Galactic structure from faint Stromgren photometry.

Abstract

A 3-color Stromgren photometric survey covering $\approx$1 square degree was conducted and yielded a kinematically unbiased in situ sample of over 1200 thick disk and halo stars to V $\approx$ 18.5. Two follow-up studies were conducted, utilizing Stromgren u-band photometry and blue spectroscopy, and yielded luminosity classes, radial velocities and improved metallicities. Survey stars were found to have a turn-off color of b $-$ y $\approx$ 0.32, similar to the turn-off colors of metal-rich globular clusters. Photometric metallicities for over 500 stars indicate a metal-poor population, with $\langle\lbrack$Fe/H) $\rangle\approx$ $-$1.26, consistent with a mixture of thick disk and halo stars, and inconsistent with the closed box model of chemical evolution. Distances were derived for thick disk turn-off stars and yielded a scaleheight of 1010 $\pm$ 150 pc, with an unknown external error. The small internal scaleheight error indicates that the thick disk is spatially discrete and implies that it was created during a brief time interval under a narrow range of physical conditions. The thick disk is not the overlap of the thin disk and halo populations, nor is it likely that accelerating thin disk stars to thick disk orbits, as discussed by Norris (1987), is feasible. The sudden collapse scenario for the creation of the Galaxy of Eggen et al. (1962) is also inconsistent with the data. If the thick disk resulted from the accretion of dwarf galaxies, such accretion must have been brief and limited to a few events. The energy-regulated galaxy formation model of Burkert et al. (1991) provides a natural scenario for understanding a spatially discrete thick disk.