New constraints on direct collapse black hole formation in the early Universe

Abstract

Direct collapse black holes (DCBH) have been proposed as a solution to the challenge of assembling supermassive black holes by z > 6 to explain the bright quasars observed at this epoch. The formation of a DCBH seed with M BH ∼ 10 4−5 M ⊙ requires a pristine atomiccooling halo to be illuminated by an external radiation field that is sufficiently strong to entirely suppress H 2 cooling in the halo. Many previous studies have attempted to constrain the critical specific intensity that is likely required to suppress H 2 cooling, denoted as J crit . However, these studies have typically assumed that the incident external radiation field can be modeled with a black-body spectrum. Under this assumption, it is possible to derive a unique value for J crit that depends only on the temperature of the black-body. In this study we consider a more realistic spectral energy distribution (SED) for the external source of radiation that depends entirely on its star formation history and age. The rate of destruction of the species responsible for suppressing molecular hydrogen cooling depends on the detailed shape of the SED. Therefore the value of J crit is tied to the shape of the incident SED of the neighbouring galaxy. We fit a parametric form to the rates of destruction of H 2 and H − that permit direct collapse. Owing to this, we find that J crit is not a fixed threshold but can lie anywhere in the range J crit ∼ 0.5-10 3 , depending on the details of the source stellar population.

Enhanced direct collapse due to Lyman α feedback

“…At low densities the gas evolves adiabatically, leading to the scaling R detach ∝ n 3/2 , whereas at higher densities the scaling is better approximated assuming the gas is isothermal, leading to the scaling R detach ∝ n 31/18 . Note that, in all cases, the photodetachment rate rises above the critical rate of ∼ 10 −5 s −1 found in calculations assuming a constant background rate and a weak H 2 -dissociating radiation field by Agarwal et al (2016a). tion, molecular cooling is less effective with higher levels of the background radiation field. However, in all cases shown here the gas is still able to cool to < ∼ 10 3 K when H − detachment is not included in the calculation.…”

“…Omukai et al 2005Omukai et al , 2008Schleicher et al 2010;Agarwal et al 2016a). The model assumes that the density of the primordial gas increases on the free-fall timescale, and the non-equilibrium chemical and thermal evolution of the collapsing gas is calculated.…”

“…We have also updated the collisional dissociation rate of H 2 to that presented in Martin et al (1996), which is now the commonly adopted rate (e.g. Shang et al 2010;Agarwal et al 2016a). The model includes the main cooling processes that are relevant for the direct collapse scenario, which are atomic A&A proofs: manuscript no.…”

“…Sugimura et al 2014;Agarwal et al 2016a;Wolcott-Green et al 2016). These works show that once R detach > ∼ 10 −7 s −1 , the photodissociation rate that is required for direct collapse decreases rapidly.…”

Unveiling the First Black Holes With JWST:Multi-wavelength Spectral Predictions

“…We estimate this abundance by extrapolating the fraction of pristine haloes that are exposed to a critically high LW flux (Dijkstra et al 2014). As a first order estimate, using the peak of the critical flux distribution from Agarwal et al (2016) to estimate the number of viable hosts for DCBH formation and assembly, in the 800 arcmin 2 area of the CANDELS fields which corresponds to a ∼ 9 × 10 −3 Gpc 3 volume, we expect about a hundred potential DCBH formation sites. However, not all these sources will be DCBHs or be in the OBG stage and be detectable by Chandra since only a fraction of them are expected to be active.…”