Observations of Thick Disks in theHubble Space TelescopeUltra Deep Field

The vertical profiles of chain and spiral galaxies in the Hubble SpaceTelescope Ultra Deep Field (UDF) are fit to sech^2(z/z_0) functions convolvedwith stellar profiles in order to measure the disk scale heights z_0 in fourpassbands. The bulge regions of the spirals are avoided. Photometric redshiftsgive absolute scales. The rms heights of the giant clumps in these galaxies arealso measured. The results indicate that UDF disks are thick with an averagez_0 of 1.0\pm0.4 kpc. The ratio of radial exponential scale length to z_0 is\~3\pm1.5. The scale heights are only 20% larger than the radii of the giantstar-forming clumps and a factor of ~10 larger than the rms clump deviationsaround the midplanes. This suggests the clumps formed from midplane gas anddissolved to make the thick disks. Redshifted stellar population models suggestages of ~1 Gy and mass column densities from 4 to 40 Msun pc^{-2}. The UDFdisks look like young versions of modern thick disks. This resemblance isdifficult to understand if galaxies grow over time or if subsequent accretionof thin disks gravitationally shrinks the observed thick disks. More likely,high redshift disks are thick because their mass column densities are low; avelocity dispersion of only 14 km/s reproduces the observed thickness. Modernthick disks require more heating at high redshift. This is possible if the gasthat eventually makes the thin disk is in place before the youngest age of amodern thick disk, and if the existing stars are heated during the delivery ofthis gas.Comment: 27 pages, 18 figures, to appear in ApJ Vol 651, November 1, 200