Supermassive black holes (SMBHs) with the order of millions to billions of solar masses (Msun) are among the most interesting objects in the universe. They ubiquitously exist at the centers of galaxies and are fed with gas. A large amount of energy release associated with the BH feeding affects star formation on galactic scales, and prevents gas supply onto the BH itself. The feedback process and self-regulating nature of BH growth lead to open questions regarding (1) the coevolution of SMBHs with their host galaxies and (2) SMBH formation — how are monster SMBHs produced in the early universe? In order to tackle the challenging puzzles in astrophysics, I have studied a variety of topics including the physics of BH accretion, formation of their seeds, and observations of gravitational waves.
Rapid BH growth
As a BH is fed with gas at higher rates, radiation produced from the accreting gas becomes intense. When the radiation force finally exceeds the gravity of the BH, the BH growth rate would be limited to the Eddington rate. We explored the possibility of BH growth via accretion exceeding the standard Eddington limit, performing 1D/2D radiation hydrodynamical simulations. We found that a global and steady solution can be realized when a large amount of gas is supplied at a rate higher than 500 times the Eddington rate. In the solution, emergent radiation is trapped within the accretion flow due to electron scattering and does not affect the gas dynamics at larger scales. The BH can be fed by cold inflowing gas with high ram pressure dominating the radiation force caused by a nuclear BH accretion disk. As a result, the BH can rapidly grow to 10^6 Msun when the BH is embedded at the center of a high-redshift protogalaxy. This process can explain rapid formation of SMBHs observed in the early universe.
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Left: Simulated gas density distribution in the outer region (Takeo +KI et al. 2018). While strong radiation collimated to the poles produces a cavity, rapid accretion through the through the equator is not suppressed. Right: Accretion rate history for different BH masses and densities of ambient gas. For higher BH masses and/or higher density surrounding the BH, steady solutions for hyper-Eddington accretion are realized (KI, Haiman & Ostriker 2016, Sakurai, KI & Haiman 2016, Sugimura +KI et al. 2018).
BH seed formation
An interesting pathway to SMBH formation is to start with massive BH seeds with 10^5 Msun. The seeds can be formed from H2-free pristine gas in protogalaxies because radiative cooling by H2 likely fragment the gas could into small clumps. We have proposed a scenario of massive seed formation via strong shocks caused by colliding inflows in galaxy formation. In the dense and hot shocked gas, H2 is collisionally dissociated even without strong FUV radiation that is generally required for massive seed formation. Next, with 3D hydrodynamical simulations including all relevant cooling/chemical processes, we have explored gravitational collapse of such a massive, H2-free cloud. We found that the end result is a single protostar without major episodes of fragmentation. The protostar can grow via gas accretion unimpeded by radiative feedback or mass loss, evolve to a supermas- sive star, and collapse directly into a massive BH (direct-collapse BH; DCBH). Subsequently, the seed BH is fed stably through a self-gravitating disk and is likely to disrupt stars formed in the disk by its tidal force. The disruption events will be detectable by future multi-wavelength observations.
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Left: Density distribution of an H2-free, massive cloud for four spatial scales: from top-left, clockwise: the large-scale gas distribution (KI, Tasker & Omukai 2014). At the center of the collapsing cloud, a single protostar forms without major episodes of fragmentation. The star rapidly grows via accretion of the surrounding material, and forms a massive seed BH with 10^6 Msun. Right: The number density of massive pristine DM halos where PopIII galaxies hosting massive seed BHs (KI, Li & Haiman 2018). Orange curves show the minimum number densities required for a PopIII galaxy to be detected by JWST within three different fields of view.
Gravitational waves from BHs
The direct detections of GWs by Advanced LIGO have opened a new era in astronomy, and the discovery of an unexpected population of massive binary BHs (BBHs) surprised us. We have discussed binary evolution of massive Population III (PopIII) stars into BBHs. Intriguingly, their typical BH masses of 30 + 30 Msun predicted by our PopIII model in 2014 was coincident with those detected by LIGO. Our results can explain the natures of GWs observed by LIGO/Virgo, in terms of the merger event rate, BH mass and spin distribution. A majority of the first binary BHs form and merge in the early universe. We have suggested that such individually unresolved GWs develop a GW background, which would be detectable by LIGO/Virgo in five years. More interestingly, the BH population producing the GW background will also be probed by the observations of the cosmic microwave background, on which stellar activities of the first stars are directly imprinted.
We also proposed a method to explore the formation channels of BBHs, using phase drifts in the GW inspiral waveform of the merging BBH caused by its center-of-mass acceleration. The acceleration strongly depends on the location where BBHs form within galaxies. Therefore, in particular, BBHs formed in dense nuclear star clusters or near an SMBH would suffer strong acceleration in the deep gravitational potential and produce large phase drifts measurable by LISA. The host galaxies of the coalescing BBHs in these scenarios can also be uniquely identified in the LISA error volume, even without electromagnetic counterparts. Therefore, multi-frequency GW observations with LIGO and LISA will enable us to test the formation channels of massive stellar binary BHs.
GWs in the nano-hertz band are great tools for understanding cosmological evolution of SMBHs in galactic nuclei. We consider ultra-luminous infrared galaxies (ULIRGs), which are good tracers of merging galaxies containing SMBHs, as sources of a stochastic GW background (GWB). Adopting a well-established observation sample of ULIRGs, we study the properties of the GWB due to coalescing binary SMBHs driven by merging ULIRGs. Even with a rare population such as high-z ULIRGs associated with a single gas-rich merger unlike multiple dry mergers at low redshift, we find a tension with the upper limits from Pulsar Timing Array (PTA) experiments.
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Left: Binary evolution pathway of PopIII stars into coalescing binary BHs that can be detectable by LIGO/Virgo (KI et al. 2017). Right: Mass function of PopIII binary BHs (Kinugawa +KI et al. 2014). Their typical masses are 30+30Msun was coincident with those detected by LIGO/Virgo.
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