Color-color diagrams for the clump and interclump emission in 10 clump-cluster galaxies of the Ultra Deep Field are made from B , V , i , and z images and compared with models to determine redshifts , star formation histories , and galaxy masses .
These galaxies are members of a class dominated by 5 to 10 giant clumps , and having no exponential disk or bulge .
The redshifts are found to be in the range from 1.6 to 3 .
The clump emission is typically 40 % of the total galaxy emission and the luminous clump mass is 19 % of the total galaxy mass .
The clump colors suggest declining star formation over the last \sim 0.3 Gy , while the interclump emission is redder than the clumps , corresponding to a greater age .
The clump luminous masses are typically 6 \times 10 ^ { 8 } M _ { \odot } and their diameters average 1.8 kpc , making their average density \sim 0.2 M _ { \odot } pc ^ { -3 } .
Including the interclump populations , assumed to begin forming at z = 6 , the total galaxy luminous masses average 6.5 \times 10 ^ { 10 } M _ { \odot } and their diameters average 19 kpc to the 2 - \sigma noise level .
The expected galaxy rotation speeds average \sim 150 km s ^ { -1 } if they are uniformly rotating disks .
The ages of the clumps are longer than their internal dynamical times by a factor of \sim 8 , so they are stable star clusters , but the clump densities are only \sim 10 times the limiting tidal densities , so they could be deformed by tidal forces .
This is consistent with the observation that some clumps have tails .
The clumps could form by gravitational instabilities in accreting disk gas and then disperse on a \sim 1 Gy time scale , building up the interclump disk emission , or they could be captured as gas-rich dwarf galaxies , flaring up with star formation at first and then dispersing .
Support for this second possibility comes from the high abundance of nearly identical clumps in the UDF field , smaller than 6 pixels , whose distributions on color-magnitude and color-color plots are the same as the galaxy clumps studied here .
The distribution of axial ratios for the combined population of chain and clump-cluster galaxies in the UDF is compared with models and shown to be consistent with a thick disk geometry .
If these galaxies evolve into today ’ s disk galaxies , then we are observing a stage where accretion and star formation are extremely clumpy and the resulting high velocity dispersions form thick-disks .
Several clump-clusters have disk densities that are much larger than in local disks , however , suggesting an alternate model where they do not survive until today , but get converted into ellipticals by collisions .