# The mass, colour, and structural evolution of today's massive galaxies since z~5

## TL;DR

This study traces the mass, colour, and structural evolution of massive galaxies from redshift 5 to the present, revealing early core formation, ongoing outer growth, and a transition in growth mechanisms around redshift 1.5.

## Contribution

It provides a detailed analysis of the evolution of massive galaxies using stacking and a novel progenitor selection method, highlighting the transition in growth processes over cosmic time.

## Key findings

- Most stellar mass in centers was in place by z~2.
- Outer regions experience largest fractional mass increase.
- A transition at z~1.5 from in-situ star formation to minor mergers.

## Abstract

In this paper, we use stacking analysis to trace the mass-growth, colour evolution, and structural evolution of present-day massive galaxies ($\log(M_{*}/M_{\odot})=11.5$) out to $z=5$. We utilize the exceptional depth and area of the latest UltraVISTA data release, combined with the depth and unparalleled seeing of CANDELS to gather a large, mass-selected sample of galaxies in the NIR (rest-frame optical to UV). Progenitors of present-day massive galaxies are identified via an evolving cumulative number density selection, which accounts for the effects of merging to correct for the systematic biases introduced using a fixed cumulative number density selection, and find progenitors grow in stellar mass by $\approx1.5~\mathrm{dex}$ since $z=5$. Using stacking, we analyze the structural parameters of the progenitors and find that most of the stellar mass content in the central regions was in place by $z\sim2$, and while galaxies continue to assemble mass at all radii, the outskirts experience the largest fractional increase in stellar mass. However, we find evidence of significant stellar mass build up at $r<3~\mathrm{kpc}$ beyond $z>4$ probing an era of significant mass assembly in the interiors of present day massive galaxies. We also compare mass assembly from progenitors in this study to the EAGLE simulation and find qualitatively similar assembly with $z$ at $r<3~\mathrm{kpc}$. We identify $z\sim1.5$ as a distinct epoch in the evolution of massive galaxies where progenitors transitioned from growing in mass and size primarily through in-situ star formation in disks to a period of efficient growth in $r_{e}$ consistent with the minor merger scenario.