An HI threshold for star cluster formation in tidal debris

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 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 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 HI column density, such that the star cluster density is highest when log N_HI >~ 20.6cm^-2, but equal to the background count rate at lower HI column density. However, for two tails (NGC4038/39 Pos A and 3921), there is no significant star cluster population despite the presence of gas at high column density. This implies that the 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.

Based in part on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

E-mail: maybhate@stsci.edu (AM); masiero@ifa.hawaii.edu (JM); jhibbard@nrao.edu (JEH); charlton@astro.psu.edu (JCC); cpalma@astro.psu.edu (CP); kknierman@as.arizona.edu (KAK); jayanne_english@umanitoba.ca (JE)