AEGIS: The Morphologies of Green Galaxies at 0.4<z<1.2

TL;DR

This study analyzes the morphologies of approximately 300 green valley galaxies at 0.4<z<1.2 to understand the mechanisms behind star formation quenching, finding they are mostly massive disk galaxies with high concentration, not predominantly mergers.

Contribution

It provides a detailed morphological analysis of green valley galaxies at intermediate redshifts, showing they are mainly massive disks with high concentration, challenging the idea that mergers are the primary quenching mechanism.

Findings

Green galaxies are morphologically intermediate between red and blue galaxies.

Most green galaxies are not classified as mergers, with a lower merger fraction than blue galaxies.

Green galaxies are predominantly massive disk galaxies with high concentration values.

Abstract

We present quantitative morphologies of ~300 galaxies in the optically-defined green valley at 0.4<z<1.2, in order to constrain the mechanism(s) responsible for quenching star formation in the bulk of this population. The sample is selected from galaxies in the All-Wavelength Extended Groth Strip International Survey (AEGIS). While the green valley is defined using optical U-B colors, we find that using a green valley sample defined using NUV-R colors does not change the results. Using HST/ACS imaging, we study several quantitative morphological parameters including CAS, B/T from GIM2D, and Gini/M_20. We find that the green galaxy population is intermediate between the red and blue galaxy populations in terms of concentration, asymmetry, and morphological type and merger fraction estimated using Gini/M_20. We find that most green galaxies are not classified as mergers; in fact, the merger fraction in the green valley is lower than in the blue cloud. We show that at a given stellar mass, green galaxies have higher concentration values than blue galaxies and lower concentration values than red galaxies. Additionally, we find that 12% of green galaxies have B/T = 0 and 21% with B/T \leq 0.05. Our results show that green galaxies are generally massive (M\ast ~ 10^10.5 M_sun) disk galaxies with high concentrations. We conclude that major mergers are likely not the sole mechanism responsible for quenching star formation in this population and that either other external processes or internal secular processes play an important role both in driving gas towards the center of these galaxies and in quenching star formation.