Caught in the Act: Strong, Active Ram Pressure Stripping in Virgo
  Cluster Spiral NGC 4330
  We present a multi-wavelength study of NGC 4330, a highly-inclined spiral
galaxy in the Virgo Cluster which is a clear example of strong, ongoing ICM-ISM
ram pressure stripping. The HI has been removed from well within the
undisturbed old stellar disk, to 50% - 65% of R_25. Multi-wavelength data (WIYN
BVR and H-alpha, VLA 21-cm HI and radio continuum, and GALEX NUV and FUV)
reveal several one-sided extraplanar features likely caused by ram pressure at
an intermediate disk-wind angle. At the leading edge of the interaction, the
H-alpha and dust extinction curve sharply out of the disk in a remarkable and
distinctive "upturn" feature that may be generally useful as a diagnostic
indicator of active ram pressure. On the trailing side, the ISM is stretched
out in a long tail which contains 10% of the galaxy's total HI emission, 6 - 9%
of its NUV-FUV emission, but only 2% of the H-alpha. The centroid of the HI
tail is downwind of the UV/H-alpha tail, suggesting that the ICM wind has
shifted most of the ISM downwind over the course of the past 10 - 300 Myr.
Along the major axis, the disk is highly asymmetric in the UV, but more
symmetric in H-alpha and HI, also implying recent changes in the distributions
of gas and star formation. The UV-optical colors indicate very different star
formation histories for the leading and trailing sides of the galaxy. On the
leading side, a strong gradient in the UV-optical colors of the gas-stripped
disk suggests that it has taken 200-400 Myr to strip the gas from a radius of
>8 to 5 kpc, but on the trailing side there is no age gradient. All our data
suggest a scenario in which NGC 4330 is falling into cluster center for first
time and has experienced a significant increase in ram pressure over the last
200-400 Myr.
