Environmental Effects on the Stellar Mass Function in a z~3.3 Overdensity of Galaxies in the COSMOS Field

TL;DR

This study compares the stellar mass functions in overdense and field regions at z~3.3, revealing a higher proportion of massive galaxies in overdensities, suggesting accelerated galaxy evolution in dense environments.

Contribution

It provides the first detailed comparison of the stellar mass function in a proto-supercluster at z~3.3, highlighting environmental effects on galaxy growth at early cosmic times.

Findings

Overdense regions show a higher ratio of high-mass to low-mass galaxies.

The high-mass slope of the SMF is steeper in overdense environments.

No significant difference in quiescent galaxy fractions between environments.

Abstract

We present an analysis of the number density of galaxies as a function of stellar mass (i.e., the stellar mass function, SMF) in the COSMOS field at z~3.3, making a comparison between the SMF in overdense environments and the SMF in the coeval field. In particular, this region contains the Elent\'ari proto-supercluster, a system of 6 extended overdensities spanning ~70 cMpc on a side. A clear difference is seen in the high-mass slope of these SMFs, with overdense regions showing an increase in the ratio of high-mass galaxies to low-mass galaxies relative to the field, indicating a more rapid build-up of stellar mass in overdense environments. This result qualitatively agrees with analyses of clusters at z~1, though the differences between protocluster and field SMFs at z~3.3 are smaller. While this is consistent with overdensities enhancing the evolution of their member galaxies, potentially through increased merger rates, whether this enhancement begins in protocluster environments or even earlier in group environments is still unclear. Though the measured fractions of quiescent galaxies between the field and overdense environments do not vary significantly, implying that this stellar mass enhancement is ongoing and any starbursts triggered by merger activity have not yet quenched, we note that spectroscopic observations are biased towards star-forming populations, particularly for low-mass galaxies. If mergers are indeed responsible, high resolution imaging of Elent\'ari and similar structures at these early epochs should then reveal increased merger rates relative to the field. Larger samples of well-characterized overdensities are necessary to draw broader conclusions in these areas.