Lick Indices in the Thin and Thick Disks of Edge-On Disk Galaxies

TL;DR

This study measures stellar ages and metallicities in the thin and thick disks of edge-on galaxies, revealing that thick disks are old and the thin disks are younger with radial age gradients, but metallicity differences are not detected.

Contribution

It provides new measurements of stellar populations in galaxy disks using Lick indices, confirming the old age of thick disks and characterizing the age gradients in thin disks.

Findings

Thick disks are old, with ages 4-10 Gyr.

Thin disks are younger and have strong radial age gradients.

No significant metallicity differences detected between thin and thick disks.

Abstract

We have measured Lick index equivalent widths to derive luminosity weighted stellar ages and metallicities for thin and thick disk dominated regions of 9 edge-on disk galaxies with the ARC 3.5 meter telescope at Apache Point Observatory. In all cases, the thick disks are confirmed to be old stellar populations, with typical ages between 4 and 10 Gyr. The thin disks are uniformly younger than the thick disks, and show strong radial age gradients, with the outer regions of the disks being younger than 1 Gyr. We do not detect any significant metallicity differences or alpha-element enhancement in the thick disk stars compared to the thin disk, due to the insensitivity of the Lick indices to these differences at low metallicity. We compare these results to thick disks measured in other systems and to predictions from thick disk formation models.