ABSTRACT
Direct collapse black hole (DCBH) formation with mass ≳105 M⊙ is a promising scenario for the origin of high-redshift supermassive black holes. It has usually been supposed that the DCBH can ...only form in the primordial gas since the metal enrichment enhances the cooling ability and causes the fragmentation into smaller pieces. What actually happens in such an environment, however, has not been explored in detail. Here, we study the impact of the metal enrichment on the clouds, conducting hydrodynamical simulations to follow the cloud evolution in cases with different degree of the metal enrichment Z/Z⊙ = 10−6 to 10−3. Below Z/Z⊙ = 10−6, metallicity has no effect and supermassive stars form along with a small number of low-mass stars. With more metallicity $Z/\mathrm{ Z}_{\odot } \gtrsim5 \times 10^{-6}$, although the dust cooling indeed promotes fragmentation of the cloud core and produces about a few thousand low-mass stars, the accreting flow preferentially feeds the gas to the central massive stars, which grows supermassive as in the primordial case. We term this formation mode as the super competitive accretion, where only the central few stars grow supermassive while a large number of other stars are competing for the gas reservoir. Once the metallicity exceeds 10−3 Z⊙ and metal-line cooling becomes operative, the central star cannot grow supermassive due to lowered accretion rate. Supermassive star formation by the super competitive accretion opens up a new window for seed BHs, which relaxes the condition on metallicity and enhances the seed BH abundance.
The Birth of a Massive First-star Binary Sugimura, Kazuyuki; Matsumoto, Tomoaki; Hosokawa, Takashi ...
Astrophysical journal. Letters,
03/2020, Letnik:
892, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We study the formation of massive Population III binary stars using a newly developed radiation hydrodynamics code with the adaptive mesh refinement and adaptive ray-tracing methods. We follow the ...evolution of a typical primordial star-forming cloud obtained from a cosmological hydrodynamics simulation. Several protostars form as a result of disk fragmentation and grow in mass by the gas accretion, which is finally quenched by the radiation feedback from the protostars. Our code enables us, for the first time, to consider the feedback by both the ionizing and dissociating radiation from the multiple protostars, which is essential for self-consistently determining their final masses. At the final step of the simulation, we observe a very wide ( 104 au) binary stellar system consisting of 60 and 70 M stars. One of the member stars also has two smaller mass (10 M ) companion stars orbiting at 200 and 800 au, making up a mini-triplet system. Our results suggest that massive binary or multiple systems are common among Population III stars.
From the first stars to the first black holes Valiante, Rosa; Schneider, Raffaella; Volonteri, Marta ...
Monthly notices of the Royal Astronomical Society,
04/2016, Letnik:
457, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The growth of the first supermassive black holes (SMBHs) at z > 6 is still a major challenge for theoretical models. If it starts from black hole (BH) remnants of Population III stars (light seeds ...with mass ∼100 M⊙), it requires super-Eddington accretion. An alternative route is to start from heavy seeds formed by the direct collapse of gas on to an ∼105 M⊙ BH. Here we investigate the relative role of light and heavy seeds as BH progenitors of the first SMBHs. We use the cosmological, data constrained semi-analytic model gamete/qsodust to simulate several independent merger histories of z > 6 quasars. Using physically motivated prescriptions to form light and heavy seeds in the progenitor galaxies, we find that the formation of a few heavy seeds (between 3 and 30 in our reference model) enables the Eddington-limited growth of SMBHs at z > 6. This conclusion depends sensitively on the interplay between chemical, radiative and mechanical feedback effects, which easily erase the conditions that allow the suppression of gas cooling in the low-metallicity gas (Z < Z
cr and J
LW > J
cr). We find that heavy seeds cannot form if dust cooling triggers gas fragmentation above a critical dust-to-gas mass ratio (
${\cal D} \ge {\cal D}_{\rm cr}$
). In addition, the relative importance of light and heavy seeds depends on the adopted mass range for light seeds, as this dramatically affects the history of cold gas along the merger tree, by both SN- and AGN-driven winds.
ABSTRACT
We study star cluster formation in a low-metallicity environment using three-dimensional hydrodynamic simulations. Starting from a turbulent cloud core, we follow the formation and growth of ...protostellar systems with different metallicities ranging from 10−6 to 0.1 Z⊙. The cooling induced by dust grains promotes fragmentation at small scales and the formation of low-mass stars with M* ∼ 0.01–0.1 M⊙. While the number of low-mass stars increases with metallicity, when Z/Z⊙ ≳ 10−5, the stellar mass distribution is still top-heavy for Z/Z⊙ ≲ 10−2 compared to the Chabrier initial mass function (IMF). In these cases, star formation begins after the turbulent motion decays and a single massive cloud core monolithically collapses to form a central massive stellar system. The circumstellar disc preferentially feeds the mass to the central massive stars, making the mass distribution top-heavy. When Z/Z⊙ = 0.1, collisions of the turbulent flows promote the onset of the star formation and a highly filamentary structure develops owing to efficient fine-structure line cooling. In this case, the mass supply to the massive stars is limited by the local gas reservoir and the mass is shared among the stars, leading to a Chabrier-like IMF. We conclude that cooling at the scales of the turbulent motion promotes the development of the filamentary structure and works as an important factor leading to the present-day IMF.
Mass-gap Mergers in Active Galactic Nuclei Tagawa, Hiromichi; Kocsis, Bence; Haiman, Zoltán ...
The Astrophysical journal,
02/2021, Letnik:
908, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
The recently discovered gravitational wave sources GW190521 and GW190814 have shown evidence of BH mergers with masses and spins outside of the range expected from isolated stellar ...evolution. These merging objects could have undergone previous mergers. Such hierarchical mergers are predicted to be frequent in active galactic nuclei (AGNs) disks, where binaries form and evolve efficiently by dynamical interactions and gaseous dissipation. Here we compare the properties of these observed events to the theoretical models of mergers in AGN disks, which are obtained by performing one-dimensional
N
-body simulations combined with semi-analytical prescriptions. The high BH masses in GW190521 are consistent with mergers of high-generation (high-g) BHs where the initial progenitor stars had high metallicity, 2g BHs if the original progenitors were metal-poor, or 1g BHs that had gained mass via super-Eddington accretion. Other measured properties related to spin parameters in GW190521 are also consistent with mergers in AGN disks. Furthermore, mergers in the lower mass gap or those with low mass ratio as found in GW190814 and GW190412 are also reproduced by mergers of 2g–1g or 1g–1g objects with significant accretion in AGN disks. Finally, due to gas accretion, the massive neutron star merger reported in GW190425 can be produced in an AGN disk.
ABSTRACT The formation of supermassive stars (SMSs) via rapid mass accretion and their direct collapse into black holes (BHs) is a promising pathway for sowing seeds of supermassive BHs in the early ...universe. We calculate the evolution of rapidly accreting SMSs by solving the stellar structure equations including nuclear burning as well as general relativistic (GR) effects up to the onset of the collapse. We find that such SMSs have a less concentrated structure than a fully convective counterpart, which is often postulated for non-accreting ones. This effect stabilizes the stars against GR instability even above the classical upper mass limit 105 M derived for the fully convective stars. The accreting SMS begins to collapse at the higher mass with the higher accretion rate. The collapse occurs when the nuclear fuel is exhausted only for cases with . With , the star becomes GR unstable during the helium-burning stage at M 2-3.5 × 105 M . In an extreme case with 10 , the star does not collapse until the mass reaches 8.0 × 105 M , where it is still in the hydrogen-burning stage. We expect that BHs with roughly the same mass will be left behind after the collapse in all the cases.
It has been proposed that supermassive black holes (SMBHs) are originated from direct-collapse black holes (DCBHs) that are formed at z ≳ 10 in the primordial gas in the case where H2 cooling is ...suppressed by strong external radiation. In this work, we study the critical specific intensity J
crit required for DCBH formation for various radiation spectral shapes by a series of one-zone calculations of a collapsing primordial-gas cloud. We calculate the critical specific intensity at the Lyman–Werner (LW) bands
$J_{\rm LW,21}^{\rm crit}$
(in units of 10−21 erg s−1 Hz−1 sr−1 cm−2) for realistic spectra of metal-poor galaxies. We find that J
crit is not sensitive to the age or metallicity for the constant star formation galaxies with
$J_{\rm LW,21}^{\rm crit}=1300-1400$
, while J
crit decreases as galaxies become older or more metal-enriched for the instantaneous starburst galaxies. However, for the young (the age < 100 Myr) and/or extremely metal poor (Z < 5 × 10−4 Z⊙) instantaneous starburst galaxies, such dependence is not strong and
$J_{\rm LW,21}^{\rm crit}= 1000-1400$
. We also find that J
crit is solely determined by the ratio between the H− and H2 photodissociation rate coefficients,
$k_{\rm H^-,pd}/k_{\rm H_2,pd}$
, with which we develop a formula to estimate J
crit for a given spectrum. The higher value of J
crit for the realistic spectra than those expected in the literature significantly reduces the estimated DCBH number density n
DCBH. By extrapolating the result of Dijkstra, Ferrara & Mesinger, we obtain n
DCBH ∼ 10−10 cMpc−3 at z = 10, which is roughly consistent with the observed number density of high-redshift SMBHs n
SMBH ∼ 10−9 cMpc−3 at z ∼ 6, considering large uncertainties in the estimation.
Abstract
Discovery of high-redshift (z > 6) supermassive black holes (BHs) may indicate that the rapid (or super-Eddington) gas accretion has aided their quick growth. Here, we study such rapid ...accretion of the primordial gas on to intermediate-mass (102–105 M⊙) BHs under anisotropic radiation feedback. We perform two-dimensional radiation hydrodynamics simulations that solve the flow structure across the Bondi radius, from far outside of the Bondi radius down to a central part that is larger than a circum-BH accretion disc. The radiation from the unresolved circum-BH disc is analytically modelled considering self-shadowing effect. We show that the flow settles into a steady state, where the flow structure consists of two distinct parts: (1) bipolar ionized outflowing regions, where the gas is pushed outward by thermal gas pressure and super-Eddington radiation pressure, and (2) an equatorial neutral inflowing region, where the gas falls towards the central BH without affected by radiation feedback. The resulting accretion rate is much higher than that in the case of isotropic radiation, far exceeding the Eddington-limited rate to reach a value slightly lower than the Bondi one. The opening angle of the equatorial inflowing region is determined by the luminosity and directional dependence of the central radiation. We find that photoevaporation from its surfaces set the critical opening angle of about 10° below which the accretion to the BH is quenched. We suggest that the shadowing effect allows even stellar-remnant BHs to grow rapidly enough to become high-redshift supermassive BHs.
ABSTRACT
We study star cluster formation in various environments with different metallicities and column densities by performing a suite of 3D radiation hydrodynamics simulations. We find that the ...photoionization feedback from massive stars controls the star formation efficiency (SFE) in a star-forming cloud, and its impact sensitively depends on the gas metallicity Z and initial cloud surface density Σ. At Z = 1 Z⊙, SFE increases as a power law from 0.03 at Σ = 10 M⊙ pc−2 to 0.3 at $\Sigma = 300\,\mathrm{M}_{\odot }\, {\rm pc^{-2}}$. In low-metallicity cases $10^{-2}\!-\!10^{-1}\, \mathrm{Z}_{\odot }$, star clusters form from atomic warm gases because the molecule formation time is not short enough with respect to the cooling or dynamical time. In addition, the whole cloud is disrupted more easily by expanding H ii bubbles that have higher temperature owing to less efficient cooling. With smaller dust attenuation, the ionizing radiation feedback from nearby massive stars is stronger and terminate star formation in dense clumps. These effects result in inefficient star formation in low-metallicity environments: the SFE drops by a factor of ∼3 at Z = 10−2 Z⊙ compared to the results for Z = 1 Z⊙, regardless of Σ. Newborn star clusters are also gravitationally less bound. We further develop a new semi-analytical model that can reproduce the simulation results well, particularly the observed dependencies of the SFEs on the cloud surface densities and metallicities.
ABSTRACT
We study star cluster formation at low metallicities of Z/Z⊙ = 10−4–10−1 using three-dimensional hydrodynamics simulations. Particular emphasis is put on how the stellar mass distribution is ...affected by the cosmic microwave background radiation (CMB), which sets the temperature floor to the gas. Starting from the collapse of a turbulent cloud, we follow the formation of a protostellar system resolving ∼au scale. In relatively metal-enriched cases of Z/Z⊙ ≳ 10−2, where the mass function resembles the present-day one in the absence of CMB, high-temperature CMB suppresses cloud fragmentation and reduces the number of low-mass stars, making the mass function more top-heavy than in the cases without CMB heating at z ≳ 10. In lower-metallicity cases with Z/Z⊙ ≲ 10−3, where the gas temperature is higher than the CMB value due to inefficient cooling, the CMB has only a minor impact on the mass distribution, which is top-heavy, regardless of the redshift. In cases either with a low metallicity of Z/Z⊙ ≲ 10−2 or at a high redshift z ≳ 10, the mass spectrum consists of a low-mass Salpeter-like component, peaking at 0.1 M⊙, and a top-heavy component with 10–50 M⊙, with the fraction in the latter increasing with increasing redshift. In galaxies forming at z ≳ 10, the major targets of the future instruments including JWST, CMB heating makes the stellar mass function significantly top-heavy, enhancing the number of supernova explosions by a factor of 1.4 (2.8) at z = 10 (20, respectively) compared to the prediction by Chabrier initial mass function when Z/Z⊙ = 0.1.
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