The Radical Transformation of Massive Galaxies Since z 2

While the most massive present-day galaxies are large systems dominated by spheroids of old stars that exhibit little on-going star formation, their progenitors at intermediate redshift are an area of active study. We investigate this issue using one of the largest, most diverse samples of massive galaxies at z=1-3 mapped by the GOODS-NICMOS survey. We find that massive (M*>5×10¹⁰ M_sun) galaxies at z=2-3, when the Universe was 2-3 billion years old, are radically different in terms of rest-frame optical structure, star formation rate (SFR), and black hole activity. (1) As much as 40% of massive galaxies at z=2-3 have ultra-compact rest-frame optical sizes (half-light radius <2 kpc), while less than 1% of massive galaxies at z 0 are that small. On average, massive galaxies at z=2-3 are more compact by a factor of 3-4. Furthermore, unlike their local counterparts, a large population of massive galaxies at z=2-3 have shapes, structural properties (Sersic index n<2), and SFR activity that all favor the presence of a massive disky component over a spheroid. (2) Up to 40% of massive galaxies at z=2-3 host active black holes (AGN), which is at least an order of magnitude higher than at z 0. Furthermore, there is a fascinating correlation between structure, SFR, and AGN activity. Most disky structures have a significant (5σ) 24 μm Spitzer detection, and such disky systems host the highest SFR (53 to 1466 M_sun yr^-1). Most ( 65%) AGN hosts have disky morphologies. Ultra-compact galaxies appear quiescent in terms of AGN activity and SFR. (3) The question of how to transform the massive galaxies present 2-3 Gyr after the Big Bang into modern bulge-dominated E/S0s remains a challenge for the current paradigm of galaxy evolution. We discuss the role of major and minor mergers as well as gas accretion along cosmological filaments.