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

Abstract

We have identified 15 extended ultraviolet (XUV) disks in a largely field sample of 38 E/S0 galaxies that have stellar masses primarily below ∼4 × 10 10 M 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% ± 9% XUV-disk frequency for these E/S0s is roughly twice the ∼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% ± 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 ∼ 5 × 10 9 M (the "gas-richness threshold mass") display UV-B disks, supporting the previously suggested association of this population with active disk growth.

Star formation sustained by gas accretion

“…Surveys of extended UV disks around both early-and latetype galaxies actually suggest on-going gas accretion and star formation in the outer parts (Lemonias et al 2011;Moffett et al 2012). These ideas are consistent with cosmological numerical simulations, where disk galaxies acquire their spin together with their mass through cold gas accretion.…”

Structural analysis of star-forming blue early-type galaxies

“…The position of star-forming blue ETGs on the colour-magnitude/colour-stellar mass diagrams has been interpreted as possible migration from the red sequence to the blue cloud after acquiring sufficient fuel for star formation or fading post-starburst galaxies (Kannappan et al 2009;Huertas-Company et al 2010;Thilker et al 2010;Marino et al 2011;Moffett et al 2012;McIntosh et al 2014;Rutkowski et al 2014;Moffett et al 2015;Wong et al 2015). The sample of a few blue ETGs from other studies was found to host a significant amount of molecular gas (∼ 10 7−9 M⊙) with a linear dependence on star formation surface density within the galaxy and a higher molecular gas star formation efficiency than in normal star-forming late-type galaxies (Wei et al 2010;Stark et al 2013).…”

Determining the Halo Mass Scale Where Galaxies Lose Their Gas*

“…In the field, the gas ejected from a merger via tidal or feedback effects may fall back onto the galaxy after a few gigayears and replenish the gas supply. In some cases this may rejuvenate star formation and regrow a disk (Kannappan et al 2009;Moffett et al 2012), but depending on the specific characteristics of the merger (e.g., mass ratio, merger geometry, merger gas fraction), it may also just add a modest amount of gas to the galaxy. While mergers may be more common in groups because of their low velocity dispersions and small distance between galaxies, the gas ejected during a merger in a dense environment is less likely to fall back onto the merged galaxy because of its interactions with the group tidal field and the intragroup medium.…”