Galaxy Disks
  The formation and evolution of galactic disks is particularly important for
understanding how galaxies form and evolve, and the cause of the variety in
which they appear to us. Ongoing large surveys, made possible by new
instrumentation at wavelengths from the ultraviolet (GALEX), via optical (HST
and large groundbased telescopes) and infrared (Spitzer) to the radio are
providing much new information about disk galaxies over a wide range of
redshift. Although progress has been made, the dynamics and structure of
stellar disks, including their truncations, are still not well understood. We
do now have plausible estimates of disk mass-to-light ratios, and estimates of
Toomre's $Q$ parameter show that they are just locally stable. Disks are mostly
very flat and sometimes very thin, and have a range in surface brightness from
canonical disks with a central surface brightness of about 21.5 $B$-mag
arcsec$^{-2}$ down to very low surface brightnesses. It appears that galaxy
disks are not maximal, except possibly in the largest systems. Their HI layers
display warps whenever HI can be detected beyond the stellar disk, with
low-level star formation going on out to large radii. Stellar disks display
abundance gradients which flatten at larger radii and sometimes even reverse.
The existence of a well-defined baryonic Tully-Fisher relation hints at an
approximately uniform baryonic to dark matter ratio. Thick disks are common in
disk galaxies and their existence appears unrelated to the presence of a bulge
component; they are old, but their formation is not yet understood. Disk
formation was already advanced at redshifts of $\sim 2$, but at that epoch
disks were not yet quiescent and in full rotational equilibrium. Downsizing is
now well-established. The formation and history of star formation in S0s is
still not fully understood.
