Measuring the vertical age structure of the Galactic disc using asteroseismology and SAGA

TL;DR

This study directly measures the vertical age gradient of the Milky Way disc using asteroseismology of red giants observed by Kepler, revealing a gradient of about 4 Gyr/kpc and insights into the Galaxy's evolutionary history.

Contribution

It introduces a method to accurately determine the vertical age structure of the Galactic disc using seismic ages, accounting for selection effects and providing new insights into Galactic evolution.

Findings

Vertical age gradient of ~4 Gyr/kpc in the Galactic disc.

Old red giants dominate at higher Galactic heights.

A mostly quiescent evolution of the Milky Way since redshift 2.

Abstract

The existence of a vertical age gradient in the Milky Way disc has been indirectly known for long. Here, we measure it directly for the first time with seismic ages, using red giants observed by Kepler. We use Stroemgren photometry to gauge the selection function of asteroseismic targets, and derive colour and magnitude limits where giants with measured oscillations are representative of the underlying population in the field. Limits in the 2MASS system are also derived. We lay out a method to assess and correct for target selection effects independent of Galaxy models. We find that low mass, i.e. old red giants dominate at increasing Galactic heights, whereas closer to the Galactic plane they exhibit a wide range of ages and metallicities. Parametrizing this as a vertical gradient returns approximately 4 Gyr/kpc for the disc we probe, although with a large dispersion of ages at all heights. The ages of stars show a smooth distribution over the last 10 Gyr, consistent with a mostly quiescent evolution for the Milky Way disc since a redshift of about 2. We also find a flat age-metallicity relation for disc stars. Finally, we show how to use secondary clump stars to estimate the present-day intrinsic metallicity spread, and suggest using their number count as a new proxy for tracing the ageing of the disc. This work highlights the power of asteroseismology for Galactic studies; however, we also emphasize the need for better constraints on stellar mass-loss, which is a major source of systematic age uncertainties in red giant stars.