From Globular Clusters to Tidal Dwarfs: Structure Formation in Tidal Tails
Jane Charlton$^{1}$
Karen Knierman$^{1,2}$
Sally Hunsberger$^{1}$
Sarah Gallagher$^{1}$
Bradley Whitmore$^{3}$
Arunav Kundu$^{4}$
\& John Hibbard$^{5}$
and
astroph.tex
%% vla2000.tex 
%%
%% tex file for Gas & Galaxy Evolution Conference 
%% PLEASE EDIT THIS FILE AS NECESSARY AND SUBMIT
%% NOTE: DO NOT DEFINE ANY COMMANDS (\def, \newcommand)
%% OF YOUR OWN FOR ANY REASON!!!
%
% For an example of this file, see the files vla2000_sample*.*
% at http://www.nrao.edu/vla2000/proceedings/ or
% ftp over the file vla2000.tar.gz via anonymous ftp to 
% ftp.cv.nrao.edu/NRAO-staff/jhibbard/vla2000
% For more detailed instructions, see the file 
% vla2000_man.ps at the same site. 

% The first item in the LaTeX file must be the \documentclass command to
% declare the overall style of the paper.  Next, the \usepackage
% commands specify that we will use the definitions from the newpasp.sty 
% style file (available from http://www.aspsky.org/pubs/authors.html
% as well as the above web and ftp sites)

\documentclass[11pt,twoside]{article}
\usepackage{newpasp}

\newcommand{\HI}{\hbox{{\rm H}\kern 0.1em{\sc i}}}
\newcommand{\MgII}{\hbox{{\rm Mg}\kern 0.1em{\sc ii}}}
\newcommand{\FeII}{\hbox{{\rm Fe}\kern 0.1em{\sc ii}}}
\newcommand{\CIV}{\hbox{{\rm C}\kern 0.1em{\sc iv}}}
\newcommand{\NV}{\hbox{{\rm N}\kern 0.1em{\sc v}}}
\newcommand{\OVI}{\hbox{{\rm O}\kern 0.1em{\sc vi}}}
\newcommand{\kms}{\hbox{km~s$^{-1}$}}
\newcommand{\cmsq}{\hbox{cm$^{-2}$}}
\newcommand{\Lya}{\hbox{{\rm Ly}\kern 0.1em$\alpha$}}
\newcommand{\CIVdblt}{{\rm C}\kern 0.1em{\sc iv}~$\lambda\lambda 1548, 1550$}
\newcommand{\MgIIdblt}{{\rm Mg}\kern 0.1em{\sc ii}~$\lambda\lambda 2796, 2803$}

% Many authors like to format figures with the commands in the
% epsf.sty, so I have included a line to include that file. Without it, 
% you can still include figures using the \plotone, \plottwo, and 
% \plotfiddle commands. See vla2000_sample.tex and vla2000_man.ps for
% more details. 
%\usepackage{epsf}

% Authors should supply running head information.  The left head contains a
% list of author surnames, usually three allowed---otherwise use et al.  
% The right head is a modified title of up to roughly 44 characters.  

\markboth{Charlton et~al.}{Globular Clusters in Tidal Debris}

% the \setcounter{page}{#} command will uncommented and filled in by 
% the editorial staff, so just ignore it for now
%%\setcounter{page}{1}

% include each authors name and initials in an index command for
% help in constructing the author index for the proceedings.

\index{Charlton, J.C.}

% This is the end of the "preamble".  Now we wish to start with the
% real material for the paper, which we indicate with \begin{document}.
% Following the \begin{document} command is the front matter for the
% paper, viz., the title, author and address data, the abstract, and
% any keywords or subject headings that are relevant.
% The \affil command should be used to give the author's full postal 
% address. The address will be broken over several lines automatically;
% do NOT use LaTex's "\\" command to indicate the line breaks.
% Please use mixed case text for ALL these fields rather than supplying 
% all capitals; the style file will convert to upper case as necessary.

\begin{document}

%\setlength{\textfraction}{0}

\title{From Globular Clusters to Tidal Dwarfs: Structure Formation in Tidal Tails}
\author{Jane Charlton$^{1}$, Karen Knierman$^{1,2}$, Sally Hunsberger$^{1}$,
Sarah Gallagher$^{1}$, Bradley Whitmore$^{3}$, Arunav Kundu$^{4}$,
\& John Hibbard$^{5}$}
\affil{$^{1}$The Pennsylvania State University, $^{2}$University of Arizona,
$^{3}$Space Telescope Science Institute, $^{4}$University of Virginia,
$^{5}$National Radio Astronomy Observatory} 

% A concise abstract is recommended.  Enter the text of the abstract in
% between the \begin{abstract} and \end{abstract} commands.  Do NOT
% include the word ``Abstract'' in your text; it is inserted
% automatically. Do NOT  make a paragraph break between \begin{abstract} 
% and the first line of the text of the abstract!  Abstracts are required 
% for all papers.

\begin{abstract}
Star clusters can be found in galaxy mergers, not only in central regions,
but also in the tidal debris.  In both the Eastern and Western tidal
tails of NGC 3256 there are dozens of young star clusters, confirmed by their 
blue colors and larger concentration index as compared to sources off 
of the tail.  Tidal tails of other galaxy pairs do not have such
widespread cluster formation, indicating environmental influences
on the process of star formation or the packaging of the stars.
\end{abstract}


% Include keywords if you wish. The keywords.apj file, found on aas.org 
% in the pubs/aastex-misc directory, contains a list of keywords used 
% with the ApJ and Letters.  

%%\keywords{infrared: galaxies -- galaxies: nuclei -- galaxies: starburst}

% That's it for the front matter.  On to the main body of the paper.

\section{Introduction}
Extended structures resembling young dwarf galaxies are found in
tidal debris from galaxy interactions (Mirabel et~al. 1992;
Duc \& Mirabel 1994; Hunsberger, Charlton, \& Zaritsky 1996).
Star clusters form in
abundance in the central regions of interacting galaxy pairs
(Schweizer et~al. 1996; Miller et~al. 1997; Whitmore et~al. 1999;
Zepf et~al. 1999).
What physical conditions drive the formation of stars and determine
the nature of structure that forms in different environments?
Can star clusters also form in tidal debris, how widespread is
star formation in the debris, and how similar is it between different
tidal environments?

\section{HST WFPC2 Images of the Tidal Debris of NGC 3256}
The image in Figure 1 is a 1000 second exposure in the F555W
filter obtained on 1999 March 24.  A F814W image was also obtained.  
Point sources with $S/N > 3.0$ are indicated, those in the tail
with white circles and those out of the tail with white squares.
Although this field is crowded with foreground stars due to its 
low galactic latitude,  it is apparent that the numerous point 
sources are preferentially in the regions containing tidal debris.

\begin{figure}[th]
\plotfiddle{proc_compress.eps}{4.4in}{0.}{76.}{76.}{-213}{-170}
\vglue 0.38in
\caption
{HST WFPC2 image of the Western tail of NGC 3256 taken with
the F555W ($V$--band) filter.  The sources within the tail are
identified with circles and those outside the tail with squares.
Sources were not detected at $S/N>3$ in the PC region.}
\end{figure}

\begin{figure}[htb]
\plotfiddle{cmdconfproc.eps}{3.in}{0.}{80.}{80.}{-254}{-370}
\vglue -0.4in
\caption
{Color Magnitude diagrams for sources in and out of the Western tail of 
NGC 3256.  The $V-I$ color is plotted vs. the absolute magnitude
on the lower horizontal scale and vs. the apparent $V$ magnitude
on the upper horizontal scale.  Sources are included if errors 
in $V$ are less than 0.25 mag, and if $-0.75 < V-I < 2.0$.  Sources
from the different WFPC2 chips are indicated by the symbols in
the legends.  Many of the ``in-tail'' sources occupy a region
in the relatively low luminosity, blue part of the diagram,
which is not heavily populated with ``out of tail'' sources.}
\end{figure}

\begin{figure}[ht]
\plotfiddle{concproc.eps}{4.0in}{0.}{75.}{75.}{-240.}{-240}
\vglue -0.8in
\caption
{ The concentration index is plotted vs. the $V-I$ color for
sources in (solid) and out (open) of the Western tail of 
NGC 3256.  The concentration
index, $\Delta_V (0.5 - 3)$, provides a rough measure of cluster size,
calculated from the difference in $V$ magnitude between aperture radii
of $0.5$ and $3.0$ pixels.
 }
\end{figure}

The colors, magnitudes, and sizes of these sources allow us to 
distinguish foreground and background contamination from star
clusters in the debris.  
Figure 2 illustrates that the brighter sources are relatively
young star clusters (i.e. hundreds of millions of years old), 
but some of the fainter ones could be individual stars.
The reddest sources are most certainly foreground stars with 
$-10 < M_V < -7.5$ apparent mostly from WF4.  There is an 
enhancement of point sources with $0 < V-I < 0.8$ and
$-12 < M_V < -9$.  The relatively large spread of the $V-I$ 
colors indicates either a range of ages, or non--uniform
extinction by dust.
The central region 
of NGC 3256 also has a large number of young clusters that
contribute 20\% of the total B--band luminosity of the galaxy
(Zepf et~al. 1999).

In Figure 3, for the Western tail, the $V-I$ color is plotted 
vs. the concentration index, defined as the difference between
$V$  magnitudes measured in a 0.5\arcsec and in a 3\arcsec aperture.
The solid circles represent sources in the tail regions while the
open circles represent sources in regions outside the tail. 
Clearly, the sources in the tail are on average larger, 
indicating that they are not point sources, and bluer.
Therefore, we confirm that there are many star clusters in
this tail.  In the Eastern tail of NGC 3256 we also find
a significant number of star clusters, but they are not
as abundant as in the Western tail.

\section{Environmental Influences on Cluster Formation in Tidal Debris}

We have also obtained HST/WFPC2 V and I band images of the tidal debris 
of three other mergers: NGC 4038/9, ``the Antennae'', NGC 7252,
``Atoms for Peace'', and NGC 3921.  
We detect several cluster candidates in NGC 7252 and NGC 4038/9,
and several super star clusters in the debris of NGC 7252 and
NGC 3921.
However, the debris of the remnant
NGC 3256 by far contains the largest number of massive star
clusters, both in the Eastern and in the Western tails.  
Apparently, the conditions in this remnant are
more conducive to formation of these clusters.

Clearly, not all tidal debris is equally conducive to the formation
of these clusters.  NGC 3256 is not distinguished from
the other pairs by age or by total {\HI} mass.
However, its two tidal tails are the only ones of the eight
we have studied that {\it do not} contain tidal dwarf galaxies.
NGC 7252, for example, contains in its Western tail a bright
dwarf with prominent patches of star formation, and in its
Eastern tail an extended low surface brightness dwarf.
In both dwarfs there are point sources with $V-I < 0.5$,
indicating that star formation continues well after the merger 
$\sim 750$ million years ago (Hibbard \& Mihos 1995).
Perhaps the formation of small stellar structures (star
clusters) and large stellar structures (tidal dwarfs) are
mutually exclusive.
Detailed comparisons of cluster positions to high resolution 21~cm 
maps of {\HI} content may also suggest factors that influence 
the formation and/or the packaging of stars.

% For examples on including figures, see the file vla2000_sample.ps
% at http://www.nrao.edu/vla2000/proceedings/. 
% For examples of figures, equations or tables, please see the file
% vla2000_man.ps at the same site. Also available as
% newpaspman.ps at http://www.aspsky.org/pubs/authors.html


% Comment this out if you want to include acknowledgements

\acknowledgements 
A detailed report of results on the tidal debris in all four
pairs, NGC 3256, NGC 7252, NGC 4038/9, and NGC 3921, has been
submitted to the Astronomical Journal (Knierman et~al. 2000).
This work was supported by NASA/STScI (grant GO-07466.01-96A).

\begin{references}
{\small

\reference Duc, P.-A., \& Mirabel, I. F. 1994, A\&A, 289, 83

\reference Hibbard, J. E., \& Mihos, J. C. 1995, AJ, 110, 140

\reference Hunsberger, S. D., Charlton, J. C., \& Zaritsky, D. 1996, ApJ, 462, 50

\reference Knierman, K. A., Hunsberger, S. D., Gallagher, S. C., Charlton, J. C., Whitmore, B. C., Kundu, A., Hibbard, J. E., \& Zaritsky, D. 2000, ApJ, submitted

\reference Miller, B. W., Whitmore, B. C., Schweizer, F., \& Fall, S. M. 1997, 
AJ, 114, 2381

\reference Mirabel, I. F., Dottori, H., \& Lutz, D. 1992, A\&A, 256, L19

\reference Schweizer, F., Miller, B. W.,  Whitmore, B. C.,\& Fall, S. M. 
1996, AJ, 112, 1839

\reference Whitmore, B. C., Zhang, Q.,  Leitherer, C., Fall, S. M.
 Schweizer, F., \& Miller, B. W. 1999, AJ, 118, 1551

\reference Zepf, S. et~al. 1999, AJ, 118, 752

}
\end{references}

\end{document}

