Paper detail

On the Origin of the Hubble Sequence: I. Insights on Galaxy Color Migration from Cosmological Simulations

An analysis of more than 3000 galaxies resolved at better than 114 pc/h at z=0.62 in a LAOZI cosmological adaptive mesh refinement hydrodynamic simulation is performed and insights gained on star formation quenching and color migration. The vast majority of red galaxies are found to be within three virial radii of a larger galaxy, at the onset of quenching when the specific star formation rate experiences the sharpest decline to fall below ~10^{-2}-10^{-1}/Gyr (depending on the redshift). We shall thus call this mechanism "environment quenching", which encompasses satellite quenching. Two physical processes are largely responsible: ram-pressure stripping first disconnects the galaxy from the cold gas supply on large scales, followed by a longer period of cold gas starvation taking place in high velocity dispersion environment, during the early part of which the existing dense cold gas in the central region (<10kpc) is consumed by in situ star formation. Quenching is found to be more efficient, but not faster, on average, in denser environment. Throughout this quenching period and the ensuing one in the red sequence galaxies follow nearly vertical tracks in the color-stellar-mass diagram. In contrast, individual galaxies of all masses grow most of their stellar masses in the blue cloud, prior to the onset of quenching, and progressively more massive blue galaxies with already relatively older mean stellar ages continue to enter the red sequence. Consequently, correlations among observables of red galaxies - such as the age-mass relation - are largely inherited from their blue progenitors at the onset of quenching. While the color makeup of the entire galaxy population strongly depends on environment, which is a direct result of environment quenching, physical properties of blue galaxies as a sub-population show little dependence on environment.