Highly irregular massive disk galaxies in the local universe are usually mergers or interacting systems. We don't know if this is also true at high redshift. In the GOODS sample, there are clump clusters and chains at low redshift that look the same as those at high redshift in the UDF. But also in GOODS there are many examples of mergers and interactions that look like their local counterparts (Elmegreen & Elmegreen 2006a, Elmegreen et al. 2007b). Thus normal mergers and interactions show up just fine in GOODS, and clump clusters are different. Clump clusters usually have no tidal features, for example, and they do not typically have double red nuclei from formerly separate galaxies.
Mergers are also not required to make a galaxy lopsided. Internal
processes can do that too.
Bournaud et
al. (2008)
observed a lopsided clump
cluster in the UDF with Sinfoni. They found that it has a smooth
rotation profile and metallicity gradient, so it does not look like a
chaotic merger or have a double rotation curve. A simulation of this
system reproduced the lopsided shape very well if the initial disk and
halo were offset from each other a little. This offset seems
reasonable if very young galaxies undergo rapid accretion from a
cosmological inflow; they should often have their disk center-of-mass
at a slightly different position than their halo center-of-mass. The
disk mass in this model was 6 × 1010
M
, with
half of the disk mass in gas.
The final piece of evidence that chains are edge-on clump clusters is that the clumps in chain galaxies are highly confined to the midplanes. Their resolution-corrected rms deviation from the midplane of the chains is less than 100 pc (Elmegreen & Elmegreen 2006b). This requires in situ clump formation, not extra-galactic clump accretion. Also some chains are curved and not straight, and the clumps in them follow the curvature too, without significant deviations (Elmegreen & Elmegreen 2006a). These may be interacting edge-on clumpy galaxies, but still the clumps formed in them and are not separate merger remnants.