Thick Disks of Nearby Galaxies

Deep CCD surface photometry of nearby disk galaxies offers the opportunity to study stellar populations which have not been accessible before because of their extreme faintness compared to the night sky. Although the brighter regions of the thick disks of galaxies were detected in both S0s and spirals using faint photographic surface photometry, the data were not accurate enough to give consistent measures of thick disk parameters. We will discuss thick disks of several nearby galaxies, including NGC 4565 and 891, and compare them with the Milky Way's thick disk. The Milky Way thick disk contains the oldest disk stars in the Galaxy, so thick disks are likely to trace the early stages of disk evolution. The program galaxy thick disks show a range of properties, for example vertical scale height, that can be naturally explained if the theory of thick disk formation via minor mergers is correct. However, all predictions of the current merger models (in particular, flaring in the outer regions) are not confirmed.