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.

We present an analysis of the response of a radiative region to waves generated by a convective region of the star; this wave treatment of the classical problem of ``overshooting'' gives extra mixing relative to the treatment traditionally used in stellar evolutionary codes. The interface between convectively stable and unstable regions is dynamic and nonspherical, so that the nonturbulent material is driven into motion, even in the absence of ``penetrative overshoot.'' These motions may be described by the theory of nonspherical stellar pulsations, and are related to motion measured by helioseismology. Multi-dimensional numerical simulations of convective flow show puzzling features which we explain by this simplified physical model. Gravity waves generated at the interface are dissipated, resulting in slow circulation and mixing seen outside the formal convection zone. The approach may be extended to deal with rotation and composition gradients. Tests of this description in the stellar evolution code TYCHO produce carbon stars on the asymptotic giant branch (AGB), an isochrone age for the Hyades and three young clusters with lithium depletion ages from brown dwarfs, and lithium and beryllium depletion consistent with observations of the Hyades and Pleiades, all without tuning parameters. The insight into the different contributions of rotational and hydrodynamic mixing processes could have important implications for realistic simulation of supernovae and other questions in stellar evolution.Comment: 27 pages, 5 figures, accepted to the Astrophysical Journa

While major mergers and their tidal debris are well studied, they are less common than minor mergers (mass ratios 0.3). The peculiar spiral NGC 2782 is the result of a merger between two disk galaxies with a mass ratio of ∼ 4 : 1 occurring ∼ 200 Myr ago. This merger produced a molecular and Hi rich, optically bright Eastern tail and an Hi-rich, optically faint Western tail. Non-detection of CO in the Western Tail by Braine et al. (2001) suggested that star formation had not yet begun to occur in that tidal tail. However, deep Hα narrowband images show evidence of recent star formation in the Western tail. Across the entire Western tail, we find the global star formation rate per unit area (Σ SFR ) several orders of magnitude less than expected from the total gas density. Together with extended FUV+NUV emission from Galaxy Evolution Explorer along the tail, this indicates a low global star formation efficiency in the tidal tail producing lower mass star clusters. The Hii region we observed has a local (few-kpc scale) Σ SFR from Hα that is less than that expected from the total gas density, which is consistent with other observations of tidal debris. The star formation efficiency of this Hii region inferred from the total gas density is low, but normal when inferred from the molecular gas density. These results suggest the presence of a very small, locally dense region in the Western tail of NGC 2782 or of a low metallicity and/or low pressure star forming region.

The peculiar spiral NGC 2782 is the result of a minor merger with a mass ratio ∼ 4 : 1 occurring ∼ 200 Myr ago. This merger produced a molecular and Hi rich, optically bright Eastern tail and an Hi-rich, optically faint Western tail. Non-detection of CO in the Western Tail by Braine et al. (2001) suggested that star formation had not yet begun. However, deep UBV R and Hα narrowband images show evidence of recent star formation in the Western tail, though it lacks massive star clusters and cluster complexes. Using Herschel PACS spectroscopy, we discover 158µm [Cii] emission at the location of the three most luminous Hα sources in the Eastern tail, but not at the location of the even brighter Hα source in the Western tail. The Western tail is found to have a normal star formation efficiency (SFE), but the Eastern tail has a low SFE. The lack of CO and [Cii] emission suggests the Western tail Hii region may have a low carbon abundance and be undergoing its first star formation. The Western tail is more efficient at forming stars, but lacks massive clusters. We propose that the low SFE in the Eastern tail may be due to its formation as a splash region where gas heating is important even though it has sufficient molecular and neutral gas to make massive star clusters. The Western tail, which has lower gas surface density and does not form high mass star clusters, is a tidally formed region where gravitational compression likely enhances star formation.

Using archival data from ATCA, WSRT, and the VLA, we have analyzed the Hi emission of 22 tidal tail regions of the Mullan et al. sample of pairwise interacting galaxies. We have measured the column densities, line-of-sight velocity dispersions, and kinetic energy densities on ∼kpc scales. We also constructed a tracer of the line-of-sight velocity gradient over ∼10 kpc scales. We compared the distributions of these properties between regions that do and do not contain massive star cluster candidates (M V < -8.5; ∼ 10 4 -10 6 M ⊙ as observed in HST WFPC2 VI data). In agreement with Maybhate et al., we find that a local, ∼kpc-scale column density of log N H i 20.6 cm −2 is frequently required for detecting clustered star formation. This Hi gas also tends to be turbulent, with line-of-sight velocity dispersions σ los ≈ 10-75 km s −1 , implying high kinetic energy densities (log Σ KE > 46 erg pc −2 ). Thus high Hi densities and pressures, partly determined by the tail dynamical age and other interaction characteristics, are connected to large-scale cluster formation in tidal tails overall. Lastly, we find that the high mechanical energy densities of the gas are likely not generally due to feedback from star formation. Rather, these properties are more likely to be a cause of star formation than a result.

We have developed an observing program using deep, multiband imaging to probe the chaotic regions of tidal tails in search of an underlying stellar population, using NGC 3256's 400 Myr twin tidal tails as a case study. These tails have different colours of u − g = 1.05 ± 0.07 and r − i = 0.13 ± 0.07 for NGC 3256W, and u − g = 1.26 ± 0.07 and r − i = 0.26 ± 0.07 for NGC 3256E, indicating different stellar populations. These colours correspond to simple stellar population ages of 288 +11 −54 Myr and 841 +125 −157 Myr for NGC 3256W and NGC 3256E, respectively, suggesting NGC 3256W's diffuse light is dominated by stars formed after the interaction, while light in NGC 3256E is primarily from stars that originated in the host galaxy. Using a mixed stellar population model, we break our diffuse light into two populations: one at 10 Gyr, representing stars pulled from the host galaxies, and a younger component, whose age is determined by fitting the model to the data. We find similar ages for the young populations of both tails, (195 −13 +0 and 170 −70 +44 Myr for NGC 3256W and NGC 3256E, respectively), but a larger percentage of mass in the 10 Gyr population for NGC 3256E (98 +1 −3 % vs 90 +5 −6 %). Additionally, we detect 31 star cluster candidates in NGC 3256W and 19 in NGC 2356E, with median ages of 141 Myr and 91 Myr, respectively. NGC 3256E contains several young (< 10 Myr), low mass objects with strong nebular emission, indicating a small, recent burst of star formation.