Bursting and Quenching in Massive Galaxies without Major Mergers or AGNs

We simulate the buildup of galaxies by spherical gas accretion through dark-matter haloes, subject to the development of virial shocks.We find that a uniform cosmological accretion rate turns into a rapidly varying disc buildup rate. The generic sequence of events (Shocked-Accretion Massive Burst & Shutdown: SAMBA) consists of four distinct phases: (a) continuous cold accretion while the halo is below a threshold mass Msh ∼ 10M⊙, (b) tentative quenching of gas supply for ∼ 2Gyr, starting abruptly once the halo is ∼Msh and growing a rapidly expanding shock, (c) a massive burst due to the big crunch of ∼ 10M⊙ gas in ∼ 0.5Gyr, when the accumulated heated gas cools and joins new infalling gas, and (d) a long-term shutdown, enhanced by a temporary shock instability in late SAMBAs, those that quench at z ∼ 2, burst at z∼ 1 and end up quenched in 10M⊙ haloes today. The quenching and bursting occur at all redshifts in galaxies of baryonic mass ∼10M⊙ and involve a substantial fraction of this mass. They arise from rather smooth accretion, or minor mergers, which, unlike major mergers, may leave the disc intact while being built in a rapid pace. The early bursts match observed maximum starbursting discs at z > ∼ 2, predicted to reside in < ∼ 10M⊙ haloes. The late bursts resemble discy LIRGs at z < ∼ 1. On the other hand, the tentative quenching gives rise to a substantial population of ∼10M⊙ galaxies with a strongly suppressed star-formation rate at z∼1-3. The predicted longterm shutdown leads to red & dead galaxies in groups. A complete shutdown in more massive clusters requires an additional quenching mechanism, as may be provided by clumpy accretion. Alternatively, the SAMBA bursts may trigger the AGN activity that couples to the hot gas above Msh and helps the required quenching. The SAMBA phenomenon is yet to be investigated using cosmological simulations.