Size evolution of normal and compact galaxies in the EAGLE simulation

TL;DR

This study uses the EAGLE simulation to analyze galaxy size evolution from redshift 2 to 0, revealing size-mass relations, differences between active and passive galaxies, and the growth of compact galaxies over time.

Contribution

It provides a detailed comparison of simulated galaxy size evolution with observations, highlighting the growth mechanisms of compact galaxies and the limitations of using population averages for passive galaxies.

Findings

Galaxy sizes increase with stellar mass, relation weakens at higher redshift.

Passive galaxies are smaller than active ones at fixed mass.

Compact galaxies at high redshift grow in size through stellar migration, star formation, and mergers.

Abstract

We present the evolution of galaxy sizes, from redshift 2 to 0, for actively star forming and passive galaxies in the cosmological hydrodynamical 1003 cMpc3 simulation of the EAGLE project. We find that the sizes increase with stellar mass , but that the relation weakens with increasing redshift. Separating galaxies by their star formation activity, we find that passive galaxies are typically smaller than active galaxies at fixed stellar mass. These trends are consistent with those found in observations and the level of agreement between the predicted and observed size - mass relation is of order 0.1 dex for z < 1 and 0.2-0.3 dex from redshift 1 to 2. We use the simulation to compare the evolution of individual galaxies to that of the population as a whole. While the evolution of the size-stellar mass relation for active galaxies provides a good proxy for the evolution of individual galaxies, the evolution of individual passive galaxies is not well represented by the observed size - mass relation due to the evolving number density of passive galaxies. Observations of z \approx 2 galaxies have revealed an abundance of massive red compact galaxies, that depletes below z \approx 1. We find that a similar population forms naturally in the simulation. Comparing these galaxies to their z = 0 descendants, we find that all compact galaxies grow in size due to the high-redshift stars migrating outwards. Approximately 60% of the compact galaxies increase in size further due to renewed star formation and/or mergers.