From Globular Clusters to Tidal Dwarfs: Structure Formation in the Tidal Tails of Merging Galaxies

Using V and I images obtained with WFPC2/HST, we investigate compact stellarstructures within tidal tails. Six regions of tidal debris in the four classic``Toomre Sequence'' mergers: NGC 4038/39 (``Antennae''), NGC 3256, NGC 3921,and NGC 7252 (``Atoms for Peace'') have been studied in order to explore howthe star formation depends upon the local and global physical conditions. Thesemergers sample a range of stages in the evolutionary sequence and tails withand without embedded tidal dwarf galaxies. The six tails are found to contain avariety of stellar structures, with sizes ranging from those of globularclusters up to those of dwarf galaxies. From V and I WFPC2 images, we measurethe luminosities and colors of the star clusters. NGC 3256 is found to have alarge population of blue clusters (0.2 < V-I < 0.9), particularly in itsWestern tail, similar to those found in the inner region of the merger. Incontrast, NGC 4038/39 has no clusters in the observed region of the tail, onlyless luminous point sources likely to be individual stars. A significantcluster population is clearly associated with the prominent tidal dwarfcandidates in the eastern and western tails of NGC 7252. The cluster-richWestern tail of NGC 3256 is not distinguished from the others by its dynamicalage or by its total HI mass. However, the mergers that have few clusters in thetail all have tidal dwarf galaxies, while NGC 3256 does not have prominenttidal dwarfs. We speculate that star formation in tidal tails may manifestitself either in small structures like clusters along the tail or in largestructures such as dwarf galaxies, but not in both. Also, NGC 3256 has thehighest star formation rate of the four mergers studied, which may contributeto the high number of star clusters in its tidal tails.Comment: Accepted to Astronomical Journal. 34 pages including 15 figures and 4 table