Observations of thick disks in the Hubble Space Telescope Ultra Deep Field

The vertical profiles of chain and spiral galaxies in the Hubble Space Telescope Ultra Deep Field (UDF) are fit to sech(2)(z/z(0)) functions convolved with stellar profiles in order to measure the disk scale heights z(0) in four passbands. The bulge regions of the spiral galaxies are avoided. Photometric redshifts give absolute scales. The rms heights of the giant clumps in these galaxies are also measured. The results indicate that UDF disks are thick, with an average z(0) = 1.0 +/- 0.4 kpc. The ratio of radial exponential scale length to z(0) is similar to 3 +/- 1.5. The scale heights are only 20% larger than the radii of the giant star-forming clumps and a factor of similar to 10 larger than the rms clump deviations around the midplanes. This suggests that the clumps formed from midplane gas and dissolved to make the thick disks. Red-shifted stellar population models suggest ages of similar to 1 Gyr and mass column densities from 4 to 40M(circle dot) pc(-2). The UDF disks look like young versions of modern thick disks. This resemblance is difficult to understand if galaxies grow over time or if subsequent accretion of thin disks gravitationally shrinks the observed thick disks. More likely, high-redshift disks are thick because their mass column densities are low; a velocity dispersion of only 14 km s(-2) reproduces the observed thickness. Modern thick disks require more heating at high redshift. This is possible if the gas that eventually makes the thin disk is in place before the youngest age of a modern thick disk, and if the existing stars are heated during the delivery of this gas.