  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 spirals 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 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 giant
star-forming clumps and a factor of ~10 larger than the rms clump deviations
around the midplanes. This suggests the clumps formed from midplane gas and
dissolved to make the thick disks. Redshifted stellar population models suggest
ages of ~1 Gy and mass column densities from 4 to 40 Msun 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 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.
