2010MNRAS.405.2253C

We present deep K-band adaptive-optics observations of eight very massive (M_*∼ 4x10¹¹M_☉) galaxies at 1 < z < 2 utilizing the Gemini NIRI/Altair Laser Guide System. These systems are selected from the Palomar Observatory Wide-Field Infrared survey, and are amongst the most massive field galaxies at these epochs. The depth and high spatial resolution of our images allow us to explore for the first time the stellar mass surface density distribution of massive distant galaxies from 1 to 15 kpc on an individual galaxy basis, rather than on stacked images. We confirm that some of these massive objects are extremely compact with measured effective radii between 0.1 and 0.2arcsec, giving sizes which are ≲2 kpc, a factor of ∼7 smaller in effective radii than similar mass galaxies today. Examining stellar mass surface densities as a function of fixed physical aperture, we find an overdensity of material within the inner profiles, and an underdensity in the outer profile, within these high-z galaxies compared with similar mass galaxies in the local universe. Consequently, massive galaxies should evolve in a way to decrease the stellar mass density in their inner region and at the same time creating more extensive outer light envelopes. We furthermore show that ∼38±20 per cent of our sample contains evidence for a disturbed outer stellar matter distribution, suggesting that these galaxies are undergoing a recent dynamical episode, such as a merger or accretion event. We calculate that massive galaxies at z < 2 will undergo on the order of 5 of these events, a much higher rate than observed for major mergers, suggesting that these galaxies are growing in size and stellar mass in part through minor mergers during this epoch.