The spatial distribution of galaxies of different spectral types in the massive intermediate-redshift cluster MACSJ0717.5+3745

TL;DR

This study analyzes the spatial distribution and spectral types of galaxies in the massive cluster MACSJ0717.5+3745 at redshift 0.55, revealing environmental effects on galaxy evolution and star formation.

Contribution

It provides the first detailed spectroscopic analysis of galaxy spectral types and their spatial distribution in this high-redshift cluster, highlighting ram-pressure stripping effects.

Findings

E+A galaxies are mostly within the ram-pressure stripping radius.

Starburst galaxies are predominantly found in the filament region.

The morphology-density relation is confirmed and flattens at low densities.

Abstract

We present the results of a wide-field spectroscopic analysis of the galaxy population of the massive cluster MACSJ0717.5+3745 and the surrounding filamentary structure (z=0.55), as part of our systematic study of the 12 most distant clusters in the MACS sample. Of 1368 galaxies spectroscopically observed in this field, 563 are identified as cluster members; of those, 203 are classified as emission-line galaxies, 260 as absorption-line galaxies, and 17 as E+A galaxies (defined by $\frac{H_{\delta}+H_{\gamma}}{2}>6$\AA and no detection of [OII] and $H_{\beta}$ in emission). The variation of the fraction of emission- and absorption-line galaxies as a function of local projected galaxy density confirms the well-known morphology-density relation, and becomes flat at projected galaxy densities less than $\sim 20Mpc^{-2}. Interestingly, 16 out of 17 E+A galaxies lie (in projection) within the ram-pressure stripping radius around the cluster core, which we take to be direct evidence of ram-pressure stripping being the primary mechanism that terminates star-formation in the E+A population of galaxy clusters. This conclusion is supported by the rarity of E+A galaxies in the filament which rules out galaxy mergers as the dominant driver of evolution for E+A galaxies in clusters. In addition, we find the 42 e(a) and 27 e(b) member galaxies, i.e., the dusty-starburst and starburst galaxies respectively, to be spread out across almost the entire study area. Their spatial distribution, which shows a strong preference for the filament region, suggests that starbursts are triggered in relatively low-density environments as galaxies are accreted from the field population.