Under pressure: Star clusters and the neutral hydrogen medium of tidal tails

Using archival data from ATCA, WHISP, and the Very Large Array, we have analyzed the H I 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; ∼10⁴-10⁶ M as observed in Hubble Space Telescope WFPC2 VI data). In agreement with Maybhate et al., we find that a local, ∼kpc-scale column density of log ≳ 20.6 cm^-2 is frequently required for detecting clustered star formation. This H I gas also tends to be turbulent, with line-of-sight velocity dispersions σlos 10-75 km s ^-1, implying high kinetic energy densities (log ΣKE > 46 erg pc^-2). Thus, high H I 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. Last, 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.