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Chain galaxies were originally identified by Cowie, Hu & Songaila (1995) using ground-based images. They are linear objects with several giant clumps along their length. There is often no central red clump, and no exponential profile as in a modern edge-on spiral galaxy. There are also many oval-shaped clumpy galaxies that resemble chains in having the same numbers, magnitudes, and colors for the clumps. More important, the relative numbers of these systems, chains versus clumpy galaxies, is consistent with the chains being edge-on clump clusters (Elmegreen et al. 2004a, Elmegreen & Elmegreen 2005). Thus we have a new morphological type of galaxy, a thoroughly clumpy disk viewed in random orientations, that occurs primarily at high redshift.

Chains and clump clusters are so common that all modern spirals could have gone through this phase at z > 1. Essentially all observed disk systems are very clumpy at z > 2. The comoving space density of chains and clump clusters larger than 10 pixels in the UDF is ~ 4 × 10-3 Mpc-3 for z < 1, decreasing to ~ 1 × 10-3 Mpc-3 out to z ~ 3 or more. For spirals larger than 10 pixels in the UDF, the space density is 4 × 10-3 Mpc-3 for z < 1, but decreasing faster with z than the clumpy types, perhaps, in part, because spirals become too red to see. Considering also that the clumpy phase is probably shorter lived than the spiral phase, the prevalence of clumpy disks at high redshift seems clear.

Most clumps are not a bandshifting artifact of rest-UV normal star formation. Clumpy and spiral types are both present at low redshift. In GOODS, there are four basic types of disk galaxies: density wave spirals and flocculent spirals resembling today's galaxies, clumpy galaxies with a red disk between the clumps, and clumpy galaxies without any evident disk between the clumps. All four types span the same range of redshifts up to z ~ 1. There are clump clusters even at z ~ 0.2. This is such a low redshift that the observed V band in GOODS corresponds to a restframe passband of B band. Local spirals do not look like clump clusters in B band, so the clumpies are intrinsically different.