A Wide-field Survey of Two Z~0.5 Galaxy Clusters: Identifying the Physical Processes Responsible for the Observed Transformation of Spirals into S0s

TL;DR

This study investigates the physical processes transforming spiral galaxies into S0s in two galaxy clusters at intermediate redshift, identifying the roles of galaxy interactions and the intra-cluster medium in this morphological evolution.

Contribution

It conclusively identifies spiral galaxies in the process of transforming into S0s and compares the transformation mechanisms across two clusters, highlighting environmental influences.

Findings

Spiral galaxies in infalling groups begin slow transformation into S0s via galaxy interactions.

Rapid S0 formation occurs in dense ICM regions through ram-pressure stripping.

Transformation processes vary with environment but consistently contribute to S0 population buildup.

Abstract

We present new results from our comprehensive comparative survey of two massive, intermediate redshift galaxy clusters, Cl0024+17 (z=0.39) and MS0451-03 (z=0.54). We identify and study several key classes of `transition objects' whose stellar populations or dynamical states indicate a recent or ongoing change in morphology and star formation rate. For the first time, we have been able to conclusively identify spiral galaxies in the process of transforming into S0 galaxies. This has been accomplished by locating both spirals whose star formation is being quenched as well as their eventual successors, the recently created S0s. Differences between the two clusters in both the timescales and spatial location of this conversion process allow us to evaluate the relative importance of several proposed physical mechanisms that could be responsible for the transformation. Combined with other diagnostics that are sensitive to either ICM-driven galaxy evolution or galaxy-galaxy interactions, we describe a self-consistent picture of galaxy evolution in clusters. We find that spiral galaxies within infalling groups have already begun a slow process of conversion into S0s primarily via gentle galaxy-galaxy interactions. The fates of spirals upon reaching the core of the cluster depend heavily on the cluster ICM, with rapid conversion of all remaining spirals into S0s via ram-pressure stripping in clusters where the ICM is dense. In the presence of a less-dense ICM, the conversion continues at a slower pace, with galaxy-galaxy interactions continuing to play a role along with `starvation' by the ICM. We conclude that the buildup of the local S0 population through the transformation of spiral galaxies is a heterogeneous process that nevertheless proceeds robustly across a variety of different environments.