# Signatures of stellar accretion in MaNGA early-type galaxies

## TL;DR

This study uses MaNGA data to analyze stellar metallicity profiles in early-type galaxies, revealing that metallicity flattening in outer regions indicates stellar accretion, with higher ex-situ fractions in more massive galaxies.

## Contribution

It demonstrates that radial metallicity profiles are complex and that their flattening in galaxy outskirts serves as evidence for stellar accretion, providing new insights into galaxy assembly.

## Key findings

- Metallicity profiles are non-linear and poorly characterized by a single gradient.
- Outer metallicity flattening correlates with stellar accretion signatures.
- Ex-situ stellar mass fractions increase with galaxy mass, reaching 80% in the most massive ETGs.

## Abstract

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.