Constraints on the in-situ and ex-situ stellar masses in nearby galaxies with Artificial Intelligence

TL;DR

This study uses machine learning trained on cosmological simulations to estimate the contributions of in-situ and ex-situ stellar mass in nearby galaxies, revealing that in-situ formation dominates except in the most massive galaxies where accretion is significant.

Contribution

The paper introduces a novel machine learning approach trained on mock data to quantify stellar mass origins in galaxies, bridging simulations and observations.

Findings

In-situ stellar mass dominates across most galaxy masses.

Accreted mass becomes significant (>35%) in galaxies with M* > 10^11 Msun.

Higher accreted fractions are found in elliptical, quenched, and slow-rotator galaxies.

Abstract

The hierarchical model of galaxy evolution suggests that the impact of mergers is substantial on the intricate processes that drive stellar assembly within a galaxy. However, accurately measuring the contribution of accretion to a galaxy's total stellar mass and its balance with in-situ star formation poses a persistent challenge, as it is neither directly observable nor easily inferred from observational properties. Here, we present theory-motivated predictions for the fraction of stellar mass originating from mergers in a statistically significant sample of nearby galaxies, using data from MaNGA. Employing a robust machine learning model trained on mock MaNGA analogs (MaNGIA) in turn obtained from a cosmological simulation (TNG50), we unveil that in-situ stellar mass dominates almost across the entire stellar mass spectrum (1e9Msun < M* < 1e12Msun). Only in more massive galaxies (M* > 1e11Msun) does accreted mass become a substantial contributor, reaching up to 35-40% of the total stellar mass. Notably, the ex-situ stellar mass in the nearby universe exhibits significant dependence on galaxy characteristics, with higher accreted fractions favored by elliptical, quenched galaxies and slow rotators, as well as galaxies at the center of more massive dark matter halos.