Insights On The Formation, Evolution, And Activity Of Massive Galaxies From Ultracompact And Disky Galaxies At Z=2-3

Abstract

We present our results on the structure and activity of massive galaxies at z = 1-3 using one of the largest (166 with M-star >= 5 x 10(10) M-circle dot) and most diverse samples of massive galaxies derived from the GOODS-NICMOS survey: (1) Sersic 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 similar to 0. Approximately 40% of massive galaxies are ultracompact (r(e) <= 2 kpc), compared to less than 1% at z similar to 0. Furthermore, most (similar to 65%) systems at z = 2-3 have a low Sersic index n <= 2, compared to similar to 13% at z similar to 0. We present evidence that the n <= 2 systems at z = 2-3 likely contain prominent disks, unlike most massive z similar to 0 systems. (2) There is a correlation between structure and star formation rates (SFRs). The majority (similar to 85%) of non-active galactic nucleus (AGN) massive galaxies at z = 2-3, with SFR high enough to yield a 5 sigma (30 mu Jy) 24 mu m Spitzer detection, have low n <= 2. Such n <= 2 systems host the highest SFR. (3) The frequency of AGNs is similar to 40% at z = 2-3. Most (similar to 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, r(e)). Dry minor and major mergers may play an important role in this process.