An HI Threshold for Star Cluster Formation in Tidal Debris

TL;DR

This study identifies a threshold HI column density of approximately 10^{20.6} cm^{-2} associated with super star cluster formation in tidal tails, highlighting that high gas density is necessary but not sufficient for cluster formation.

Contribution

It demonstrates a correlation between HI column density and super star cluster presence in tidal tails, emphasizing the role of gas density thresholds in star cluster formation.

Findings

Super star clusters are more common where HI column density exceeds 10^{20.6} cm^{-2}.

Some high-density regions lack clusters, indicating other factors influence formation.

HI density threshold is necessary but not sufficient for cluster formation.

Abstract

Super star 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 HI 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 HI distribution with the location of super star clusters found from the optical images. For most of the tails, we find that there is an increase in super star cluster density with increasing projected HI column density, such that the star cluster density is highest when log N(HI) >= 20.6 cm^{-2}, but equal to the background count rate at lower HI column density. However, for two tails (NGC 4038/39 Pos A and NGC 3921), there is no significant star cluster population despite the presence of gas at high column density. This implies that the N(HI) 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 HI thresholds needed for formation of different types of stellar structures await higher resolution HI and optical observations of larger numbers of interacting galaxies.