Signatures of stellar accretion in MaNGA early-type galaxies
  The late assembly of massive galaxies is thought to be dominated by stellar
accretion in their outskirts (beyond 2 effective radii Re) due to dry, minor
galaxy mergers. We use observations of 1010 passive early-type galaxies (ETGs)
within z<0.15 from SDSS IV MaNGA to search for evidence of this accretion. The
outputs from the stellar population fitting codes FIREFLY, pPXF, and Prospector
are compared to control for systematic errors in stellar metallicity (Z)
estimation. We find that the average radial logZ/Zsun profiles of ETGs in
various stellar mass (M) bins are not linear. As a result, these profiles are
poorly characterized by a single gradient value, explaining why weak trends
reported in previous work can be difficult to interpret. Instead, we examine
the full radial extent of stellar metallicity profiles and find them to flatten
in the outskirts of M>10^{11}Msun ETGs. This is a signature of stellar
accretion. Based on a toy model for stellar metallicity profiles, we infer the
ex-situ stellar mass fraction in ETGs as a function of M and galactocentric
radius. We find that ex-situ stars at 2Re make up 20% of the projected stellar
mass of M<10^{10.5}Msun ETGs, rising up to 80% for M>10^{11.5}Msun ETGs.
