# Formation and assembly history of stellar components in galaxies as a function of stellar and halo mass

## TL;DR

This paper investigates how the assembly history of stellar components in galaxies depends on halo and stellar mass, highlighting the increasing importance of mergers in mass growth for massive galaxies within a cosmological framework.

## Contribution

It extends previous models by analyzing the dependence of stellar assembly on halo environment and mass using semi-analytic methods, emphasizing the role of mergers in galaxy evolution.

## Key findings

- Ex situ fraction increases with halo mass, sharply rising above 10^11 solar masses.
- Mergers dominate mass growth in galaxies with halo masses above 10^13 solar masses.
- Satellite galaxies rarely become merger-dominated despite environmental effects.

## Abstract

Galaxy mass assembly is an end product of structure formation in the $\Lambda$CDM cosmology. As an extension of Lee \& Yi (2013), we investigate the assembly history of stellar components in galaxies as a function of halo environments and stellar mass using semi-analytic approaches. In our fiducial model, halo mass intrinsically determines the formation and assembly of the stellar mass. Overall, the ex situ fraction slowly increases in central galaxies with increasing halo mass but sharply increases for $\log M_{*}/M_{\odot}\gtrsim11$. A similar trend is also found in satellite galaxies, which implies that mergers are essential to build stellar masses above $\log M_{*}/M_{\odot}\sim11$. We also examine the time evolution of the contribution of mass growth channels. Mergers become the primary channel in the mass growth of central galaxies when their host halo mass begins to exceed $\log M_{200}/M_{\odot}\sim13$. However, satellite galaxies seldom reach the merger-dominant phase despite their reduced star formation activities due to environmental effects.