada et~al.(2009)]{2009E}
---. 2009, in prep.
\bibitem[Haynes \& Giovanelli(1984)]{1984AJ.....89..758H}
{Haynes}, M.~P., \& {Giovanelli}, R. 1984, \aj, 89, 758
\bibitem[Haynes et~al.(1998)]{1998AJ....115...62H}
{Haynes}, M.~P., {van Zee}, L., {Hogg}, D.~E., {Roberts}, M.~S., \&
  {Maddalena}, R.~J. 1998, \aj, 115, 62
  \bibitem[Karachentseva(1973)]{1973AISAO...8....3K} Karachentseva, V.~E.\ 
1973, Astrofizicheskie Issledovaniia Izvestiya Spetsial'noj 
Astrofizicheskoj Observatorii, 8, 3 
\bibitem[Matthews et~al.(1998)]{1998AJ....116.1169M}
{Matthews}, L.~D., {van Driel}, W., \& {Gallagher}, III, J.~S. 1998, \aj, 116,
  1169
\bibitem[Richter \& Sancisi(1994)]{1994A&A...290L...9R}
{Richter}, O.-G., \& {Sancisi}, R. 1994, \aap, 290, L9
\bibitem[Swaters et~al. (1999)]{1999MNRAS.304..330S}
{Swaters}, R.~A., {Schoenmakers}, R.~H.~M., {Sancisi}, R., \& {van Albada},
  T.~S. 1999, \mnras, 304, 330
\bibitem[Verdes-Montenegro et~al.(2005)]{2005A&A...436..443V}
{Verdes-Montenegro}, L., {Sulentic}, J., {Lisenfeld}, U., {Leon}, S., {Espada},
  D., {Garcia}, E., {Sabater}, J., \& {Verley}, S. 2005, \aap, 436, 443
\bibitem[Verley et al.(2007) ]{2007A&A...470..505V} Verley, S., et al.\ 2007$a$, \aap, 470, 505 
\bibitem[Verley et al.(2007)]{2007A&A...472..121V} Verley, S., et al.\ 2007$b$, \aap, 472, 121 
\end{thebibliography}


%%% MAIN BODY OF TEXT GOES HERE. CONSULT "INSTRUCTIONS FOR AUTHORS USING
%%% LATEX2E MARKUP", SECTIONS 2.3-2.6 FOR HELP WITH EQUATIONS, FIGURES,
%%% AND TABLES.

%\section{}   %%% Top level section head (remove "%" symbol)
%\subsection{}   %%% Second level section head (remove "%" symbol)
%\subsubsection{}   %%% Lowest level section head (remove "%" symbol)
%\section*{}    %%% Unnumbered top level section head (remove "%" symbol)
%\subsection*{}   %%% Unnumbered second level section head (remove "%" symbol)
%
%\section{Summary}

%
% In the context of the AMIGA project, we use \hi\ global velocity profiles for a large sample of strictly isolated galaxies in order to: $i$ give the reference to study normalcy in the HI content in galaxies, $ii)$ quantify  the rate of \hi\ asymmetries, and $iii)$ seek for the origin of asymmetries in isolated galaxies. Our sample is large (N = 837 galaxies), homogeneous, including survival analysis, and permit discrimination on revised isolation quantities, morphological types, $L_B$, $L_FIR$ and M$_{H_2}$.   We restrict our study to a still large  subsample for which we minimize undesired pointing offsets and other uncertainties modifying the shape of the profile. We calculate a flux ratio asymmetry parameter and found that the distribution is well described by a Gaussian function with an intrinsic width of 0.13. This is lower than in previous studies that use other samples, not necessarily with a strict isolation criterion. This is likely the lowest value that one can find in the Universe. The most common asymmetry is different heights in double horn profiles.
%We find that there are no tidal tails nor companions in some of the most asymmetric galaxies observed with the VLA. Since the \hi\ asymmetry rate  in field galaxies with respect to isolated galaxies is  $\sim$ 20\% different, 
% it is argued that short-lived ($\sim$ 1~Gyr) tidal interactions must be a measurable and fundamental mechanism to produce
%large \hi\ asymmetries in galaxies. 

%{\bf Some discussion.}

%The small fraction of asymmetric HI profiles in the AMIGA sample confirms
%its value as an almost  nurture-free sample of galaxies. The sample should contain isolated galaxies where we can be assure a
%3~Gyr period of time since the last encounter with another galaxy.



 %The fact that the correlation is stronger in environmental density rather than tidal strength may indicate that it is minor mergers the responsible for such asymmetries. 
%We have also compared with other samples characterizing field/isolated galaxies, finding a noticeable smaller rate of galaxies with asymmetric profiles, that we interpret as a result of the strict isolation criteria of our sample.

%\acknowledgements %%% Text of acknowledgements runs on after this command.
%DE has been partially supported by a I3P-CSIC scholarship funded by European Social Fund and a Marie Curie International fellowship within the 6$\rm ^{th}$ European Community Framework Programme


%\bibliography{AMIGAXI-12272008}








%\begin{figure}
%\begin{center}
%\includegraphics[width=16cm]{}
%\end{center}
%\caption{HI distribution maps for 2 of the galaxies in the dozen of galaxies observed by us at the VLA. CIG~... was selected to be asymmetric, CIG~... to be symmetric.}
%\label{fig4}
%\end{figure}
\end{document}

0909.2736.extracted_bib
\begin{thebibliography}{}
%\expandafter\ifx\csname natexlab\endcsname\relax\def\natexlab#1{#1}\fi
\bibitem[Baldwin et~al.(1980)]{1980MNRAS.193..313B}
{Baldwin}, J.~E., {Lynden-Bell}, D., \& {Sancisi}, R. 1980, \mnras, 193, 313
\bibitem[Beale \& Davies(1969)]{1969Natur.221..531B}
{Beale}, J.~S., \& {Davies}, R.~D. 1969, \nat, 221, 531
%\bibitem[Block et~al.(1994)]{1994A&A...288..365B}
%{Block}, D.~L., {Bertin}, G., {Stockton}, A., {Grosbol}, P., {Moorwood},
%  A.~F.~M., \& {Peletier}, R.~F. 1994, \aap, 288, 365
  \bibitem[Zaritsky 
\& Rix(1997)]{1997ApJ...477..118Z} Zaritsky, D., \& Rix, H.-W.\ 1997, \apj, 477, 118 
\bibitem[Bournaud et~al.(2005)]{2005A&A...438..507B}
{Bournaud}, F., {Combes}, F., {Jog}, C.~J., \& {Puerari}, I. 2005, \aap, 438,
  507
\bibitem[Davies \& Lewis(1973)]{1973MNRAS.165..231D}
{Davies}, R.~D., \& {Lewis}, B.~M. 1973, \mnras, 165, 231
\bibitem[Espada et~al.(2005)]{2005A&A...442..455E}
{Espada}, D., {Bosma}, A., {Verdes-Montenegro}, L., {Athanassoula}, E., {Leon},
  S., {Sulentic}, J., \& {Yun}, M.~S. 2005, \aap, 442, 455
\bibitem[Espada et~al.(2009)]{2009E}
---. 2009, in prep.
\bibitem[Haynes \& Giovanelli(1984)]{1984AJ.....89..758H}
{Haynes}, M.~P., \& {Giovanelli}, R. 1984, \aj, 89, 758
\bibitem[Haynes et~al.(1998)]{1998AJ....115...62H}
{Haynes}, M.~P., {van Zee}, L., {Hogg}, D.~E., {Roberts}, M.~S., \&
  {Maddalena}, R.~J. 1998, \aj, 115, 62
  \bibitem[Karachentseva(1973)]{1973AISAO...8....3K} Karachentseva, V.~E.\ 
1973, Astrofizicheskie Issledovaniia Izvestiya Spetsial'noj 
Astrofizicheskoj Observatorii, 8, 3 
\bibitem[Matthews et~al.(1998)]{1998AJ....116.1169M}
{Matthews}, L.~D., {van Driel}, W., \& {Gallagher}, III, J.~S. 1998, \aj, 116,
  1169
\bibitem[Richter \& Sancisi(1994)]{1994A&A...290L...9R}
{Richter}, O.-G., \& {Sancisi}, R. 1994, \aap, 290, L9
\bibitem[Swaters et~al. (1999)]{1999MNRAS.304..330S}
{Swaters}, R.~A., {Schoenmakers}, R.~H.~M., {Sancisi}, R., \& {van Albada},
  T.~S. 1999, \mnras, 304, 330
\bibitem[Verdes-Montenegro et~al.(2005)]{2005A&A...436..443V}
{Verdes-Montenegro}, L., {Sulentic}, J., {Lisenfeld}, U., {Leon}, S., {Espada},
  D., {Garcia}, E., {Sabater}, J., \& {Verley}, S. 2005, \aap, 436, 443
\bibitem[Verley et al.(2007) ]{2007A&A...470..505V} Verley, S., et al.\ 2007$a$, \aap, 470, 505 
\bibitem[Verley et al.(2007)]{2007A&A...472..121V} Verley, S., et al.\ 2007$b$, \aap, 472, 121 
\end{thebibliography}