Hubble Space Telescope Observations of Accretion-Induced Star Formation   in the Tadpole Galaxy Kiso 5639
  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. 
Debra Meloy Elmegreen, Bruce G. Elmegreen, Jorge Sanchez Almeida,   Casiana Munoz-Tunon, Jairo Mendez-Abreu, John S. Gallagh...
astro-ph.GA