Origin of Bimodality in Galaxy Properties: Cold and Hot Flows, Clustering and Feedback

We address the origin of the robust bi-modality observed in galaxy properties about a characteristic stellar mass ∼ 3× 10 10 M ⊙. Less massive galaxies tend to be ungrouped blue star-forming disks, with properties correlated along a " fundamental line". More massive galaxies are typically grouped red old-star spheroids on a " fundamental plane" and hosting AGNs. M/L is at a minimum near the critical mass. Color-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 bi-modality 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 halos below a critical shock-heating mass M shock ∼ 6 × 10 11 M ⊙ , disks are built by cold streams, not heated by a virial shock, yielding efficient early star formation (SFR). It is regulated by supernova and radiative feedbacks into a long sequence of bursts in blue galaxies constrained to the fundamental line. Cold streams penetrating through hot media in > M shock halos at z ≥ 2 lead to massive starbursts in > L * galaxies. At z < 2, in > M shock halos hosting groups, the gas is heated by a virial shock, and being dilute it becomes vulnerable to feedback from energetic sources such as AGNs. This shuts off gas supply and prevents further star formation, preferentially in spheroids formed by mergers in groups. Subsequent passive evolution leads to " red-and-dead" massive spheroids starting at z ∼ 1. The SFR is high just below M shock , where the feedbacks are weak, leading to a minimum in M/L. Convolved with the clustering scale growth, this explains the observed SFR broad peak at z > ∼ 1 and the drop toward z ∼ 0. When these processes are incorporated in the modeling of galaxy formation, they should recover the bi-modality features and solve other open puzzles.