Extended UV Disks and UV-Bright Disks in Low-Mass E/S0 Galaxies
  We have identified 15 XUV disks in a largely field sample of 38 E/S0 galaxies
with stellar masses primarily below ~4 x 10^10 M_sun and comparable numbers on
the red and blue sequences. We use a new purely quantitative XUV disk
definition requiring UV extension relative to a UV-defined star formation
threshold radius. The 39(+-9)% XUV-disk frequency for these E/S0s is roughly
twice the ~20% reported for late types, possibly indicating that XUV disks are
associated with galaxies experiencing weak or inefficient star formation.
Consistent with this interpretation, the XUV disks in our sample do not
correlate with enhanced outer-disk star formation as traced by blue optical
outer-disk colors. However, UV-Bright (UV-B) disk galaxies with blue UV colors
outside their optical 50% light radii do display enhanced optical outer-disk
star formation as well as enhanced atomic gas content. UV-B disks occur with a
42(+9/-8)% frequency, and the combined XUV/UV-B frequency is 61(+-9)%. For both
types, UV colors typically imply <1 Gyr ages. XUV disks occur over the full
sample mass range and on both sequences, suggesting an association with galaxy
interactions or another general evolutionary process. In contrast, UV-B disks
favor the blue sequence and may also prefer low masses, perhaps reflecting the
onset of cold-mode accretion or another mass-dependent evolutionary process.
Virtually all blue E/S0s in the gas-rich regime below stellar mass M_t ~ 5 x
10^9 M_sun (the "gas-richness threshold mass") display UV-B disks. [abridged]
