Mapping Accreted Stars in Early-Type Galaxies Across the Mass-Size Plane

TL;DR

This study maps the distribution of ex-situ stars in early-type galaxies using MUSE spectroscopy, revealing that ex-situ fractions increase with radius and are higher in more massive, extended galaxies, aligning with theoretical predictions.

Contribution

It provides the first spatially resolved mapping of ex-situ stars in a sample of massive early-type galaxies, confirming theoretical models of galaxy assembly.

Findings

Ex-situ fraction increases with radius in all galaxies.

More massive and extended galaxies show a faster increase in ex-situ fraction.

Ex-situ fractions reach up to 100% at 2 effective radii in some galaxies.

Abstract

Galaxy mergers are instrumental in dictating the final mass, structure, stellar populations, and kinematics of galaxies. Cosmological galaxy simulations indicate that the most massive galaxies at z=0 are dominated by high fractions of `ex-situ' stars, which formed first in distinct independent galaxies, and then subsequently merged into the host galaxy. Using spatially resolved MUSE spectroscopy we quantify and map the ex-situ stars in thirteen massive Early Type galaxies. We use full spectral fitting together with semi-analytic galaxy evolution models to isolate the signatures in the galaxies' light which are indicative of ex-situ populations. Using the large MUSE field of view we find that all galaxies display an increase in ex-situ fraction with radius, with massive and more extended galaxies showing a more rapid increase in radial ex-situ fraction, (reaching values between 30% to 100% at 2 effective radii) compared to less massive and more compact sources (reaching between 5% to 40% ex-situ fraction within the same radius). These results are in line with predictions from theory and simulations which suggest ex-situ fractions should increase significantly with radius at fixed mass for the most massive galaxies.