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In the absence of  H 2   molecules, the primordial gas in early dark matter haloes with virial temperatures just above   T  vir ≳ 10 4  K  cools by collisional excitation of atomic H. Although it cools efficiently, this gas remains relatively hot, at a temperature near   T ∼ 8000 K , and consequently might be able to avoid fragmentation and collapse directly into a supermassive black hole. In order for  H 2   formation and cooling to be strongly suppressed, the gas must be irradiated by a sufficiently intense ultraviolet (UV) flux. We performed a suite of three‐dimensional hydrodynamical adaptive mesh refinement (AMR) simulations of gas collapse in three different protogalactic haloes with   T  vir ≳ 10 4  K , irradiated by a UV flux with various intensities and spectra. We determined the critical specific intensity,   J  crit  21  , required to suppress  H 2   cooling in each of the three haloes. For a hard spectrum representative of metal‐free stars, we find (in units of  10 −21  erg s −1  Hz −1  sr −1  cm −2 ) 10 4  &lt;  J  crit  21  &lt; 10 5  , while for a softer spectrum, which is characteristic of a normal stellar population, and for which  H −   dissociation is important, we find  30 &lt;  J  crit  21  &lt; 300 . These values are a factor of 3–10 lower than previous estimates. We attribute the difference to the higher, more accurate  H 2   collisional dissociation rate we adopted. The reduction in   J  crit  21   exponentially increases the number of rare haloes exposed to supercritical radiation. When  H 2   cooling is suppressed, gas collapse starts with a delay, but it ultimately proceeds more rapidly. The infall velocity is near the increased sound speed, and an object as massive as   M ∼ 10 5  M ⊙   may form at the centre of these haloes, compared to the   M ∼ 10 2  M ⊙   stars forming when  H 2   cooling is efficient.

monthly notices of the royal astronomical society

Supermassive black hole formation by direct collapse: keeping protogalactic gas H 2  free in dark matter haloes with virial temperatures  T   vir   &gt;  rsim   10 4   K