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_* ≥ 5 × 10^(10) M_☉) and most diverse samples of massive galaxies derived from the GOODS-NICMOS survey: (1) Sérsic 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 ≤ 2 kpc), compared to less than 1% at z ~ 0. Furthermore, most (~65%) systems at z = 2-3 have a low Sérsic 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 5σ (30 μJy) 24 μm 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, r_e). Dry minor and major mergers may play an important role in this process.

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