THE THICK DISKS OF SPIRAL GALAXIES AS RELICS FROM GAS-RICH, TURBULENT, CLUMPY DISKS AT HIGH REDSHIFT

Abstract

The formation of thick stellar disks in spiral galaxies is studied.\nSimulations of gas-rich young galaxies show formation of internal clumps by\ngravitational instabilities, clump coalescence into a bulge, and disk\nthickening by strong stellar scattering. The bulge and thick disks of modern\ngalaxies may form this way. Simulations of minor mergers make thick disks too,\nbut there is an important difference. Thick disks made by internal processes\nhave a constant scale height with galactocentric radius, but thick disks made\nby mergers flare. The difference arises because in the first case,\nperpendicular forcing and disk-gravity resistance are both proportional to the\ndisk column density, so the resulting scale height is independent of this\ndensity. In the case of mergers, perpendicular forcing is independent of the\ncolumn density and the low density regions get thicker; the resulting flaring\nis inconsistent with observations. Late-stage gas accretion and thin disk\ngrowth are shown to preserve the constant scale heights of thick disks formed\nby internal evolution. These results reinforce the idea that disk galaxies\naccrete most of their mass smoothly and acquire their structure by internal\nprocesses, in particular through turbulent and clumpy phases at high redshift.\n