Transformation of a Virgo Cluster Dwarf Irregular Galaxy by Ram Pressure
  Stripping: IC3418 and its Fireballs
  We present optical imaging and spectroscopy and HI imaging of the Virgo
Cluster galaxy IC 3418, which is likely a "smoking gun" example of the
transformation of a dwarf irregular into a dwarf elliptical galaxy by ram
pressure stripping. IC 3418 has a spectacular 17 kpc length UV-bright tail
comprised of knots, head-tail, and linear stellar features. The only H-alpha
emission arises from a few HII regions in the tail, the brightest of which are
at the heads of head-tail UV sources whose tails point toward the galaxy
("fireballs"). Several of the elongated tail sources have H-alpha peaks
outwardly offset by 80-150 pc from the UV peaks, suggesting that gas clumps
continue to accelerate through ram pressure, leaving behind streams of newly
formed stars which have decoupled from the gas. Absorption line strengths,
measured from Keck DEIMOS spectra, together with UV colors, show star formation
stopped 300+/-100 Myr ago in the main body, and a strong starburst occurred
prior to quenching. While neither H-alpha nor HI emission are detected in the
main body of the galaxy, we have detected 4x10^7 M_sun of HI from the tail with
the VLA. The gas consumption timescale in the tail is relatively long, implying
that most of the stripped gas does not form stars but joins the ICM. The
velocities of tail HII regions, measured from Keck LRIS spectra, extend only a
small fraction of the way to the cluster velocity, suggesting that star
formation does not happen in more distant parts of the tail. Stars in the outer
tail have velocities exceeding the escape speed, but some in the inner tail
should fall back into the galaxy, forming halo streams. One likely fallback
stream is identified.
