Origin of Bimodality in Galaxy Properties: Cold and Hot Flows, Clustering and Feedback
Abstract
We address the origin of the robust bi-modality in galaxies about a critical stellar mass ~3x10^10 Msol. Less massive galaxies tend to be ungrouped blue star-forming discs correlated along a fundamental line. More massive galaxies are typically grouped red old-star spheroids on a fundamental plane 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 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 associated with a similar scale. In haloes below a critical shock-heating mass Ms~6x10^11 Msol, discs 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 along the fundamental line. Cold streams penetrating through hot media in >Ms haloes at z>2 lead to massive starbursts in >L* galaxies. At zMs haloes hosting groups, the gas is heated by a virial shock, and being dilute it becomes vulnerable to AGN feedback. This shuts off gas supply and further star formation, preferentially in spheroids formed by mergers in groups. Subsequent passive evolution leads to red & dead massive spheroids starting at z~1. The SFR is high just below Ms, where the feedbacks are weak, leading to a minimum in M/L. Convolved with the clustering scale growth, this explains the observed SFR history. When these processes are incorporated in the modeling of galaxy formation, they should recover the bi-modality features and solve other open puzzles.Keywords
All Related Versions
This publication has 0 references indexed in Scilit: