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تسجيل مستخدم جديدOn the formation of direct collapse black hole seeds: Impact of gas spin and Lyman Werner flux
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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.
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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 ionis
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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 $rm Z/Z_{odot} leq 10^{-5}$.
Direct collapse within dark matter (DM) halos is a promising path to form supermassive black hole (SMBH) seeds at high redshifts. The outer part of this collapse remains optically thin, and has been studied intensively using numerical simulations. Ho
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adiative transfer has been implemented in the flux-limited diffusion (FLD) approximation. Adiabatic models were run for comparison. We find that (a) the FLD flow forms an irregular central structure and does not exhibit fragmentation, contrary to adiabatic flow which forms a thick disk, driving a pair of spiral shocks, subject to Kelvin-Helmholtz shear instability forming fragments; (b) the growing central core in the FLD flow quickly reaches ~10 Mo and a highly variable luminosity of 10^{38}-10^{39} erg/s, comparable to the Eddington luminosity. It experiences massive recurrent outflows driven by radiation force and thermal pressure gradients, which mix with the accretion flow and transfer the angular momentum outwards; and (c) the interplay between these processes and a massive accretion, results in photosphere at ~10 AU. We conclude that in the FLD model (1) the central object exhibits dynamically insignificant rotation and slower than adiabatic temperature rise with density; (2) does not experience fragmentation leading to star formation, thus promoting the fast track formation of a supermassive black hole (SMBH) seed; (3) inclusion of radiation force leads to outflows, resulting in the mass accumulation within the central 10^{-3} pc, which is ~100 times larger than characteristic scale of star formation. The inclusion of radiative transfer reveals complex early stages of formation and growth of the central structure in the direct collapse scenario of SMBH seed formation.
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,
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