The rise and fall of stellar discs across the peak of cosmic star formation history: mergers versus smooth accretion

TL;DR

This study uses cosmological simulations to analyze how mergers and smooth accretion influence galaxy morphology, revealing that accretion fosters disk formation while mergers tend to create elliptical galaxies and increase their sizes.

Contribution

It provides a detailed statistical analysis of the effects of mergers and smooth accretion on galaxy shapes and sizes across cosmic time, highlighting the different impacts based on merger mass ratios and gas content.

Findings

Smooth accretion flattens small galaxies, promoting disk formation.

Mergers, including minor ones, tend to puff up and destroy stellar disks.

Elliptical galaxies grow more in size through mergers than disk galaxies.

Abstract

Building galaxy merger trees from a state-of-the-art cosmological hydrodynamics simulation, Horizon-AGN, we perform a statistical study of how mergers and smooth accretion drive galaxy morphologic properties above $z > 1$. More specifically, we investigate how stellar densities, effective radii and shape parameters derived from the inertia tensor depend on mergers of different mass ratios. We find strong evidence that smooth accretion tends to flatten small galaxies over cosmic time, leading to the formation of disks. On the other hand, mergers, and not only the major ones, exhibit a propensity to puff up and destroy stellar disks, confirming the origin of elliptical galaxies. We also find that elliptical galaxies are more susceptible to grow in size through mergers than disc galaxies with a size-mass evolution $r \prop M^{1.2}$ instead of $r \prop M^{-0.5} - M^{0.5}$ depending on the merger mass ratio. The gas content drive the size-mass evolution due to merger with a faster size growth for gas-poor galaxies $r \prop M^2$ than for gas-rich galaxies $r \prop M$.