3.1.4. New classes of galaxies
The possibility that many morphologically peculiar high-redshift galaxies may be more-or-less conventional mergers (ie. interactions between established galaxies) should not blind us to the likelihood that many of the observed galaxy forms probably correspond to entirely new classes of objects, or perhaps to the initial merging events forming the first generation of luminous galaxies. For example, it has been pointed out that a fairly large proportion of faint peculiar systems have knotty, linear forms . Several examples are seen in Figure 5. The nature of these ``chain galaxies'' is controversial. Some authors suggest that they may be edge-on spiral or low surface-brightness disk systems . However, I believe the evidence (at least for the most striking chain galaxies) strongly suggests that these deserve to be considered a bona fide new class of object. For example, the colours of the knots in some relatively low-redshift chains are remarkably synchronized - giant complexes of star-formation appear to propagate along the body of these system like a string of fireworks . While intrinsically linear systems are dynamically unstable on timescales ~ 100 Myr, a straight-forward comparison of the internal colours of these galaxies with spectral synthesis models suggests that the unweighted mean age of the starlight in chain galaxies is indeed of order 100 Myr, and thus comparable with the dynamical timescale. The age difference between the youngest and oldest knots in the chain is only around 30-50 Myr. If these properties prove universally true (and one needs to be cautious, as only a few chain galaxies have been examined in detail so far), then chain galaxies certainly seem to be systems with no local analogue.
How many other entirely new classes of peculiar galaxy exist at high redshifts? We will not know until such systems can be distinguished from more straight-forward mergers (perhaps through their dynamical properties), and until an inventory of the different classes of peculiar systems is undertaken over a broad range of redshifts. Until such studies are undertaken it will be difficult to disentangle the physical mechanisms responsible for the myriad spectacular forms of the distant galaxies seen on deep HST images.
Acknowledgments I am grateful to Olivier Le Fèvre and the other organizers of this school for giving me a welcome excuse to spend many pleasurable hours studying the Hubble Atlas. I thank my collaborators Richard Ellis, Jarle Brinchmann, Karl Glazebrook, Andy Fabian, Sidney van den Bergh, Nial Tanvir, and Basilio Santiago for their many contributions to the projects described in this article. I am also grateful to Simon Lilly and the rest of the CFRS team for useful discussions, and for permission to describe results in advance of publication. I also thank Mike Merrifield for interesting discussions on the ages and colours of bulges.