INSIGHTS ON THE FORMATION, EVOLUTION, AND ACTIVITY OF MASSIVE GALAXIES FROM ULTRACOMPACT AND DISKY GALAXIES AT z = 2–3

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) 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 (re ⩽ 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, re). Dry minor and major mergers may play an important role in this process.