Using V and I images obtained with the Wide Field Planetary Camera 2 (WFPC2) of the Hubble Space Telescope, we investigate compact stellar structures 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 how the star formation depends upon the local and global physical conditions. These mergers sample a range of stages in the evolutionary sequence and tails with and without embedded tidal dwarf galaxies. The six tails are found to contain a variety of stellar structures, with sizes ranging from those of globular clusters up to those of dwarf galaxies. From V and I WFPC2 images, we measure the luminosities and colors of the star clusters. NGC 3256 is found to have a large population of blue clusters (0.2 V − I 0.9), particularly in its Western tail, similar to those found in the inner region of the merger. In contrast, NGC 4038/39 has no clusters in the observed region of the tail, only less luminous point sources likely to be individual stars. NGC 3921 and NGC 7252 have small populations of clusters along their tails. A significant cluster population

We have searched for compact stellar structures within 17 tidal tails in 13 different interacting galaxies using F606W -and F814W -band images from the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST ). The sample of tidal tails includes a diverse population of optical properties, merging galaxy mass ratios, Hi content, and ages. Combining our tail sample with Knierman et al. (2003), we find evidence of star clusters formed in situ with M V < -8.5 and V -I < 2.0 in 10 of 23 tidal tails; we are able to identify cluster candidates to M V = -6.5 in the closest tails. Three tails offer clear examples of "beads on a string" star formation morphology in V -I color maps. Two tails present both tidal dwarf galaxy (TDG) candidates and cluster candidates. Statistical diagnostics indicate that clusters in tidal tails may be drawn from the same power-law luminosity functions (with logarithmic slopes ≈ -2 --2.5) found in quiescent spiral galaxies and the interiors of interacting systems. We find that the tail regions with the largest number of observable clusters are relatively young ( 250 Myr old) and bright (V 24 mag arcsec −2 ), probably attributed to the strong bursts of star formation in interacting systems soon after periapse. Otherwise, we find no statistical difference between cluster-rich and cluster-poor tails in terms of many observable characteristics, though this analysis suffers from complex, unresolved gas dynamics and projection effects.

Superstar clusters are young, compact star clusters found in the central regions of interacting galaxies. Recently, they have also been reported to preferentially form in certain tidal tails, but not in others. In this paper, we have used 21‐cm H i maps and the Hubble Space Telescope Wide Field Planetary Camera 2 images of eight tidal tail regions of four merging galaxy pairs to compare the kiloparsec scale H i distribution with the location of superstar clusters found from the optical images. For most of the tails, we find that there is an increase in superstar cluster density with increasing projected H i column density, such that the star cluster density is highest when log NH i≳ 20.6 cm−2, but equal to the background count rate at lower H i column density. However, for two tails (NGC 4038/39 Pos A and 3921), there is no significant star cluster population despite the presence of gas at high column density. This implies that the NH i threshold is a necessary but not sufficient condition for cluster formation. Gas volume density is likely to provide a more direct criterion for cluster formation, and other factors such as gas pressure or strength of encounter may also have an influence. Comparison of H i thresholds needed for formation of different types of stellar structures await higher resolution H i and optical observations of larger numbers of interacting galaxies.