Mass assembly of galaxies: Smooth accretion versus mergers

TL;DR

This study uses cosmological simulations to quantify the roles of smooth accretion and mergers in galaxy mass assembly, finding that most galaxies grow primarily through accretion, with mergers being significant mainly for the most massive galaxies.

Contribution

It provides a detailed analysis of mass assembly modes in galaxies over cosmic time, highlighting the dominance of smooth accretion and the mass-dependent nature of galaxy growth.

Findings

Most galaxies assemble 77% of their mass via smooth accretion.

Massive galaxies grow significantly through mergers, accounting for 23%.

Galaxy assembly history shows a downsizing trend, with massive galaxies forming earlier.

Abstract

Galaxies accrete their mass by means of both smooth accretion from the cosmic web, and the mergers of smaller entities. We wish to quantify the respective role of these two modes of accretion, which could determine the morphological types of galaxies observed today. Multi-zoom cosmological simulations are used to estimate as a function of time the evolution of mass in bound systems, for dark matter as well as baryons. The baryonic contents of dark matter haloes are studied. Merger histories are followed as a function of external density, and the different ways in which mass is assembled in galaxies and the stellar component accumulated are quantified. We find that most galaxies assemble their mass through smooth accretion, and only the most massive galaxies also grow significantly through mergers. The mean fraction of mass assembled by accretion is 77 %, and by mergers 23 %. We present typical accretion histories of hundreds of galaxies: masses of the most massive galaxies increase monotonically in time, mainly through accretion, many intermediate-mass objects also experience mass-loss events such as tidal stripping and evaporation. However, our simulations suffer from the overcooling of massive galaxies caused by the neglect of active galaxy nuclei (AGN) feedback. The time by which half of the galay mass has assembled, both in dark matter and baryons, is a decreasing function of mass, which is compatible with the observations of a so-called downsizing. At every epoch in the universe, there are low-mass galaxies actively forming stars, while more massive galaxies form their stars over a shorter period of time within half the age of the universe.