The Physical Origins of The Morphology-Density Relation: Evidence for Gas Stripping from the SDSS

TL;DR

This study links galaxy morphology and environment, showing gas stripping in dense regions leads to quiescent, flatter disk galaxies, supporting slow gas removal as the main driver of the morphology-density relation.

Contribution

It provides a quantitative interpretation of the morphology-density relation, emphasizing gas stripping as the key physical process behind galaxy evolution in dense environments.

Findings

Quiescent fraction increases with dark matter halo mass.

Quiescent galaxies are flatter in dense environments.

Gas removal explains the morphology-density relation.

Abstract

We provide a physical interpretation and explanation of the morphology-density relation for galaxies, drawing on stellar masses, star formation rates, axis ratios and group halo masses from the Sloan Digital Sky Survey. We first re-cast the classical morphology-density relation in more quantitative terms, using low star formation rate (quiescence) as a proxy for early-type morphology and dark matter halo mass from a group catalog as a proxy for environmental density: for galaxies of a given stellar mass the quiescent fraction is found to increase with increasing dark matter halo mass. Our novel result is that - at a given stellar mass - quiescent galaxies are significantly flatter in dense environments, implying a higher fraction of disk galaxies. Supposing that the denser environments differ simply by a higher incidence of quiescent disk galaxies that are structurally similar to star-forming disk galaxies of similar mass, explains simultaneously and quantitatively these quiescence -nvironment and shape-environment relations. Our findings add considerable weight to the slow removal of gas as the main physical driver of the morphology-density relation, at the expense of other explanations.