A relation between characteristic stellar age of galaxies and their intrinsic shape

TL;DR

This study uses large integral field spectroscopic surveys to explore how a galaxy's stellar age correlates with its intrinsic shape and kinematic properties, revealing a strong link between stellar age, rotation, and ellipticity.

Contribution

It is the first to connect spatially-resolved stellar populations with intrinsic galaxy shapes across a large, diverse galaxy sample using IFS data.

Findings

Stellar age correlates strongly with galaxy position in the ($V / \sigma$, _{ m{e}}$) diagram.

Characteristic stellar age follows the intrinsic ellipticity in oblate rotating spheroids.

The age-shape correlation persists across early-type and late-type galaxy classifications.

Abstract

Stellar population and stellar kinematic studies provide unique but complementary insights into how galaxies build-up their stellar mass and angular momentum. A galaxy's mean stellar age reveals when stars were formed, but provides little constraint on how the galaxy's mass was assembled. Resolved stellar dynamics trace the change in angular momentum and orbital distribution of stars due to mergers, but major mergers tend to obscure the effect of earlier interactions. With the rise of large multi-object integral field spectroscopic (IFS) surveys, such as SAMI and MaNGA, and single-object IFS surveys (e.g., ATLAS$^{\rm{3D}}$, CALIFA, MASSIVE), it is now feasible to connect a galaxy's star formation and merger history on the same resolved physical scales, over a large range in galaxy mass, and across the full range of optical morphology and environment. Using the SAMI Galaxy Survey, here we present the first study of spatially-resolved stellar kinematics and global stellar populations in a large IFS galaxy survey. We find a strong correlation of stellar population age with location in the ($V / \sigma$, $\epsilon_{\rm{e}}$) diagram that links the ratio of ordered rotation to random motions in a galaxy to its observed ellipticity. For the large majority of galaxies that are oblate rotating spheroids, we find that characteristic stellar age follows the intrinsic ellipticity of galaxies remarkably well. This trend is still observed when galaxies are separated into early-type and late-type samples.