When did the Hubble sequence appear?: Morphology, color, and number-density evolution of the galaxies in the Hubble deep field north

Using the HST WFPC2/NICMOS archival data of the Hubble Deep Field North, we constructed a nearly complete sample of the M-V < -20 (similar to L* + 1) galaxies to z = 2, and investigate when the Hubble sequence appeared, namely, the evolution of the morphology, colors, and the comoving number density of the sample. Even if taking into account the uncertainty of the photometric redshift technique, the number density of relatively bright bulge-dominated galaxies in the HDF-N decreases significantly at z > 1, and their rcst-frame U - V color distribution is wide-spread over 0.5 < z < 2. On the other hand, while the number density of both disk-dominated and irregular galaxies does not show a significant change at 0.5 < z < 2, their distribution of the rest-frame U - V color alters at z similar to 1.5; there is no relatively red (rest U - V greater than or similar to 0.3) galaxies at z > 1.5, while a significant fraction of these red disk-dominated or irregular galaxies exist at z < 1.5. These results suggest that the significant evolution of the Hubble sequence, which is seen in the present Universe, occurs at 1 < z < 2.