Galaxy Disks

The disks of disk galaxies contain a substantial fraction of their baryonic matter and angular momentum, and much of the evolutionary activity in these galaxies, such as the formation of stars, spiral arms, bars and rings, and the various forms of secular evolution, takes place in their disks. The formation and evolution of galactic disks are therefore 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 UV (Galaxy Evolution Explorer), via optical (Hubble Space Telescope and large groundbased telescopes) and IR (Spitzer Space Telescope), 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 they 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 that flatten at larger radii and sometimes even reverse. The existence of a well-defined baryonic (stellar + Hi) 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 ∼2, but at that epoch disks were not yet quiescent and in full rotational equilibrium. Downsizing (the gradual reduction with time in the mass of the most actively star-forming galaxies) is now well-established. The formation and history of star formation in S0s are still not fully understood.