Wenxiu Li

Bio: Wenxiu Li is an academic researcher. The author has contributed to research in topics: Star formation & Quasar. The author has an hindex of 1, co-authored 2 publications receiving 1 citations.

Evolution of high-redshift quasar hosts and promotion of massive black hole seed formation

Abstract: High-redshift luminous quasars powered by accreting supermassive black holes (SMBHs) with mass $\gtrsim 10^9 M_\odot$ constrain their formation pathways. We investigate the formation of heavy seeds of SMBHs through gas collapse in the quasar host progenitors, using merger trees to trace the halo growth in highly-biased, overdense regions of the universe. The progenitor halos are likely irradiated by intense H$_2$-photodissociating radiation from nearby star-forming galaxies and heat the interior gas by successive mergers. The kinetic energy of the gas originating from mergers as well as baryonic streaming motion prevents gas collapse and delays prior star formation. With a streaming velocity higher than the root-mean-square value, gas clouds in nearly all $10^4$ realizations of merger trees enter the atomic-cooling stage and begin to collapse isothermally with $T \simeq 8000 K$ via Ly$\alpha$ cooling. The fraction of trees which host isothermal gas collapse is $14\%$ and increases with streaming velocity, while the rest form H$_2$-cooled cores after short isothermal phases. If the collapsing gas is enriched to $Z_{crit}\sim 2\times 10^{-3} Z_\odot$, requiring efficient metal mixing, this fraction could be reduced by additional cooling via metal fine-structure lines. In the massive collapsing gas, the accretion rate onto a newly-born protostar ranges between $3 \times 10^{-3}-5 M_\odot yr^{-1}$, among which a large fraction exceeds the critical rate suppressing stellar radiative feedback. As a result, we expect a distribution of stellar mass (presumably BH mass) ranging from several hundred to above $10^5 M_\odot$, potentially forming massive BH binary mergers and yielding gravitational wave events.

Evolution of High-redshift Quasar Hosts and Promotion of Massive Black Hole Seed Formation

Abstract: High-redshift luminous quasars powered by accreting supermassive black holes (SMBHs) with mass $\gtrsim 10^9 M_\odot$ constrain their formation pathways. We investigate the formation of heavy seeds of SMBHs through gas collapse in the quasar host progenitors, using merger trees to trace the halo growth in highly-biased, overdense regions of the universe. The progenitor halos are likely irradiated by intense H$_2$-photodissociating radiation from nearby star-forming galaxies and heat the interior gas by successive mergers. The kinetic energy of the gas originating from mergers as well as baryonic streaming motion prevents gas collapse and delays prior star formation. With a streaming velocity higher than the root-mean-square value, gas clouds in nearly all $10^4$ realizations of merger trees enter the atomic-cooling stage and begin to collapse isothermally with $T \simeq 8000 K$ via Ly$\alpha$ cooling. The fraction of trees which host isothermal gas collapse is $14\%$ and increases with streaming velocity, while the rest form H$_2$-cooled cores after short isothermal phases. If the collapsing gas is enriched to $Z_{crit}\sim 2\times 10^{-3} Z_\odot$, requiring efficient metal mixing, this fraction could be reduced by additional cooling via metal fine-structure lines. In the massive collapsing gas, the accretion rate onto a newly-born protostar ranges between $3 \times 10^{-3}-5 M_\odot yr^{-1}$, among which a large fraction exceeds the critical rate suppressing stellar radiative feedback. As a result, we expect a distribution of stellar mass (presumably BH mass) ranging from several hundred to above $10^5 M_\odot$, potentially forming massive BH binary mergers and yielding gravitational wave events.

Rapid growth of seed black holes during early bulge formation

Abstract: 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 $M_\bullet \sim 10^5~M_\odot$ is embedded in dense metal-poor gas ($Z=0.01~Z_\odot$) with a density of $\gtrsim 100~{\rm cm}^{-3}$ and bulge stars with a total mass of $M_\star \gtrsim 100~M_\bullet$, a massive gaseous disk feeds the BH efficiently at rates of $\gtrsim 0.3-1~M_\odot~{\rm yr}^{-1}$ and the BH mass increases nearly tenfold within $\sim 2$ 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 $\gtrsim 10^9~M_\odot$ at $z\sim 15-20$ (the virial temperature is $T_{\rm vir}\simeq 10^5~{\rm K}$). The host halos are heavier and rarer than those of typical first galaxies, but are more likely to end up in quasar hosts by $z\simeq 6$. This mechanism naturally yields a mass ratio of $M_\bullet/M_\star >0.01$ higher than the value seen in the local universe and the existence of such overmassive BHs provides us a unique opportunity of detecting highly accreting seed BHs at $z\sim 15$ with AB magnitude of $m_{\rm AB} \sim26 - 29$ mag at $2~\mu{\rm m}$ (rest-frame 10 eV) by the upcoming observations by the James Webb Space Telescope and Nancy Grace Roman Space Telescope.

Rapid Growth of Seed Black Holes during Early Bulge Formation

Abstract: 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 $M_\bullet \sim 10^5~M_\odot$ is embedded in dense metal-poor gas ($Z=0.01~Z_\odot$) with a density of $\gtrsim 100~{\rm cm}^{-3}$ and bulge stars with a total mass of $M_\star \gtrsim 100~M_\bullet$, a massive gaseous disk feeds the BH efficiently at rates of $\gtrsim 0.3-1~M_\odot~{\rm yr}^{-1}$ and the BH mass increases nearly tenfold within $\sim 2$ 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 $\gtrsim 10^9~M_\odot$ at $z\sim 15-20$ (the virial temperature is $T_{\rm vir}\simeq 10^5~{\rm K}$). The host halos are heavier and rarer than those of typical first galaxies, but are more likely to end up in quasar hosts by $z\simeq 6$. This mechanism naturally yields a mass ratio of $M_\bullet/M_\star >0.01$ higher than the value seen in the local universe and the existence of such overmassive BHs provides us a unique opportunity of detecting highly accreting seed BHs at $z\sim 15$ with AB magnitude of $m_{\rm AB} \sim26 - 29$ mag at $2~\mu{\rm m}$ (rest-frame 10 eV) by the upcoming observations by the James Webb Space Telescope and Nancy Grace Roman Space Telescope.

Long-term Evolution of Supercritical Black Hole Accretion with Outflows: A Subgrid Feedback Model for Cosmological Simulations

Abstract: We study the long-term evolution of the global structure of axisymmetric accretion flows onto a black hole (BH) at rates substantially higher than the Eddington value ( ṀEdd ), performing 2D hydrodynamical simulations with and without radiative diffusion. In the high-accretion optically thick limit, where the radiation energy is efficiently trapped within the inflow, the accretion flow becomes adiabatic and comprises turbulent gas in the equatorial region and strong bipolar outflows. As a result, the mass inflow rate decreases toward the center as Ṁin∝rp with p ∼ 0.5–0.7 and a small fraction of the inflowing gas feeds the nuclear BH. Thus, super-Eddington accretion is sustained only when a larger amount of gas is supplied from larger radii at ≳100–1000 ṀEdd . The global structure of the flow settles down to a quasi-steady state in millions of the orbital timescale at the BH event horizon, which is ≳10–100 times longer than that addressed in previous (magneto-)RHD simulation studies. Energy transport via radiative diffusion accelerates the outflow near the poles in the inner region but does not change the overall properties of the accretion flow compared to the cases without diffusion. Based on our simulation results, we provide a mechanical feedback model for super-Eddington accreting BHs. This can be applied as a subgrid model in large-scale cosmological simulations that do not sufficiently resolve galactic nuclei, and to the formation of the heaviest gravitational-wave sources via accretion in dense environments.

Probing the z ≳ 6 quasars in a universe with IllustrisTNG physics: Impact of gas-based black hole seeding models

Abstract: We explore implications of a range of black hole (BH) seeding prescriptions on the formation of the brightest z ≳ 6 quasars in cosmological hydrodynamic simulations. The underlying galaxy formation model is the same as in the IllustrisTNG simulations. Using constrained initial conditions, we study the growth of BHs in rare overdense regions (forming ≳ 1012 M⊙/h haloes by z = 7) using a (9 Mpc/h)3 simulated volume. BH growth is maximal within haloes that are compact and have a low tidal field. For these haloes, we consider an array of gas-based seeding prescriptions wherein Mseed = 104 − 106 M⊙/h seeds are inserted in haloes above critical thresholds for halo mass and dense, metal-poor gas mass (defined as $\tilde{M}_{\mathrm{h}}$ and $\tilde{M}_{\mathrm{sf,mp}}$, respectively, in units of Mseed). We find that a seed model with $\tilde{M}_{\mathrm{sf,mp}}=5$ and $\tilde{M}_{\mathrm{h}}=3000$ successfully produces a z ∼ 6 quasar with ∼109 M⊙ mass and ∼1047 erg s−1 luminosity. BH mergers play a crucial role at z ≳ 9, causing an early boost in BH mass at a time when accretion-driven BH growth is negligible. With more stringent seeding conditions ( e.g. $\tilde{M}_{\mathrm{sf,mp}}=1000$), the relative paucity of BH seeds results in a much lower merger rate. In this case, z ≳ 6 quasars can only be formed if we enhance the maximum allowed BH accretion rates (by factors ≳ 10) compared to the accretion model used in IllustrisTNG. This can be achieved either by allowing for super-Eddington accretion, or by reducing the radiative efficiency. Our results demonstrate that progenitors of z ∼ 6 quasars have distinct BH merger histories for different seeding models, which will be distinguishable with LISA observations.

Supercritical Growth Pathway to Overmassive Black Holes at Cosmic Dawn: Coevolution with Massive Quasar Hosts

Abstract: Observations of the most luminous quasars at high redshifts (z > 6) have revealed that the largest supermassive black holes (SMBHs) at those epochs tend to be substantially overmassive relative to their host galaxies compared to the local relations, suggesting they experienced rapid early growth phases. We propose an assembly model for the SMBHs that end up in rare massive ∼1012 M ⊙ host halos at z ∼ 6–7, applying a kinetic feedback prescription for BHs accreting above the Eddington rate, provided by radiation hydrodynamic simulations for the long-term evolution of the accretion-flow structure. The large inflow rates into these halos during their assembly enable the formation of >109 M ⊙ SMBHs by z ∼ 6, even starting from stellar-mass seeds at z ∼ 30, and even in the presence of outflows that reduce the BH feeding rate, especially at early times. This mechanism also naturally yields a high BH-to-galaxy mass ratio of >0.01 before the SMBH mass reaches M BH > 109 M ⊙ by z ∼ 6. These fast-growing SMBH progenitors are bright enough to be detected by upcoming observations with the James Webb Space Telescope over a wide range of redshift (7 < z < 15), regardless of how they were seeded.