  We investigate the origin of star formation activity in early-type galaxies
with current star formation using spatially resolved spectroscopic data from
the Mapping Nearby Galaxies at APO (MaNGA) in the Sloan Digital Sky Survey
(SDSS). We first identify star-forming early-type galaxies from the SDSS
sample, which are morphologically early-type but show current star formation
activity in their optical spectra. We then construct comparison samples with
different combinations of star formation activity and morphology, which include
star-forming late-type galaxies, quiescent early-type galaxies and quiescent
late-type galaxies. Our analysis of the optical spectra reveals that the
star-forming early-type galaxies have two distinctive episodes of star
formation, which is similar to late-type galaxies but different from quiescent
early-type galaxies with a single star formation episode. Star-forming
early-type galaxies have properties in common with star-forming late-type
galaxies, which include stellar population, gas and dust content, mass and
environment. However, the physical properties of star-forming early-type
galaxies derived from spatially resolved spectroscopy differ from those of
star-forming late-type galaxies in the sense that the gas in star-forming
early-type galaxies is more concentrated than their stars, and is often
kinematically misaligned with stars. The age gradient of star-forming
early-type galaxies also differs from those of star-forming late-type galaxies.
Our findings suggest that the current star formation in star-forming early-type
galaxies has an external origin including galaxy mergers or accretion gas from
the cosmic web.
