H2 suppression with shocking inflows: testing a pathway for supermassive black hole formation

The presence of quasars at redshifts z > 6 indicates the existence of supermassive black holes (SMBHs) as massive as a few times 10⁹ M_⊙, challenging models for SMBH formation. One pathway is through the direct collapse of gas in T_vir ≳ 10⁴ K haloes; however, this requires the suppression of H₂ cooling to prevent fragmentation. In this paper, we examine a proposed new mechanism for this suppression which relies on cold-mode accretion flows leading to shocks at high densities (n > 10⁴ cm^-3) and temperatures (T > 10⁴ K). In such gas, H₂ is efficiently collisionally dissociated. We use high-resolution numerical simulations to test this idea, demonstrating that such haloes typically have lower temperature progenitors, in which cooling is efficient. Those haloes do show filamentary flows; however, the gas shocks at or near the virial radius (at low densities), thus preventing the proposed collisional mechanism from operating. We do find that if we artificially suppress H₂ formation with a high-UV background, so as to allow gas in the halo centre to enter the high-temperature, high-density `zone of no return', it will remain there even if the UV flux is turned off, collapsing to high density at high temperature. Due to computational limitations, we simulated only three haloes. However, we demonstrate, using Monte Carlo calculations of 10⁶ halo merger histories, that a few rare haloes could assemble rapidly enough to avoid efficient H₂ cooling in all of their progenitor haloes, provided that the UV background exceeds J₂₁ ∼ few at redshifts as high as z ∼ 20.