INSIGHTS ON THE FORMATION, EVOLUTION, AND ACTIVITY OF MASSIVE GALAXIES FROM ULTRACOMPACT AND DISKY GALAXIES AT z=2-3
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We present our results on the structure and activity of massive galaxies at z = 1-3 using one of the largest (166 with M* 5 1010 M ) and most diverse samples of massive galaxies derived from the GOODS-NICMOS survey: (1) Srsic fits to deep NIC3 F160W images indicate that the rest-frame optical structures of massive galaxies are very different at z = 2-3 compared to z 0. Approximately 40% of massive galaxies are ultracompact (r e 2kpc), compared to less than 1% at z 0. Furthermore, most (65%) systems at z = 2-3 have a low Srsic index n 2, compared to 13% at z 0. We present evidence that the n 2 systems at z = 2-3 likely contain prominent disks, unlike most massive z 0 systems. (2) There is a correlation between structure and star formation rates (SFRs). The majority (85%) of non-active galactic nucleus (AGN) massive galaxies at z = 2-3, with SFR high enough to yield a 5 (30 Jy) 24 m Spitzer detection, have low n 2. Such n 2 systems host the highest SFR. (3) The frequency of AGNs is 40% at z = 2-3. Most (65%) AGN hosts have disky (n 2) morphologies. Ultracompact galaxies appear quiescent in terms of both AGN activity and star formation. (4) Large stellar surface densities imply massive galaxies at z = 2-3 formed via rapid, highly dissipative events at z > 2. The large fraction of n 2 disky systems suggests cold mode accretion complements gas-rich major mergers at z > 2. In order for massive galaxies at z = 2-3 to evolve into present-day massive E/S0s, they need to significantly increase (n, re ). Dry minor and major mergers may play an important role in this process. 2011. The American Astronomical Society. All rights reserved.