Extended UV Disks and UV-Bright Disks in Low-Mass E/S0 Galaxies

TL;DR

This study identifies and characterizes extended UV disks in low-mass E/S0 galaxies, revealing their frequency, association with star formation, and potential links to galaxy evolution processes like interactions and accretion.

Contribution

It introduces a new quantitative definition for XUV disks and provides the first comprehensive analysis of their occurrence in low-mass E/S0 galaxies.

Findings

XUV disks are present in about 39% of the sample, roughly twice the rate in late-type galaxies.

UV-B disks are found in approximately 42% of the sample, often in gas-rich, low-mass galaxies.

UV colors suggest these disks are typically younger than 1 Gyr, indicating recent or ongoing star formation.

Abstract

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]