The Kinematics of Thick Disks in Nine External Galaxies

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 Gemini-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 counterrotating 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 in which thick-disk stars are accreted or formed during merger events as opposed to models that form thick disks through gradual thin- disk heating.