The Kinematics of Thick Disks in Nine External Galaxies

TL;DR

This study measures the kinematics of thin and thick disks in nine edge-on galaxies, revealing diverse behaviors that support formation through mergers rather than gradual heating.

Contribution

It provides the first detailed kinematic comparison of thin and thick disks across multiple external galaxies, highlighting the diversity in thick disk dynamics.

Findings

Higher mass galaxies show no kinematic difference between thin and thick disks.

Lower mass galaxies exhibit a wide range of thick disk behaviors, including counter-rotation.

Results support formation scenarios involving mergers or accretion rather than gradual heating.

Abstract

We present kinematic measurements of thin and thick disk components in a sample of nine edge-on galaxies. We extract stellar and ionized gas rotation curves at and above the galaxies' midplanes using the Ca II triplet absorption features and H-alpha emission lines measured with the GMOS spectrographs on Gemini North and South. For the higher mass galaxies in the sample, we fail to detect differences between the thin and thick disk kinematics. In the lower mass galaxies, there is a wide range of thick disk behavior including thick disks with substantial lag and one counter-rotating thick disk. We compare our rotation curves with expectations from thick disk formation models and conclude that the wide variety of thick disk kinematics favors a formation scenario where thick disk stars are accreted or formed during merger events as opposed to models that form thick disks through gradual thin disk heating.