Star Formation Signatures in Optically Quiescent Early-type Galaxies
  In recent years argument has been made that a high fraction of early-type
galaxies in the local universe experience low levels (< 1 M_sun/yr) of star
formation (SF) that causes strong excess in UV flux, yet leaves the optical
colors red. Many of these studies were based on GALEX imaging of SDSS galaxies
(z~0.1), and were thus limited by its 5" FWHM. Poor UV resolution left other
possibilities for UV excess open, such as the old populations or an AGN. Here
we study high-resolution far-ultraviolet HST/ACS images of optically quiescent
early-type galaxies with strong UV excess. The new images show that
three-quarters of these moderately massive (~5x10^10 M_sun) early-type galaxies
shows clear evidence of extended SF, usually in form of wide or concentric UV
rings, and in some cases, striking spiral arms. SDSS spectra probably miss
these features due to small fiber size. UV-excess early-type galaxies have on
average less dust and larger UV sizes (D>40 kpc) than other green-valley
galaxies, which argues for an external origin for the gas that is driving the
SF. Thus, most of these galaxies appear `rejuvenated' (e.g., through minor
gas-rich mergers or IGM accretion). For a smaller subset of the sample, the
declining SF (from the original internal gas) cannot be ruled out. SF is rare
in very massive early-types (M_* > 10^11 M_sun), a possible consequence of AGN
feedback. In addition to extended UV emission, many galaxies show a compact
central source, which may be a weak, optically inconspicuous AGN.
