We study the early growth of massive seed black holes (BHs) via accretion in protogalactic nuclei where the stellar bulge component is assembled, performing axisymmetric two-dimensional radiation hydrodynamical simulations. We find that when a seed BH with M • ∼ 105 M ⊙ is embedded in dense metal-poor gas (Z = 0.01 Z ⊙) with a density of ≳ 100 cm−3 and bulge stars with a total mass of M ⋆ ≳ 100 M •, a massive gaseous disk feeds the BH efficiently at rates of ≳ 0.3–1 M ⊙ yr−1, and the BH mass increases nearly tenfold within ∼2 Myr. This rapid accretion phase lasts until a good fraction of the gas bounded within the bulge accretes onto the BH, although the feeding rate is regulated owing to strong outflows driven by ionizing radiation emitted from the accreting BH. The transient growing mode can be triggered for seed BHs formed in massive dark-matter halos with masses of ≳ 109 M ⊙ at z ∼ 15–20 (the virial temperature is T vir ≃ 105 K). The host halos are heavier and rarer than those of typical first galaxies, but are more likely to end up in quasar hosts by z ≃ 6. This mechanism naturally yields a mass ratio of M •/M ⋆ > 0.01 higher than the value seen in the local universe. The existence of such overmassive BHs provides us with a unique opportunity to detect highly accreting seed BHs at z ∼ 15 with AB magnitude of m AB ∼ 26–29 mag at 2 μm (rest frame 10 eV) by the upcoming observations by the James Webb Space Telescope and Nancy Grace Roman Space Telescope.
The super-Eddington accretion onto intermediate seed BHs is a potential formation mode of supermassive black holes exceeding 10 9 M in the early universe. We here investigate how such rapid accretion may occur with finite amounts of heavy elements contained in the gas and dust. In our 1D radiation-hydrodynamics simulations, the radiative transfer is solved for both the direct UV lights emitted by an accretion disk and the diffuse IR lights thermally emitted by dust grains. Our results show that the radiative force by the IR lights causes a strong feedback to regulate the mass accretion. The resulting mean accretion rate is lower with the higher metallicity, and there is the critical metallicity Z ∼ 10 −2 Z , above which the super-Eddington accretion is prevented by the radiation pressure of the IR lights. With this taken into account, we examine if the dusty super-Eddington accretion occurs in young galaxies using a simple model. We show that a sufficient number of galaxies at z 10 can be such potential sites if BHs accrete the cold dense gas with T ∼ 10 2 K, approximately the thermal equilibrium value at Z = 10 −2 Z . We argue that the efficiency of the BH growth via the rapid accretion depends on the metallicity, and that the metallicity slightly lower than 10 −2 Z provides a chance for the most efficient growth.
We investigate the formation history of the stellar disk component in the Milky Way (MW) based on our new chemical evolution model. Our model considers several fundamental baryonic processes, including gas infall, re-accretion of outflowing gas, and radial migration of disk stars. Each of these baryonic processes in the disk evolution is characterized by model parameters, which are determined by fitting to various observational data of the stellar disk in the MW, including the radial dependence of the metallicity distribution function (MDF) of the disk stars, which has recently been derived in the APOGEE survey. We succeeded to obtain the best set of model parameters, which well reproduces the observed radial dependences of the mean, standard deviation, skewness, and kurtosis of the MDFs for the disk stars. We analyze the basic properties of our model results in detail to get new insights into the important baryonic processes in the formation history of the MW. One of the remarkable findings is that outflowing gas, containing much heavy elements, preferentially re-accretes onto the outer disk parts, and this recycling process of metal-enriched gas is a key ingredient to reproduce the observed narrower MDFs at larger radii. Moreover, important implications for the radial dependence of gas infall and the influence of radial migration on the MDFs are also inferred from our model calculation. Thus, the MDF of disk stars is a useful clue for studying the formation history of the MW.
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