Star Formation Signatures in Optically Quiescent Early-type Galaxies

TL;DR

High-resolution UV imaging reveals that most optically quiescent early-type galaxies with UV excess exhibit extended star formation, often in rings or spiral arms, indicating external gas accretion and rejuvenation processes.

Contribution

This study provides high-resolution UV evidence that most UV-excess early-type galaxies show extended star formation, challenging previous assumptions based on lower-resolution data.

Findings

75% of UV-excess early-type galaxies show extended star formation

UV sizes are larger and dust content is lower compared to other green-valley galaxies

Star formation is less common in very massive early-type galaxies, possibly due to AGN feedback

Abstract

In recent years argument has been made that a high fraction of early-type galaxies in the local universe experience low levels (< 1 M_sun/yr) of star formation (SF) that causes strong excess in UV flux, yet leaves the optical colors red. Many of these studies were based on GALEX imaging of SDSS galaxies (z~0.1), and were thus limited by its 5" FWHM. Poor UV resolution left other possibilities for UV excess open, such as the old populations or an AGN. Here we study high-resolution far-ultraviolet HST/ACS images of optically quiescent early-type galaxies with strong UV excess. The new images show that three-quarters of these moderately massive (~5x10^10 M_sun) early-type galaxies shows clear evidence of extended SF, usually in form of wide or concentric UV rings, and in some cases, striking spiral arms. SDSS spectra probably miss these features due to small fiber size. UV-excess early-type galaxies have on average less dust and larger UV sizes (D>40 kpc) than other green-valley galaxies, which argues for an external origin for the gas that is driving the SF. Thus, most of these galaxies appear `rejuvenated' (e.g., through minor gas-rich mergers or IGM accretion). For a smaller subset of the sample, the declining SF (from the original internal gas) cannot be ruled out. SF is rare in very massive early-types (M_* > 10^11 M_sun), a possible consequence of AGN feedback. In addition to extended UV emission, many galaxies show a compact central source, which may be a weak, optically inconspicuous AGN.