Under Pressure: Star Clusters and the Neutral Hydrogen Medium of Tidal Tails

TL;DR

This study analyzes the neutral hydrogen properties in tidal tails of interacting galaxies, finding that high HI density and turbulence are linked to star cluster formation, with gas conditions likely causing rather than resulting from star formation.

Contribution

It provides the first detailed analysis of HI properties in tidal tails, linking gas density and turbulence to star cluster formation, and suggests gas conditions drive star formation rather than feedback.

Findings

High HI column density (~10^20.6 cm^(-2)) correlates with star cluster presence.

Turbulent HI gas with velocity dispersions 10-75 km/s is common in star-forming regions.

High kinetic energy densities are more likely causes of star formation than effects.

Abstract

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.