The Morphological Diversities among Star‐forming Galaxies at High Redshifts in the Great Observatories Origins Deep Survey

We have used the HST/ACS images to identify 4700 Lyman break galaxies (LBGs)in GOODS. We present the results from a parametric analysis of the 2-D surfacebrightness profiles, for 1333 LBGs at z > 2.5 with rest-frame UV(1600 Angstrom)AB magnitude < -20.5. Based on the Sersic index, n, which measures the profileshape, we find that about 40% of LBGs at z=3 have light profiles close toexponential, and only 30% have the high concentrations seen for spheroids.About 30% of LBGs appear to have multiple cores or disturbed morphologiessuggestive of close pairs or on-going mergers. The fraction of spheroid-like (n> 2.5) LBGs decreases by about 15% from z = 5 to 3. A comparison of LBGs withthe starburst galaxies at z = 1.2, shows that the fraction of spheroid-likeprofiles is about 20% higher among LBGs. The ellipticity distribution for LBGsexhibits a pronounced skew towards high ellipticities (> 0.5), which cannot beexplained by morphologies similar to the local disks and spheroids viewed atrandom orientations. The peak of the distribution evolves toward lowerellipticities, from 0.7 at z = 4 to 0.5 at z = 3. At z = 1.2 the distributionis relatively flat as seen among the present-day galaxies. The dominance ofelongated morphologies among LBGs suggests that in a significant fraction ofthem we may be witnessing star-formation in clumps along gas-rich filaments, orthe earliest gas-rich bars that encompass essentially the entire visiblegalaxy. Similar features are found to be ubiquitous in hydrodynamicalsimulations in which galaxy formation at high redshifts occurs in filamentaryinflows of dynamically cold gas within the dark matter halos, and involves gas-rich mergers.Comment: Accepted for publication in the Astrophysical Journal (10 October, 2006 issue), 15 pages + 12 JPEG figure