The similar stellar populations of quiescent spiral and elliptical galaxies

TL;DR

This study compares the stellar populations of quiescent spiral and elliptical galaxies, revealing similarities in age and alpha-element enhancement, with metallicity differences linked to bulge mass rather than morphology.

Contribution

It demonstrates that stellar population properties are primarily governed by bulge mass, not galaxy morphology, challenging traditional distinctions between spiral and elliptical galaxies.

Findings

Older ages and higher metallicities correlate with larger velocity dispersions.

Metallicity in spiral bulges is slightly higher than in ellipticals at fixed velocity dispersion.

Galaxy mass influences metal retention and star formation history.

Abstract

We compare the stellar population properties in the central regions of visually classified non-starforming spiral and elliptical galaxies from Galaxy Zoo and SDSS DR7. The galaxies lie in the redshift range $0.04<z<0.1$ and have stellar masses larger than $logM_*=10.4$. We select only face-on spiral galaxies in order to avoid contamination by light from the disk in the SDSS fiber and enabling the robust visual identification of spiral structure. Overall, we find that galaxies with larger central stellar velocity dispersions, regardless of morphological type, have older ages, higher metallicities, and an increased overabundance of alpha-elements. Age and alpha-enhancement, at fixed velocity dispersion, do not depend on morphological type. The only parameter that, at a given velocity dispersion, correlates with morphological type is metallicity, where the metallicity of the bulges of spiral galaxies is 0.07 dex higher than that of the ellipticals. However, for galaxies with a given total stellar mass, this dependence on morphology disappears. Under the assumption that, for our sample, the velocity dispersion traces the mass of the bulge alone, as opposed to the total mass (bulge+disk) of the galaxy, our results imply that the formation epoch of galaxy and the duration of its star-forming period are linked to the mass of the bulge. The extent to which metals are retained within the galaxy, and not removed as a result of outflows, is determined by the total mass of the galaxy.