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Register a new userBlack holes formed by direct collapse: observational evidences
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I.F. Mirabel
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Binary black holes as the recently detected sources of gravitational waves can be formed from massive stellar binaries in the field or by dynamical interactions in clusters of high stellar density, if the black holes are the remnants of massive stars that collapsed without natal kicks that would disrupt the binary system or eject the black holes from the cluster before binary black hole formation. Here are summarized and discussed the kinematics in three dimensions of space of five Galactic black hole X-ray binaries. For Cygnus X-1 and GRS 1915+105 it is found that the black holes of ~15 and ~10 solar masses in these sources were formed in situ, without energetic kicks. These observations suggest that binary black holes with components of ~10 solar masses may have been prolifically produced in the universe.
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We propose a mechanism of producing a new type of primordial perturbations that collapse to primordial black holes whose mass can be as large as necessary for them to grow to the supermassive black holes observed at high redshifts, without contradicting COBE/FIRAS upper limits on cosmic microwave background (CMB) spectral distortions. In our model, the observable Universe consists of two kinds of many small patches which experienced different expansion histories during inflation. Primordial perturbations large enough to form primordial black holes are realized on patches that experienced more Hubble expansion than the others. By making these patches the minor component, the rarity of supermassive black holes can be explained. On the other hand, most regions of the Universe experienced the standard history and, hence, only have standard almost-scale-invariant adiabatic perturbations confirmed by observations of CMB or large-scale structures of the Universe. Thus, our mechanism can evade the constraint from the nondetection of the CMB distortion set by the COBE/FIRAS measurement. Our model predicts the existence of supermassive black holes even at redshifts much higher than those observed. Hence, our model can be tested by future observations peeking into the higher-redshift Universe.
We explore the possibility to detect the continuum radio signal from direct collapse black holes (DCBHs) by upcoming radio telescopes such as the SKA and ngVLA, assuming that after formation they can launch and sustain powerful jets at the accretion stage. We assume that the high-$z$ DCBHs have similar jet properties as the observed radio-loud AGNs, then use a jet model to predict their radio flux detectability. If the jet power $P_{rm jet}gtrsim10^{42-43}$ erg s$^{-1}$, it can be detectable by SKA/ngVLA, depending on the jet inclination angle. Considering the relation between jet power and black hole mass and spin, generally, jetted DCBHs with mass $gtrsim10^5~M_odot$ can be detected. For a total jetted DCBH number density of $sim2.5times10^{-3}$ Mpc$^{-3}$ at $z=10$, about 100 deg$^{-2}z^{-1}$ DCBHs are expected to be above the detection threshold of SKA1-mid (100 hours integration). If the jet blob emitting most of the radio signal is dense and highly relativistic, then the DCBH would only feebly emit in the SKA-low band, because of self-synchrotron absorption (SSA) and blueshift. Moreover, the free-free absorption in the DCBH envelope may further reduce the signal in the SKA-low band. Thus, combining SKA-low and SKA-mid observations might provide a potential tool to distinguish a DCBH from a normal star-forming galaxy.
We analyze the early growth stage of direct-collapse black holes (DCBHs) with $sim 10^{5} rm M_odot$, which are formed by collapse of supermassive stars in atomic-cooling halos at $z gtrsim 10$. A nuclear accretion disk around a newborn DCBH is gravitationally unstable and fragments into clumps with a few $10 rm M_odot$ at $sim 0.01-0.1 rm pc$ from the center. Such clumps evolve into massive population III stars with a few $10-100 rm M_odot$ via successive gas accretion and a nuclear star cluster is formed. Radiative and mechanical feedback from an inner slim disk and the star cluster will significantly reduce the gas accretion rate onto the DCBH within $sim 10^6 rm yr$. Some of the nuclear stars can be scattered onto the loss cone orbits also within $lesssim 10^6 rm yr$ and tidally disrupted by the central DCBH. The jet luminosity powered by such tidal disruption events can be $L_{rm j} gtrsim 10^{50} rm erg s^{-1}$. The prompt emission will be observed in X-ray bands with a peak duration of $delta t_{rm obs} sim 10^{5-6} (1+z) rm s$ followed by a tail $propto t_{rm obs}^{-5/3}$, which can be detectable by Swift BAT and eROSITA even from $z sim 20$. Follow-up observations of the radio afterglows with, e.g., eVLA and the host halos with JWST could probe the earliest AGN feedback from DCBHs.
Rapid infall of gas in the nuclei of galaxies could lead to the formation of black holes by direct collapse, without first forming stars. Black holes formed in this way would have initial masses of a few solar masses, but would be embedded in massive envelopes that would allow them to grow at a highly super-Eddington rate. Thus, seed black holes as large as 10^3-10^4 solar masses could form very rapidly. I will sketch the basic physics of the direct collapse process and the properties of the accreting envelopes.
Direct-collapse black holes (DCBHs) are currently one of the leading contenders for the origins of the first quasars in the universe, over 300 of which have now been found at $z >$ 6. But the birth of a DCBH in an atomically-cooling halo does not by itself guarantee it will become a quasar by $z sim$ 7, the halo must also be located in cold accretion flows or later merge with a series of other gas-rich halos capable of fueling the BHs rapid growth. Here, we present near infrared luminosities for DCBHs born in cold accretion flows in which they are destined to grow to 10$^9$ M$_{odot}$ by $z sim$ 7. Our observables, which are derived from cosmological simulations with radiation hydrodynamics with Enzo, reveal that DCBHs could be found by the James Webb Space Telescope at $z lesssim$ 20 and strongly-lensed DCBHs might be found in future wide-field surveys by Euclid and the Wide-Field Infrared Space Telescope at $z lesssim$ 15.
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