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]
We identify a high frequency of Type 1 XUV disks, reflecting recent outer disk star formation, in a sample of 31 E/S0s with stellar masses primarily below M_* ~ 4 x 10^10 M_sun. Our ~40% identification rate is roughly twice the 20% fraction reported
for late-type galaxies. Intriguingly, in the dwarf mass regime (below M_* ~ 5 x 10^9 M_sun) where gas fractions clearly rise, Type 1 XUV disks occur in ~70% of red-sequence E/S0s but only ~20% of blue-sequence E/S0s, a population recently linked to active disk rebuilding, especially in the dwarf regime. Our statistics are preliminary, but could indicate that for dwarf E/S0s Type 1 XUV disks are primarily related to weak or inefficient outer-disk star formation rather than to star formation capable of driving substantial disk growth. Substantial growth may instead be associated with populations that have low XUV-disk frequency, possibly explaining the similar ~20% frequencies for normal late types and low-mass blue-sequence E/S0s.
The recent discovery of extended ultraviolet (XUV) disks around a large fraction of late-type galaxies provides evidence for unexpectedly large-scale disk building at recent epochs. Combining GALEX UV observations with deep optical and Spitzer IR ima
ging, we search for XUV disks in a sample of nearby low-to-intermediate mass E/S0 galaxies to explore evidence for disk rebuilding after mergers. Preliminary visual classification yields ten XUV-disk candidates from the full sample of 30, intriguingly similar to the ~30% frequency for late-type galaxies. These XUV candidates occur at a wide range of masses and on both the red and blue sequences in color vs. stellar mass, indicating a possible association with processes like gas accretion and/or galaxy interactions that would affect the galaxy population broadly. We go on to apply the quantitative Type 1 and Type 2 XUV-disk definitions to a nine-galaxy subsample analyzed in detail. For this subsample, six of the nine are Type 1 XUVs, i.e., galaxies with UV structure beyond the expected star formation threshold. The other three come close to satisfying the Type 2 definition, but that definition proves problematic to apply to this sample: the NUV-derived star formation threshold radii for our E/S0s often lie inside the 80% Ks-band light (K80) radii, violating an implicit assumption of the Type 2 definition, or lie outside but not as far as the definition requires. Nonetheless, the three otherwise Type 2-like galaxies (modified Type 2 XUVs) have higher star formation rates and bluer FUV - NUV colors than the Type 1 XUVs in the sample. We propose that Type 1 XUVs may reflect early or inefficient stages of star formation, while modified Type 2 XUVs perhaps reflect inside-out disk regrowth.
