Under Pressure: Star Clusters and the Neutral Hydrogen Medium of Tidal
  Tails
  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_(sun) 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_(HI) = 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 ~10--75 km/s,
implying high kinetic energy densities (>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.
