Galaxy Evolution in all Five CANDELS Fields and IllustrisTNG: Morphological, Structural, and the Major Merger Evolution to $z \sim 3$

TL;DR

This study analyzes the structural evolution of 16,778 galaxies up to redshift 3 across five CANDELS fields, revealing how galaxy concentration and asymmetry change over time and estimating merger rates using simulations.

Contribution

It provides the first large-scale quantitative analysis of galaxy structural evolution, including merger timescales and rates, using observational data combined with IllustrisTNG simulations.

Findings

Median merger timescale is approximately 0.56 Gyr, consistent across redshifts.

Merger rates increase with redshift as (1+z)^{1.87}, aligning with recent methods.

Galaxies are more concentrated at higher redshifts, indicating inside-out growth.

Abstract

A fundamental feature of galaxies is their structure, yet we are just now understanding the evolution of structural properties in quantitative ways. As such, we explore the quantitative non-parametric structural evolution of 16,778 galaxies up to $z\sim3$ in all five CANDELS fields, the largest collection of high resolution images of distant galaxies to date. Our goal is to investigate how the structure, as opposed to size, surface brightness, or mass, changes with time. In particular, we investigate how the concentration and asymmetry of light evolve in the rest-frame optical. To interpret our galaxy structure measurements, we also run and analyse 300 simulation realisations from IllustrisTNG to determine the timescale of mergers for the CAS system. We measure that from $z=0-3$, the median asymmetry merger timescale is $0.56^{+0.23}_{-0.18}$Gyr, and find it does not vary with redshift. Using this data, we find that galaxies become progressively asymmetric at a given mass at higher redshifts and we derive merger rates which scale as $\sim(1+z)^{1.87\pm0.04}$Gyr$^{-1}$, which agrees well with recent machine learning and galaxy pair approaches, removing previous inconsistencies. We also show that far-infrared selected galaxies that are invisible to \textit{HST} have a negligible effect on our measurements. We also find that galaxies are more concentrated at higher redshifts. We interpret this as a sign of how their formation occurs from a smaller initial galaxy that later grows into a larger one through mergers, consistent with the size growth of galaxies from `inside-out', suggesting that the centres are the oldest parts of most galaxies.