STELLAR POPULATIONS of EARLY-TYPE GALAXIES with MID-INFRARED EXCESS EMISSION

We present a stellar population analysis of quiescent (without Hα emission) and bright (M_r < -21.5) early-type galaxies (ETGs) with recent star formation. The ETGs are selected from a spectroscopic sample of SDSS galaxies at 0.04 < z < 0.11 with WISE mid-infrared (IR) and GALEX near-ultraviolet (UV) emissions. We stack the optical spectra of ETGs with different amounts of mid-IR and near-UV excess emissions to measure the strength of 4000 break D_n4000 and the width of Balmer absorption line Hδ_A that are indicative of recent (∼1 Gyr) star formation activity. The WISE [3.4]-[12] colors show stronger correlations with the spectral features than NUV-r colors. We fit to the stacked spectra with a spectral fitting code, STARLIGHT, and find that the mass fraction of young (≤Gyr) and intermediate-age (∼1-5 Gyr) stars in the ETGs with mid-IR excess emission is ∼4%-11%, depending on the template spectrum used for the fit. These results show that the ETGs with mid-IR excess emission have experienced star formation within the last 1-5 Gyr and that the mid-IR emission is a useful diagnostic tool for probing recent star formation activity in ETGs.