The Kinematics of Thick Disks in External Galaxies

Abstract

We present kinematic measurements of the thick and thin disks in two edge-on galaxies. We have derived stellar rotation curves at and above the galaxies' midplanes using Ca II triplet features measured with the GMOS spectrograph on Gemini North. In one galaxy, FGC 1415, the kinematics above the plane shows clear rotation that lags that of the midplane by ~20%-50%, similar to the behavior seen in the Milky Way. However, the kinematics of the second galaxy, FGC 227, is quite different. The rotation above the plane is extremely slow, showing 25% of the rotation speed of the stars at the midplane. We decompose the observed rotation curves into a superposition of thick- and thin-disk kinematics, using two-dimensional fits to the galaxy images to determine the fraction of thick-disk stars at each position. We find that the thick disk of FGC 1415 rotates at 30%-40% of the rotation speed of the thin disk. In contrast, the thick disk of FGC 227 is very likely counterrotating if it is rotating at all. These observations are consistent with the velocity dispersion profiles that we measure for each galaxy. The detection of counterrotating thick disks conclusively rules out models in which the thick disk forms either during monolithic collapse or from vertical heating of an earlier thin disk. Instead, the data strongly support models in which the thick disk forms from direct accretion of stars from infalling satellites.