MORPHOLOGICAL EVOLUTION OF GALAXIES FROM ULTRA-DEEP HUBBLE SPACE TELESCOPE WIDE FIELD CAMERA 3 IMAGING: THE HUBBLE SEQUENCE AT z ～ 2

摘要

We use ultra-deep Hubble Space Telescope Wide Field Camera 3/infrared imaging of the Hubble Ultra Deep Field to investigate the rest-frame optical morphologies of a mass-selected sample of galaxies at z ～ 2. We find a large variety of galaxy morphologies, ranging from large, blue, disk-like galaxies to compact, red, early-type galaxies. We derive rest-frame u～g color profiles for these galaxies and show that most z ～ 2 galaxies in our sample have negative color gradients such that their cores are red. Although these color gradients may partly be caused by radial variations in dust content, they point to the existence of older stellar populations in the centers of z ～ 2 galaxies. This result is consistent with an “inside-out” scenario of galaxy growth. We find that the median color gradient is fairly constant with redshift: (Δ(u-g_(rest))/Δ(log r))_(median) = ～0.47, ～0.33, and ～0.46 for z ～ 2, z ～ 1, and z = 0, respectively. Using structural parameters derived from surface brightness profiles we confirm that at z ～ 2 galaxy morphology correlates well with specific star formation rate. At the same mass, star-forming galaxies have larger effective radii, bluer rest-frame u～g colors, and lower S′ersic indices than quiescent galaxies. These correlations are very similar to those at lower redshift, suggesting that the relations that give rise to the Hubble sequence at z = 0 are already in place for massive galaxies at this early epoch.