When the Hubble Sequence Appeared ?: 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 construct the nearly complete sample of the Mv <-20 (~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 of the uncertainty of the photometric redshift technique, the number density of relatively bright bulge-dominated galaxies in the HDF-N decrease significantly at z>1, and their rest-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 significant change at z<2, their distribution of the rest-frame U-V color alters at z~1.5: there is no relatively red (rest U-V>0.3) galaxies at z>1.5, while the significant fraction of these red disk-dominated or irregular galaxies exists 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.

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