Two Growing Modes and the Morphology-Quiescence Relation in Galaxies

TL;DR

This study identifies two distinct evolutionary pathways in galaxies—inside-out growth and core-building via compaction—that influence their morphology and quiescence, using MaNGA survey data on stellar profiles and metallicity.

Contribution

It provides observational evidence distinguishing two galaxy evolution pathways contributing to the morphology-quiescence relation, linking stellar profiles, metallicity, and AGN activity.

Findings

Galaxies with low central density show inside-out growth characteristics.

Galaxies with high central density exhibit signs of core-building via compaction.

AGN activity peaks in the core-building pathway area of the parameter space.

Abstract

Quiescence in galaxies correlates strongly with the central density/morphology of the stellar distribution. We investigate two possible explanations for this morphology-quiescence relation: 1) the central density results from a dissipative core-building event ("compaction") that feeds an AGN that quenches the galaxy and 2) the central density results from inside-out growth by galaxy-wide star formation that is quenched by processes unrelated to the central density. We aim to distinguish these two scenarios using the MaNGA survey to determine profiles of stellar age, specific star formation rate (sSFR) and gas phase metallicity (O/H) as a function of stellar mass surface density within 1 kpc (Sigma_1kpc) and total stellar mass (M*). We find that gradients in age, sSFR and O/H depend on the galaxy's position on the Sigma_1kpc-M* diagram, suggesting at least two evolutionary pathways. The first pathway consists of galaxies with low Sigma_1kpc for their M* whose centres are old, metal-rich and suppressed in sSFR compared to their outskirts, consistent with the inside-out growth scenario. The second pathway, consistent with a compaction-like core-building scenario, consists of galaxies with higher Sigma_1kpc for their M*, whose centres are younger, enhanced in sSFR and metal-deficient compared to their outskirts. Moreover, the WISE-selected AGN fraction peaks in the same area of the Sigma_1kpc-M* diagram as the core-building pathway. The sSFR profiles of the quiescent population suggest that galaxies on the compaction-like path quench uniformly, while those on the inside-out growing path quench their centres first. Our results imply that both pathways contribute to the morphology-quiescence relation.