The Build-Up of the Hubble Sequence in the COSMOS Field

TL;DR

This study examines how the morphological composition of massive galaxies in the COSMOS field evolves from redshift 1 to 0.2, revealing mass-dependent trends and the role of mergers and secular processes in shaping the Hubble sequence.

Contribution

It provides a detailed analysis of galaxy morphological evolution across different mass scales, highlighting the dominant processes at each stage and the stability of the most massive galaxies since z=1.

Findings

Massive ellipticals dominate since z=1 with no evolution.

Bulge-dominated disks grow significantly at 5e10-1e11 Msol.

Bulge-less disks virtually disappear by z=0.2.

Abstract

We use ~8,600 >5e10 Msol COSMOS galaxies to study how the morphological mix of massive ellipticals, bulge-dominated disks, intermediate-bulge disks, bulge-less disks and irregular galaxies evolves from z=0.2 to z=1. The morphological evolution depends strongly on mass. At M>3e11 Msol, no evolution is detected in the morphological mix: ellipticals dominate since z=1, and the Hubble sequence has quantitatively settled down by this epoch. At the 1e11 Msol mass scale, little evolution is detected, which can be entirely explained with major mergers. Most of the morphological evolution from z=1 to z=0.2 takes place at masses 5e10 - 1e11 Msol, where: (i) The fraction of spirals substantially drops and the contribution of early-types increases. This increase is mostly produced by the growth of bulge-dominated disks, which vary their contribution from ~10% at z=1 to >30% at z=0.2 (cf. the elliptical fraction grows from ~15% to ~20%). Thus, at these masses, transformations from late- to early-types result in disk-less elliptical morphologies with a statistical frequency of only 30% - 40%. Otherwise, the processes which are responsible for the transformations either retain or produce a non-negligible disk component. (ii) The bulge-less disk galaxies, which contribute ~15% to the intermediate-mass galaxy population at z=1, virtually disappear by z=0.2. The merger rate since z=1 is too low to account for the disappearance of these massive bulge-less disks, which most likely grow a bulge via secular evolution.