Early-type galaxy spin evolution in the Horizon-AGN simulation

TL;DR

This study uses the Horizon-AGN simulation to analyze how mergers and environmental factors influence the spin evolution of early-type galaxies, highlighting the dominant role of tidal perturbations over mergers in most cases.

Contribution

It provides new insights into the relative importance of mergers and tidal effects in shaping the spin of early-type galaxies post-$z \, 1$, emphasizing non-merger tidal perturbations.

Findings

Galaxy mergers cause spin changes in 94% of central ETGs with massive halos.

Satellite ETGs lose spin gradually due to environmental effects.

Non-merger tidal perturbations correlate better with spin-down in satellites.

Abstract

Using the Horizon-AGN simulation data, we study the relative role of mergers and environmental effects in shaping the spin of early-type galaxies (ETGs) after $z \simeq 1$. We follow the spin evolution of 10,037 color-selected ETGs more massive than 10$^{10} \rm \, M_{\odot}$ that are divided into four groups: cluster centrals (3%), cluster satellites (33%), group centrals (5%), and field ETGs (59%). We find a strong mass dependence of the slow rotator fraction, $f_{\rm SR}$, and the mean spin of massive ETGs. Although we do not find a clear environmental dependence of $f_{\rm SR}$, a weak trend is seen in the mean value of spin parameter driven by the satellite ETGs as they gradually lose their spin as their environment becomes denser. Galaxy mergers appear to be the main cause of total spin changes in 94% of central ETGs of halos with $M_{vir} > 10^{12.5}\rm M_{\odot}$, but only 22% of satellite and field ETGs. We find that non-merger induced tidal perturbations better correlate with the galaxy spin-down in satellite ETGs than mergers. Given that the majority of ETGs are not central in dense environments, we conclude that non-merger tidal perturbation effects played a key role in the spin evolution of ETGs observed in the local ($z < 1$) universe.