Forming Early-type Galaxies in Groups Prior to Cluster Assembly

TL;DR

This study investigates how galaxy morphological transformations occur in a supergroup before it becomes a cluster, showing that significant early-type galaxy fractions are established in group environments prior to cluster assembly.

Contribution

It provides evidence that morphological transformation of galaxies predominantly occurs in group environments before cluster formation, based on observations of a unique proto-cluster at z~0.37.

Findings

Early-type galaxy fraction in supergroup is ~60%.

S0 to elliptical ratio is 0.5, similar to clusters at similar redshift.

Pre-processing in groups largely explains early-type fractions in clusters.

Abstract

We study a unique proto-cluster of galaxies, the supergroup SG1120-1202. We quantify the degree to which morphological transformation of cluster galaxies occurs prior to cluster assembly in order to explain the observed early-type fractions in galaxy clusters at z=0. SG1120-1202 at z~0.37 is comprised of four gravitationally bound groups that are expected to coalesce into a single cluster by z=0. Using HST ACS observations, we compare the morphological fractions of the supergroup galaxies to those found in a range of environments. We find that the morphological fractions of early-type galaxies (~60 %) and the ratio of S0 to elliptical galaxies (0.5) in SG1120-1202 are very similar to clusters at comparable redshift, consistent with pre-processing in the group environment playing the dominant role in establishing the observed early-type fraction in galaxy clusters.