We have identified 15 XUV ( extended ultraviolet ) disks in a largely field sample of 38 E/S0 galaxies that have stellar masses primarily below \sim 4 \times 10 ^ { 10 } M _ { \odot } and comparable numbers on the red and blue sequences .
We use a new purely quantitative XUV disk definition designed with reference to the “ Type 1 ” XUV disk definition found in the literature , requiring UV extension relative to a UV-defined star formation threshold radius .
The 39 \pm 9 % XUV-disk frequency for these E/S0s is roughly twice the \sim 20 % reported for late-type galaxies ( although differences in XUV-disk criteria complicate the comparison ) , possibly indicating that XUV disks are preferentially associated with galaxies experiencing weak or inefficient star formation .
Consistent with this interpretation , we find that 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 in our E/S0s with a 42 ^ { +9 } _ { -8 } % frequency and need not coincide with XUV disks , thus their combined frequency is 61 \pm 9 % .
For both XUV and UV-B disks , UV colors typically imply < 1 Gyr ages , and most such disks extend beyond the optical R _ { 25 } radius .
XUV disks occur over the full sample mass range and on both the red and blue sequences , suggesting an association with galaxy interactions or another similarly 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 gas accretion or another mass-dependent evolutionary process .
Virtually all blue E/S0s in the gas-rich regime below stellar mass M _ { t } \sim 5 \times 10 ^ { 9 } M _ { \odot } ( the “ gas-richness threshold mass ” ) display UV-B disks , supporting the previously suggested association of this population with active disk growth .