FORMING COMPACT MASSIVE GALAXIES

In this paper we study a key phase in the formation of massive galaxies: the transition of star-forming galaxies into massive (M-stars similar to 10(11)M(circle dot)), compact (r(e) similar to 1 kpc) quiescent galaxies, which takes place from z similar to 3 to z similar to 1.5. We use HST grism redshifts and extensive photometry in all five 3D-HST/CANDELS fields, more than doubling the area used previously for such studies, and combine these data with Keck MOSFIRE and NIRSPEC spectroscopy. We first confirm that a population of massive, compact, star-forming galaxies exists at z greater than or similar to 2, using K-band spectroscopy of 25 of these objects at 2.0 < z < 2.5. They have a median [N II]/H alpha ratio of 0.6, are highly obscured with SFR(tot)/SFR(H alpha) similar to 10, and have a large range of observed line widths. We infer from the kinematics and spatial distribution of H alpha that the galaxies have rotating disks of ionized gas that are a factor of similar to 2 more extended than the stellar distribution. By combining measurements of individual galaxies, we find that the kinematics are consistent with a nearly Keplerian fall-off from V-rot similar to 500 km s(-1) at 1 kpc to V-rot similar to 250 km s(-1) at 7 kpc, and that the total mass out to this radius is dominated by the dense stellar component. Next, we study the size and mass evolution of the progenitors of compact massive galaxies. Even though individual galaxies may have had complex histories with periods of compaction and mergers, we show that the population of progenitors likely followed a simple inside-out growth track in the size-mass plane of Delta log r(e) similar to 0.3 Delta log M-stars. This mode of growth gradually increases the stellar mass within a fixed physical radius, and galaxies quench when they reach a stellar density or velocity dispersion threshold. As shown in other studies, the mode of growth changes after quenching, as dry mergers take the galaxies on a relatively steep track in the size-mass plane.