Radiative feedback for supermassive star formation in a massive cloud with H2 molecules in an atomic-cooling halo

ABSTRACT Recent three-dimensional cosmological simulations of protogalaxy formation have suggested that supermassive stars (SMSs) can form in gas clouds in which H2 cooling is suppressed by dynamical heating prior to the activation of atomic cooling, but they stopped short of the following growth of a central protostar. Here, we examine whether accretion on the protostellar core in this cloud is sufficiently rapid, in the face of the radiation feedback, to produce an SMS. We perform one-dimensional radiation-hydrodynamical simulations of the hot collapsing cloud with non-equilibrium chemical reactions directly adopting the cloud properties from Wise et al. as an initial condition. We find that the stellar Lyman–Werner (LW) radiation from the SMS dissociates H2 in the inner regions of the gas flow, increasing gas temperature and thermal pressure, and temporarily stopping the accretion. However, this negative feedback ceases when the self-gravity and inward ram pressure force on larger scales push the gas inwards. The central protostar is unable to expand an H ii region due to the high density, and grows to a mass of ${\gtrsim}10^5\, {\rm M}_{\odot }$. Our results suggests the successful formation of SMSs, and resulting massive (${\sim}10^5\, {\rm M}_{\odot }$) remnant black holes in the clouds, but need to be confirmed in two- or three-dimensional simulations.

Direct collapse to supermassive black hole seeds: the critical conditions for suppression of H2 cooling

ABSTRACT Observations of high-redshift quasars imply the presence of supermassive black holes (SMBHs) already at $z$ ∼ 7.5. An appealing and promising pathway to their formation is the direct collapse scenario of a primordial gas in atomic-cooling haloes at $z$ ∼ 10–20, when the $\rm H_{2}$ formation is inhibited by a strong background radiation field, whose intensity exceeds a critical value, Jcrit. To estimate Jcrit, typically, studies have assumed idealized spectra, with a fixed ratio of $\rm H_{2}$ photodissociation rate $k_{\rm H_2}$ to the $\rm H^-$ photodetachment rate $k_{\rm H^-}$. This assumption, however, could be too narrow in scope as the nature of the background radiation field is not known precisely. In this work we argue that the critical condition for suppressing the H2 cooling in the collapsing gas could be described in a more general way by a combination of $k_{\rm H_2}$ and $k_{\rm H^-}$ parameters, without any additional assumptions about the shape of the underlying radiation spectrum. By performing a series of cosmological zoom-in simulations covering a wide range of relevant $k_{\rm H_2}$ and $k_{\rm H^-}$ parameters, we examine the gas flow by following evolution of basic parameters of the accretion flow. We test under what conditions the gas evolution is dominated by $\rm H_{2}$ and/or atomic cooling. We confirm the existence of a critical curve in the $k_{\rm H_2}{\!-\!}k_{\rm H^-}$ plane and provide an analytical fit to it. This curve depends on the conditions in the direct collapse, and reveals domains where the atomic cooling dominates over the molecular cooling. Furthermore, we have considered the effect of $\rm H_{2}$ self-shielding on the critical curve, by adopting three methods for the effective column density approximation in $\rm H_{2}$. We find that the estimate of the characteristic length scale for shielding can be improved by using λJeans25, which is 0.25 times that of the local Jeans length, which is consistent with previous one-zone modelling.

The emergence of the first star-free atomic cooling haloes in the Universe

ABSTRACT Using the Renaissance suite of simulations, we examine the emergence of pristine atomic cooling haloes that are both metal free and star free in the early universe. The absence of metals prevents catastrophic cooling, suppresses fragmentation, and may allow for the formation of massive black hole seeds. Here we report on the abundance of pristine atomic cooling haloes found and on the specific physical conditions that allow for the formation of these direct-collapse-black hole (DCBH) haloes. In total, in our simulations we find that 79 DCBH haloes form before a redshift of 11.6. We find that the formation of pristine atomic haloes is driven by the rapid assembly of the atomic cooling haloes with mergers, both minor and/or major, prior to reaching the atomic cooling limit a requirement. However, the ability of assembling haloes to remain free of (external) metal enrichment is equally important and underlines the necessity of following the transport of metals in such simulations. The candidate DCBH-hosting haloes we find have been exposed to mean Lyman–Werner radiation fields of J21 ∼1 and typically lie at least 10 kpc (physical) from the nearest massive galaxy. The growth rates of the haloes reach values of greater than 107$\rm {M_{\odot }}~$ per unit redshift, leading to significant dynamical heating and the suppression of efficient cooling until the halo crosses the atomic cooling threshold. Finally, we also find five synchronized halo candidates where pairs of pristine atomic cooling haloes emerge that are both spatially and temporally synchronized.

Maximally accreting supermassive stars: a fundamental limit imposed by hydrostatic equilibrium

Context. Major mergers of gas-rich galaxies provide promising conditions for the formation of supermassive black holes (SMBHs; ≳105 M⊙) by direct collapse because they can trigger mass inflows as high as 104 − 105 M⊙ yr−1 on sub-parsec scales. However, the channel of SMBH formation in this case, either dark collapse (direct collapse without prior stellar phase) or supermassive star (SMS; ≳104 M⊙), remains unknown. Aims. Here, we investigate the limit in accretion rate up to which stars can maintain hydrostatic equilibrium. Methods. We compute hydrostatic models of SMSs accreting at 1–1000 M⊙ yr−1, and estimate the departures from equilibrium a posteriori by taking into account the finite speed of sound. Results. We find that stars accreting above the atomic cooling limit (≳10 M⊙ yr−1) can only maintain hydrostatic equilibrium once they are supermassive. In this case, they evolve adiabatically with a hylotropic structure, that is, entropy is locally conserved and scales with the square root of the mass coordinate. Conclusions. Our results imply that stars can only become supermassive by accretion at the rates of atomically cooled haloes (∼0.1 − 10 M⊙ yr−1). Once they are supermassive, larger rates are possible.

Phat ELVIS: The inevitable effect of the Milky Way’s disc on its dark matter subhaloes

ABSTRACT We introduce an extension of the ELVIS project to account for the effects of the Milky Way galaxy on its subhalo population. Our simulation suite, Phat ELVIS, consists of 12 high-resolution cosmological dark matter-only (DMO) zoom simulations of Milky Way-size ΛCDM haloes [Mv = (0.7−2) × 1012 M⊙] along with 12 re-runs with embedded galaxy potentials grown to match the observed Milky Way disc and bulge today. The central galaxy potential destroys subhalos on orbits with small pericentres in every halo, regardless of the ratio of galaxy mass to halo mass. This has several important implications. (1) Most of the Disc runs have no subhaloes larger than Vmax = 4.5 km s−1 within 20 kpc and a significant lack of substructure going back ∼8 Gyr, suggesting that local stream-heating signals from dark substructure will be rare. (2) The pericentre distributions of Milky Way satellites derived from Gaia data are remarkably similar to the pericentre distributions of subhaloes in the Disc runs, while the DMO runs drastically overpredict galaxies with pericentres smaller than 20 kpc. (3) The enhanced destruction produces a tension opposite to that of the classic ‘missing satellites’ problem: in order to account for ultra-faint galaxies known within 30 kpc of the Galaxy, we must populate haloes with Vpeak ≃ 7 km s−1 (M ≃ 3 × 107 M⊙ at infall), well below the atomic cooling limit of $V_\mathrm{peak}\simeq 16 \,{\rm km} \, {\rm s}^{-1}$ (M ≃ 5 × 108M⊙ at infall). (4) If such tiny haloes do host ultra-faint dwarfs, this implies the existence of ∼1000 satellite galaxies within 300 kpc of the Milky Way.