Galaxy bimodality due to cold flows and shock heating

Abstract

We address the origin of the robust bimodality observed in galaxy properties about a characteristic stellar mass ∼3 × 1010 M⊙. Less massive galaxies tend to be ungrouped blue star forming discs, while more massive galaxies are typically grouped red old-star spheroids. Colour–magnitude data show a gap between the red and blue sequences, extremely red luminous galaxies already at z∼ 1, a truncation of today's blue sequence above L*, and massive starbursts at z∼ 2–4. We propose that these features are driven by the thermal properties of the inflowing gas and their interplay with the clustering and feedback processes, all functions of the dark matter halo mass and associated with a similar characteristic scale. In haloes below a critical shock-heating mass Mshock≲ 1012 M⊙, discs are built by cold streams, not heated by a virial shock, yielding efficient early star formation. It is regulated by supernova feedback into a long sequence of bursts in blue galaxies constrained to a ‘fundamental line’. Cold streams penetrating through hot media in M≥Mshock haloes preferentially at z≥ 2 lead to massive starbursts in L > L* galaxies. At z < 2, in M > Mshock haloes hosting groups, the gas is heated by a virial shock, and being dilute it becomes vulnerable to feedback from energetic sources such as active galactic nuclei. This shuts off gas supply and prevents further star formation, leading by passive evolution to ‘red-and-dead’ massive spheroids starting at z∼ 1. A minimum in feedback efficiency near Mshock explains the observed minimum in M/L and the qualitative features of the star formation history. The cold flows provide a hint for solving the angular momentum problem. When these processes are incorporated in simulations they recover the main bimodality features and solve other open puzzles.