Star Formation Signatures in Optically Quiescent Early-type Galaxies

Abstract

In recent years, an argument has been made that a high fraction of early-type galaxies (ETGs) in the local universe experience low levels (lsim1 M _sun yr^-1) of star formation (SF) that causes strong excess in UV flux, yet leaves the optical colors red. Many of these studies were based on Galaxy Evolution Explorer imaging of Sloan Digital Sky Survey (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 active galactic nucleus (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 (~5 × 10¹⁰ M _sun) ETGs 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 ETGs 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 intergalactic medium 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 _* > 10¹¹ 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.