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.
