Stellar Populations of Early-type Galaxies with Mid-Infrared Excess
  Emission
  We present a stellar population analysis of quiescent (without H$\alpha$
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 {\it WISE} mid-infrared (IR) and
{\it 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 \AA{} break $D_{n}$4000 and the width of Balmer absorption
line H$\delta_{A}$ that are indicative of recent ($\sim$1 Gyr) star formation
activity. The {\it WISE} [3.4]$-$[12] colors show stronger correlations with
the spectral features than NUV$-r$ colors. We fit to the stacked spectra with a
spectral fitting code, STARLIGHT, and find that the mass fraction of young
($\leq$1 Gyr) and intermediate-age ($\sim$1$-$5 Gyr) stars in the ETGs with
mid-IR excess emission is $\sim$4$-$11\%, depending on the template spectrum
used for the fit. These results show that the ETGs with mid-IR excess emission
have experienced star formation within the last 1$-$5 Gyr and that the mid-IR
emission is a useful diagnostic tool for probing recent star formation activity
in ETGs.
