A mass threshold for galactic gas discs by spin flips

Abstract

We predict, analytically and by simulations, that gas discs tend to survive only in haloes above a threshold mass ∼ 2×10 11 M (stellar mass ∼ 10 9 M ), with only a weak redshift dependence. At lower masses, the disc spins typically flip in less than an orbital time due to mergers associated with a change in the pattern of the feeding cosmicweb streams. This threshold arises from the halo merger rate when accounting for the mass dependence of the ratio of galactic baryons and halo mass. Above the threshold, wet compactions lead to massive central nuggets that allow the longevity of extended clumpy gas rings. Supernova feedback has a major role in disrupting discs below the critical mass, by driving the stellar-to-halo mass ratio that affects the merger rate, by stirring up turbulence and suppressing high-angular-momentum gas supply, and by confining major compactions to the critical mass. Our predictions seem consistent with current observed fractions of gas discs, to be explored by future observations that will resolve galaxies below 10 9 M at high redshifts, e.g. by JWST.

Origin of star-forming rings around massive centres in massive galaxies at z < 4

“…The colour refers to the alternative measure of disciness by shape using R d /H d , showing consistency between the two alternative measures of disciness. In both figures we see a marked transition from non-discs to discs or rings at a threshold mass of M v ≃ 10 11 M ⊙ , with negligible redshift dependence, as predicted analytically in Dekel et al (2020). merger) rate of mass and AM that could damage a disc is primarily determined by the total halo mass, the AM of the existing central galaxy that it is affecting is increasing with its baryonic mass, so the damaging relative change in AM is expected to be larger for lower-mass galaxies and at lower redshifts.…”

“…This is expected to be the case in mergers of high-sigma nodes of the cosmic web, when the pattern of feeding streams drastically changes. Figs 4 and 5 of Dekel et al (2020) demonstrate that the disruption below the critical mass is largely due to merger-driven spin flips in less than an orbital time. The mass threshold is derived by a simple analytic model, contrary to the naive expectation of a redshift threshold based on halo merger rates, where the time between mergers with respect to the halo orbital time is t mer /t orb ∝ (1 + z) −1 (Neistein & Dekel 2008; D e k e le ta l .2013).…”

“…In Dekel et al (2020), we showed that in haloes below a critical virial mass of M v ∼ 10 11 M ⊙ , the merger-driven spin flips are indeed disruptive as they tend to be gas rich and more frequent than the disc/ring orbital frequency. In more massive haloes, the mergers are less frequent, thus possibly allowing the rings/discs to survive for many orbital times.…”

“…The extended rings that form are expected to be fragile. In Dekel et al (2020) we used the simulations and analytic estimates to explore how discs and rings populate the M v -z plane. The disc disruption below a characteristic mass is shown in Fig.…”

“…In parallel to our analytic modelling, we utilize a suite of VELA zoom-in hydro-cosmological simulations, which are described in Appendix A and Table A1 (available in the Supporting Information) and in earlier papers (e.g. Ceverino et al 2014;Zolotov et al 2015;Dekel et al 2019aDekel et al , 2020. Here, we bring only a brief summary of the relevant features of these simulations.…”