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Register a new userImpact of dust cooling on direct collapse black hole formation
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Muhammad Abdul Latif
Publication date
2015
fields
Physics
and research's language is
English
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Observations of quasars at $ z > 6$ suggest the presence of black holes with a few times $rm 10^9 ~M_{odot}$. Numerous models have been proposed to explain their existence including the direct collapse which provides massive seeds of $rm 10^5~M_{odot}$. The isothermal direct collapse requires a strong Lyman-Werner flux to quench $rm H_2$ formation in massive primordial halos. In this study, we explore the impact of trace amounts of metals and dust enrichment. We perform three dimensional cosmological simulations for two halos of $rm > 10^7~M_{odot}$ with $rm Z/Z_{odot}= 10^{-4}-10^{-6}$ illuminated by an intense Lyman Werner flux of $rm J_{21}=10^5$. Our results show that initially the collapse proceeds isothermally with $rm T sim 8000$ K but dust cooling becomes effective at densities of $rm 10^{8}-10^{12} ~cm^{-3}$ and brings the gas temperature down to a few 100-1000 K for $rm Z/Z_{odot} geq 10^{-6}$. No gravitationally bound clumps are found in $rm Z/Z_{odot} leq 10^{-5}$ cases by the end of our simulations in contrast to the case with $rm Z/Z_{odot} = 10^{-4}$. Large inflow rates of $rm geq 0.1~M_{odot}/yr$ are observed for $rm Z/Z_{odot} leq 10^{-5}$ similar to a zero-metallicity case while for $rm Z/Z_{odot} = 10^{-4}$ the inflow rate starts to decline earlier due to the dust cooling and fragmentation. For given large inflow rates a central star of $rm sim 10^4~M_{odot}$ may form for $rm Z/Z_{odot} leq 10^{-5}$.
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Direct collapse models for black hole (BH) formation predict massive ($sim 10^5 M_{odot}$) seeds, which hold great appeal as a means to rapidly grow the observed $sim 10^9 M_{odot}$ quasars by $zgtrsim 7$; however, their formation requires fine-tuned conditions. In this work, we use cosmological zoom simulations to study systematically the impact of requiring: 1) low gas angular momentum, and 2) a minimum incident Lyman Werner (LW) flux radiation in order to form direct-collapse BH seeds. We start with a baseline model (introduced in Bhowmick et. al. 2021) that restricts black hole seed formation (with seed masses of $M_{mathrm{seed}}=1.25times10^{4},1times10^{5} & 8times10^{5}M_{odot}/h$) to occur only in haloes with a minimum total mass ($3000times M_{mathrm{seed}}$) and star forming, metal poor gas mass ($5times M_{mathrm{seed}}$). When seeding is further restricted to halos with low gas spins (i.e. smaller than the minimum value required for the gas disc to be gravitationally stable), the seeding frequency is suppressed by factors of $sim6$ compared to the baseline model regardless of the mass threshold used. In contrast, imposing a minimum LW flux ($>10J_{21}$) disproportionately suppresses seed formation in $lesssim10^9M_{odot}/h$ halos, by factors of $sim100$ compared to the baseline model. Very few BH merger events occur in the models with a LW flux criterion, and because early BH growth is dominated by mergers in our models, this results in only the most massive ($8times10^{5}M_{odot}/h$) seeds being able to grow to the supermassive regime ($gtrsim10^6M_{odot}/h$) by $z=7$. Our results therefore suggest that producing the bulk of the $zgtrsim7$ BH population requires alternate seeding channels, early BH growth dominated by rapid or super-eddington accretion, massive seeding scenarios that do not depend on LW flux, or a combination of these possibilities.
A nearby source of Lyman-Werner (LW) photons is thought to be a central component in dissociating H$_2$ and allowing for the formation of a direct collapse black hole seed. Nearby sources are also expected to produce copious amounts of hydrogen ionising photons and X-ray photons. We study here the feedback effects of the X-ray photons by including a spectrum due to high-mass X-ray binaries on top of a galaxy with a stellar spectrum. We explicitly trace photon packages emerging from the nearby source and track the radiative and chemical effects of the multi-frequency source $(E_{rm photon} = rm{0.76 eV rightarrow 7500 eV}$). We find that X-rays have a strongly negative feedback effect, compared to a stellar only source, when the radiative source is placed at a separation greater than $gtrsim 1 rm kpc$. The X-rays heat the low and medium density gas in the envelope surrounding the collapsing halo suppressing the mass inflow. The result is a smaller enclosed mass compared to the stellar only case. However, for separations of $lesssim 1 rm kpc$, the feedback effects of the X-rays becomes somewhat neutral. The enhanced LW intensity at close separations dissociates more H$_2$ and this gas is heated due to stellar photons alone, the addition of X-rays is then not significant. This distance dependence of X-ray feedback suggests that a Goldilocks zone exists close to a forming galaxy where X-ray photons have a much smaller negative feedback effect and ideal conditions exist for creating massive black hole seeds.
The leading contenders for the seeds of the first quasars are direct collapse black holes (DCBHs) formed during catastrophic baryon collapse in atomically-cooled halos at $z sim$ 20. The discovery of the Ly$alpha$ emitter CR7 at $z =$ 6.6 was initially held to be the first detection of a DCBH, although this interpretation has since been challenged on the grounds of Spitzer IRAC and Very Large Telescope X-Shooter data. Here we determine if radio flux from a DCBH in CR7 could be detected and discriminated from competing sources of radio emission in the halo such as young supernovae and H II regions. We find that a DCBH would emit a flux of 10 - 200 nJy at 1.0 GHz, far greater than the sub-nJy signal expected for young supernovae but on par with continuum emission from star-forming regions. However, radio emission from a DCBH in CR7 could be distinguished from free-free emission from H II regions by its spectral evolution with frequency and could be detected by the Square Kilometer Array in the coming decade.
Direct-collapse black holes (DCBHs) are currently one of the leading contenders for the origins of the first quasars in the universe, over 300 of which have now been found at $z >$ 6. But the birth of a DCBH in an atomically-cooling halo does not by itself guarantee it will become a quasar by $z sim$ 7, the halo must also be located in cold accretion flows or later merge with a series of other gas-rich halos capable of fueling the BHs rapid growth. Here, we present near infrared luminosities for DCBHs born in cold accretion flows in which they are destined to grow to 10$^9$ M$_{odot}$ by $z sim$ 7. Our observables, which are derived from cosmological simulations with radiation hydrodynamics with Enzo, reveal that DCBHs could be found by the James Webb Space Telescope at $z lesssim$ 20 and strongly-lensed DCBHs might be found in future wide-field surveys by Euclid and the Wide-Field Infrared Space Telescope at $z lesssim$ 15.
Observations of high-redshift quasars imply the presence of supermassive black holes 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, $J_{rm crit}$. To estimate $J_{rm crit}$, typically, studies have assumed idealized spectra, with a fixed ratio of $rm H_{2}$ photo-dissociation rate $k_{rm H_2}$ to the $rm H^-$ photo-detachment 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 $rm H_2$ 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. By performing a series of cosmological zoom-in simulations with an encompassing set of $k_{rm H_2}$ and $k_{rm H^-}$, 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 $lambda_{rm Jeans25}$, which is 0.25 times that of the local Jeans length.
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