B. The Nature of Chain Galaxies
Figure 5 shows a montage of resolved color-color diagrams for the three "chain galaxies" (as visually classified by R. Ellis) with known redshifts in the Hubble Deep Field (2). The morphophotometric analysis shown in this figure indicates that these systems are likely to be very young galaxies recovering from their first star-formation episodes. Unlike the case for late-type galaxies exhibiting knots of star-formation superposed on a disk (as shown in Figure 4), there is no evidence for an underlying "old" component in any of these systems. Using the surface brightness detection threshold for the I814 data, we can place an upper limit of <10% for the contribution of old stars to the total baryonic mass of two systems at z < 1.5 systems shown in this figure (3). All components of the chain galaxies lie close to the "pure" starburst track: evidently rather little mixing of young components within the body of the galaxy as occurred. (This mixing would show up as a dispersion along the exponential or constant star formation tracks, as populations with different ages blend together). Intriguingly, the knots of star-formation in the lower-redshift chains appear to be synchronized, both spatially and temporally. Star-formation has been triggered along the body of these system like a string of fireworks. The oldest knots in the z < 1.5 systems appear to have ignited the other knots in sequence along the body of the galaxies. In all cases shown the unweighted mean age of the starlight in the galaxy is 100-200 Myr (comparable with the dynamical timescale of the galaxy), with the youngest and oldest knots in the chain differing in age by around 30-50 Myr. This morphophotometric analysis indicates that these chain galaxies are likely to be stochastically ignited very young galaxies, and not edge-on low-surface brightness spirals, as has been claimed. Although it is important to bear in mind that the three systems presented in this article may not be representative of the class (being taken from an incomplete redshift survey, with strong biases toward strong emission-line systems), it appears that at least some chain galaxies (i.e. all chain galaxies in the redshift sample to date) are protogalactic starburst systems.
Figure 5. Morphophotometric color-color diagrams for the three "chain galaxies" in the public database of HDF redshifts. The right-hand panels show the distribution of pixel colors within the regions of the galaxy defined on the left-hand panels. Also shown on the right are models corresponding to constant star formation, exponential star formation, and an instantaneous starburst. See also the caption for the previous figure.
3 Strong upper limits cannot be placed on a putative old component underlying the "hot dog" Lyman limit system at z = 2.803 shown in Fig. 5, because of strong K-corrections for red light at z > 2. However, depending on , restrictions on such a component may be imposed by the age of the Universe in the rest frame. Back.