Shape Evolution of Massive Early-Type Galaxies: Confirmation of Increased Disk Prevalence at z>1

TL;DR

This study uses high-resolution imaging to analyze the shape evolution of massive early-type galaxies since z~1, revealing increased disk features at higher redshifts and the role of merging in their morphological transformation.

Contribution

It provides new evidence for the evolution of galaxy shapes over time, especially the increased prevalence of disk-like structures at z>1, based on axis-ratio measurements from a large, mass-selected sample.

Findings

Early-type galaxies at z>1 are flatter than at z<1.

The median projected axis ratio decreases with redshift.

Most massive early-type galaxies are round regardless of epoch.

Abstract

We use high-resolution K-band VLT/HAWK-I imaging over 0.25 square degrees to study the structural evolution of massive early-type galaxies since z~1. Mass-selected samples, complete down to log(M/M_sun)~10.7 such that `typical' L* galaxies are included at all redshifts, are drawn from pre-existing photometric redshift surveys. We then separated the samples into different redshift slices and classify them as late- or early-type galaxies on the basis of their specific star-formation rate. Axis-ratio measurements for the ~400 early-type galaxies in the redshift range 0.6<z<1.8 are accurate to 0.1 or better. The projected axis-ratio distributions are then compared with lower redshift samples. We find strong evidence for evolution of the population properties: early-type galaxies at z>1 are, on average, flatter than at z<1 and the median projected axis ratio at a fixed mass decreases with redshift. However, we also find that at all epochs z<~2 the very most massive early-type galaxies (log(M/M_sun)>11.3) are the roundest, with a pronounced lack among them of galaxies that are flat in projection. Merging is a plausible mechanism that can explain both results: at all epochs merging is required for early-type galaxies to grow beyond log(M/M_sun)~11.3, and all early types over time gradually and partially loose their disk-like characteristics.