Hubble Space Telescope Observations of Accretion-Induced Star Formation in the Tadpole Galaxy Kiso 5639

TL;DR

This study uses Hubble Space Telescope observations to investigate how accretion of metal-poor gas triggers star formation in the Tadpole Galaxy Kiso 5639, revealing detailed properties of its star-forming regions and clusters.

Contribution

It provides new multi-wavelength imaging data and analysis of star formation triggered by gas accretion in Kiso 5639, including star cluster properties and gas dynamics.

Findings

Star-forming head contains ~10^6 solar masses of young stars.

Star clusters follow a power-law mass distribution with slope -1.73.

Star formation rate exceeds typical rates unless accreted gas density is significantly higher.

Abstract

The tadpole galaxy Kiso 5639 has a slowly rotating disk with a drop in metallicity at its star-forming head, suggesting that star formation was triggered by the accretion of metal-poor gas. We present multi-wavelength HST WFC3 images of UV through I band plus Halpha to search for peripheral emission and determine the properties of various regions. The head has a mass in young stars of ~10^6 Mo and an ionization rate of 6.4x10^51 s^{-1}, equivalent to ~2100 O9-type stars. There are four older star-forming regions in the tail, and an underlying disk with a photometric age of ~1 Gyr. The mass distribution function of 61 star clusters is a power law with a slope of -1.73+-0.51. Fourteen young clusters in the head are more massive than 10^4 Mo, suggesting a clustering fraction of 30%-45%. Wispy filaments of Halpha emission and young stars extend away from the galaxy. Shells and holes in the head HII region could be from winds and supernovae. Gravity from the disk should limit the expansion of the HII region, although hot gas might escape through the holes. The star formation surface density determined from Halpha in the head is compared to that expected from likely pre-existing and accreted gas. Unless the surface density of the accreted gas is a factor of ~3 or more larger than what was in the galaxy before, the star formation rate has to exceed the usual Kennicutt-Schmidt rate by a factor of >5.