arXiv subjects

Takamitsu L. Tanaka

Publications and source records attributed to Takamitsu L. Tanaka.

12 recordsLinked to original sources

arXiv2019

Titans of the Early Universe: The Prato Statement on the Origin of the First Supermassive Black Holes

In recent years, the discovery of massive quasars at z~7 has provided a striking challenge to our understanding of the origin and growth of supermassive black holes in the early Universe. Mounting observational and theoretical evidence indicates the viability of massive seeds, formed by the collapse of supermassive stars, as a progenitor model for such early, massive accreting black holes. Although considerable progress has been made in our theoretical understanding, many questions remain regarding how (and how often) such objects may form, how they live and die, and how next generation observatories may yield new insight into the origin of these primordial titans. This review focusses on our present understanding of this remarkable formation scenario, based on discussions held at the Monash Prato Centre from November 20--24, 2017, during the workshop "Titans of the Early Universe: The Origin of the First Supermassive Black Holes."

astro-ph.GA↗

arXiv2017

Population III X-Ray Binaries

Understanding of the role of X-rays for driving the thermal evolution of the intergalactic medium (IGM) at high redshifts is one of important questions in astrophysics. High-mass X-ray binaries (HMXBs) in early stellar populations are prime X-ray source; however, their formation efficiency is not well understood. Using $N$-body simulations, we estimate the HMXB formation rate via mutual gravitational interactions of nascent, small groups of the Population~III stars. We find that HMXBs form at a rate of one per $\gtrsim 10^{4}M_{\odot}$ in newly born stars, and that they emit with a power of $\sim 10^{41} {\rm erg}~{\rm s}^{-1}$ in the $2-10$ keV band per star formation rate (SFR). This value is a factor $\sim 10^{2}$ larger than what is observed in star forming galaxies at lower redshifts; the X-ray production from early HMXBs would have been even more copious, if they also formed \textit{in situ} or via migration in protostellar disks. Combining our results with earlier studies suggests that early HMXBs were highly effective at heating the IGM and leaving a strong 21 cm signature. We discuss broader implications of our results, such as the rate of long gamma-ray bursts from Population~III stars and the direct collapse channel for massive black hole formation.

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arXiv2016

Flares in Gamma Ray Bursts: Disc Fragmentation and Evolution

Flaring activity following gamma-ray bursts (GRBs), observed in both long and short GRBs, signals a long-term activity of the central engine. However, its production mechanism has remained elusive. Here we develop a quantitative model of the idea proposed by Perna et al. of a disc whose outer regions fragment due to the onset of gravitational instability. The self-gravitating clumps migrate through the disc and begin to evolve viscously when tidal and shearing torques break them apart. Our model consists of two ingredients: theoretical bolometric flare lightcurves whose shape (width, skewness) is largely insensitive to the model parameters, and a spectral correction to match the bandpass of the available observations, that is calibrated using the observed spectra of the flares. This simple model reproduces, with excellent agreement, the empirical statistical properties of the flares as measured by their width-to-arrival time ratio and skewness (ratio between decay and rise time). We present model fits to the observed lightcurves of two well-monitored flares, of GRB 060418 and of GRB 060904B. To the best of our knowledge, this is the first quantitative model able to reproduce the flare lightcurves and explain their global statistical properties.

astro-ph.HE↗

arXiv2016

Intermediate-mass black holes from Population III remnants in the first galactic nuclei

We report the formation of intermediate-mass black holes (IMBHs) in suites of numerical $N$-body simulations of Population III remnant black holes (BHs) embedded in gas-rich protogalaxies at redshifts $z\gtrsim10$. We model the effects of gas drag on the BHs' orbits, and allow BHs to grow via gas accretion, including a mode of hyper-Eddington accretion in which photon trapping and rapid gas inflow suppress any negative radiative feedback. Most initial BH configurations lead to the formation of one (but never more than one) IMBH in the center of the protogalaxy, reaching a mass of $10^{3-5}\mathrm{M}_{\odot}$ through hyper-Eddington growth. Our results suggest a viable pathway to forming the earliest massive BHs in the centers of early galaxies. We also find that the nuclear IMBH typically captures a stellar-mass BH companion, making these systems observable in gravitational waves as extreme mass-ratio inspirals (EMRIs) with \textit{eLISA}.

astro-ph.GA↗

arXiv2015

The imprint of the cosmic supermassive black hole growth history on the 21 cm background radiation

The redshifted 21 cm transition line of hydrogen tracks the thermal evolution of the neutral intergalactic medium (IGM) at "cosmic dawn," during the emergence of the first luminous astrophysical objects (~100 Myr after the Big Bang) but before these objects ionized the IGM (~400-800 Myr after the Big Bang). Because X-rays, in particular, are likely to be the chief energy courier for heating the IGM, measurements of the 21 cm signature can be used to infer knowledge about the first astrophysical X-ray sources. Using analytic arguments and a numerical population synthesis algorithm, we argue that the progenitors of supermassive black holes (SMBHs) should be the dominant source of hard astrophysical X-rays---and thus the primary driver of IGM heating and the 21 cm signature---at redshifts $z < 20$, if (i) they grow readily from the remnants of Population III stars and (ii) produce X-rays in quantities comparable to what is observed from active galactic nuclei and high-mass X-ray binaries. We show that models satisfying these assumptions dominate over contributions to IGM heating from stellar populations, and cause the 21 cm brightness temperature to rise at $z > 20$. An absence of such a signature in the forthcoming observational data would imply that SMBH formation occurred later (e.g. via so-called direct collapse scenarios), that it was not a common occurrence in early galaxies and protogalaxies, or that it produced far fewer X-rays than empirical trends at lower redshifts, either due to intrinsic dimness (radiative inefficiency) or Compton-thick obscuration close to the source.

astro-ph.CO↗

arXiv2015

Formation, disruption and energy output of Population III X-ray binaries

The first astrophysical objects shaped the cosmic environment by reionizing and heating the intergalactic medium (IGM). In particular, X-rays are very efficient at heating the IGM before it became completely ionized, an effect that can be measured through the 21 cm line of neutral hydrogen. High-mass X-ray binaries (HMXBs), known to be prolific X-ray sources in star-forming galaxies at lower redshifts, are prime candidates for driving the thermal evolution of the IGM at redshifts $z > 20$. Despite their importance, the formation efficiency of HMXBs from the first stellar populations is not well understood---as such, their collective X-ray emission and the subsequent imprint on the 21 cm signature are usually evaluated using free parameters. Using $N$-body simulations, we estimate the rate of HMXB formation via mutual gravitational interactions of nascent, small groups of the first stars (Pop III stars). We run two sets of calculations: one in which stars form in small groups of five in nearly Keplerian initial orbits, and another in which two such groups collide (expected from mergers of host protogalaxies). We find that HMXBs form at a rate of one per $~10^{4}~{\rm M}_{\odot}$ in newly born stars, and that they emit with a power of $\sim 10^{41} ~{\rm erg}~{\rm s} ^{-1}$ in the $2-10$ keV band per solar mass per year of star formation. This value is a factor $\sim 10^{2}$ larger than what is observed in star forming galaxies at lower redshifts; the X-ray production from early HMXBs would have been even more copious, if they also formed $in$ $situ$ or via migration in protostellar disks. Combining our results with earlier studies suggests that early HMXBs were highly effective at heating the IGM and leaving a strong 21 cm signature. We discuss broader implications of our results, such as the rate of long GRBs from PopIII stars and the direct collapse black hole formation.

astro-ph.GA↗

arXiv2015

The suppression of direct collapse black hole formation by soft X-ray irradiation

The origin of supermassive black holes (SMBHs) in galactic nuclei is one of the major unsolved problems in astrophysics. One hypothesis is that they grew from >10^5 M_sun black holes that formed in the `direct collapse' of massive gas clouds that have low concentrations of both metals and molecular hydrogen (H_2). Such clouds could form in the early (z>10) Universe if pre-galactic gas is irradiated by H_2-photodissociating, far-ultraviolet (FUV) light from a nearby star-forming galaxy. In this work, we re-examine the critical FUV flux J_crit that is required to keep H_2 photodissociated and lead to direct collapse. We submit that the same galaxies that putatively supply the extraordinary FUV fluxes required for direct collapse should also produce copious amounts of soft X-rays, which work to offset H_2 photodissociation by increasing the ionization fraction and promoting H_2 formation. Accounting for this effect increases the value of J_crit by a factor of at least 3-10, depending on the brightness temperature of FUV radiation. This enhancement of J_crit suppresses the abundance of potential direct collapse sites at z>10 by several orders of magnitude. Recent studies---without accounting for the soft X-rays from the FUV source galaxies---had already arrived at large values of J_crit that implied that direct collapse may occur too rarely to account for the observed abundance of high-redshift quasars. Our results suggest that J_crit should be even higher than previously estimated, and pose an additional challenge for the direct collapse scenario via strong FUV radiation to explain the high-redshift quasar population.

astro-ph.GA↗

arXiv2014

Driving the Growth of the Earliest Supermassive Black Holes with Major Mergers of Host Galaxies

The formation mechanism of supermassive black holes (SMBHs) in general, and of $\sim 10^9\,{\rm M}_{\odot}$ SMBHs observed as luminous quasars at redshifts $z> 6$ in particular, remains an open fundamental question. The presence of such massive BHs at such early times, when the Universe was less than a billion years old, implies that they grew via either super-Eddington accretion, or nearly uninterrupted gas accretion near the Eddington limit; the latter, at first glance, is at odds with empirical trends at lower redshifts, where quasar episodes associated with rapid BH growth are rare and brief. In this work, I examine whether and to what extent the growth of the $z> 6$ quasar SMBHs can be explained within the standard quasar paradigm, in which major mergers of host galaxies trigger episodes of rapid gas accretion below or near the Eddington limit. Using a suite of Monte Carlo merger tree simulations of the assembly histories of the likely hosts of the $z> 6$ quasars, I investigate (i) their growth and major merger rates out to $z\sim 40$, and (ii) how long the feeding episodes induced by host mergers must last in order to explain the observed $z> 6$ quasar population without super-Eddington accretion. The halo major merger rate scales roughly as $\propto (1+z)^{5/2}$, with quasar hosts typically experiencing $> 10$ major mergers between $15> z > 6$ ($\approx 650\,{\rm Myr}$), compared to $\sim 1$ for typical massive galaxies at $3>z > 0$ ($\approx 11 \,{\rm Gyr}$). An example of a viable sub-Eddington SMBH growth model is one where a host merger triggers feeding for a duration comparable to the halo dynamical time. These findings suggest that the growth mechanisms of the earliest quasar SMBHs need not have been drastically different from their counterparts at lower redshifts.

astro-ph.GA↗

arXiv2014

The formation of massive black holes in z~30 dark matter haloes with large baryonic streaming velocities

The origins of the ~10^9 Msun quasar supermassive black holes (BHs) at redshifts z > 6 remain a theoretical puzzle. One possibility is that they grew from ~10^5 Msun BHs formed in the 'direct collapse' of pristine, atomic-cooling (temperatures T ~ 8000 K; PAC) gas that did not fragment to form ordinary stars due to a lack of molecular hydrogen and metals. We propose that baryonic streaming---the relic relative motion of gas with respect to dark matter from cosmological recombination---provides a natural mechanism for establishing the conditions necessary for direct collapse. This effect delays the formation of the first stars by inhibiting the infall of gas into dark matter haloes; streaming velocities more than twice the root-mean-square value could forestall star formation until halo virial temperatures Tvir ~ 8000 K. The resulting PAC gas can proceed to form massive BHs by any of the mechanisms proposed in the literature to induce direct collapse in the absence of a ultraviolet background. This scenario produces haloes containing PAC gas at a characteristic redshift z ~ 30. It can explain the abundance of the most luminous quasars at z = 6, regardless of whether direct collapse occurs in nearly all or less than 1 per cent of PAC haloes.

astro-ph.CO↗

arXiv2013

Electromagnetic signatures of supermassive black hole binaries resolved by PTAs

Pulsar timing arrays (PTAs) may eventually be able to detect not only the stochastic gravitational-wave (GW) background of SMBH binaries, but also individual, particularly massive binaries whose signals stick out above the background. In this contribution, we discuss the possibility of identifying and studying such `resolved' binaries through their electromagnetic emission. The host galaxies of such binaries are themselves expected to be also very massive and rare, so that out to redshifts z~2 a unique massive galaxy may be identified as the host. At higher redshifts, the PTA error boxes are larger and may contain as many as several hundred massive-galaxy interlopers. In this case, the true counterpart may be identified, if it is accreting gas efficiently, as an active galactic nucleus (AGN) with a peculiar spectrum and variable emission features. Specifically, the binary's tidal torques expel the gas from the inner part of the accretion disk, making it unusually dim in X-ray and UV bands and in broad optical emission lines. The tails of the broad wings of any FeK-alpha emission line may also be `clipped' and missing. The binary's orbital motion, as well as the gas motions it induces, may trigger quasiperiodic variations. These include coherent flux variability, such as luminous, multi-wavelength flares, as well as Doppler shifts of broad emission lines and `see-saw' oscillations in the FeK-alpha line. Additional features, such as evidence for a recent major merger or dual collimated jets, could also corroborate the counterpart. These properties would make resolved PTA sources stand out among AGN with similar overall luminosities and allow their identification.

astro-ph.CO↗

arXiv2013

The effect of baryonic streaming motions on the formation of the first supermassive black holes

Observations of quasars at redshifts z > 6 reveal that 10^9 Msol supermassive black holes (SMBHs) had already formed when the Universe was < 0.9 Gyr old. One hypothesis for the origins of these SMBHs is that they grew from the remnants of the first generation of massive stars, which formed in low-mass (~ 10^5 to 10^6 Msol) dark matter minihaloes at z > 20. This is the regime where baryonic streaming motions--the relative velocities of baryons with respect to dark matter in the early Universe--most strongly inhibit star formation by suppressing gas infall and cooling. We investigate the impact of this effect on the growth of the first SMBHs using a suite of high-fidelity, ellipsoidal-collapse Monte Carlo merger-tree simulations. We find that the suppression of seed BH formation by the streaming motions significantly reduces the number density of the most massive BHs at z > 15, but the residual effect at lower redshifts is essentially negligible. The streaming motions can reduce by a factor of few the number density of the most luminous quasars at z ~ 10-11, where such objects could be detected by the James Webb Space Telescope. We conclude, with minor theoretical caveats, that baryonic streaming motions are unlikely to pose a significant additional obstacle to the formation of the observed high-redshift quasar SMBHs. Nor do they appreciably affect the heating and reionization histories of the Universe or the merger rates of nuclear BHs in the mass and redshift ranges of interest for proposed gravitational-wave detectors.

astro-ph.CO↗

arXiv2013

Recurring flares from supermassive black hole binaries: implications for tidal disruption candidates and OJ 287

I discuss the possibility that accreting supermassive black hole (SMBH) binaries with sub-parsec separations produce periodically recurring luminous outbursts that interrupt periods of relative quiescence. This hypothesis is motivated by two characteristics found generically in simulations of binaries embedded in prograde accretion discs: (i) the formation of a central, low-density cavity around the binary, and (ii) the leakage of gas into this cavity, occurring once per orbit via discrete streams on nearly radial trajectories. The first feature would reduce the emergent optical/UV flux of the system relative to active galactic nuclei powered by single SMBHs, while the second can trigger quasiperiodic fluctuations in luminosity. I argue that the quasiperiodic accretion signature may be much more dramatic than previously thought, because the infalling gas streams can strongly shock-heat via self-collision and tidal compression, thereby enhancing viscous accretion. Any optically thick gas that is circularized about either SMBH can accrete before the next pair of streams is deposited, fueling transient, luminous flares that recur every orbit. Due to the diminished flux in between accretion episodes, such cavity-accretion flares could plausibly be mistaken for the tidal disruptions of stars in quiescent nuclei. The flares could be distinguished from tidal disruption events if their quasiperiodic recurrence is observed, or if they are produced by very massive SMBHs that cannot disrupt solar-type stars. They may be discovered serendipitously in surveys such as LSST or eROSITA. I present a heuristic toy model as a proof of concept for the production of cavity-accretion flares, and generate mock light curves and specta. I also apply the model to the active galaxy OJ 287, whose production of quasiperiodic pairs of optical flares has long fueled speculation that it hosts a SMBH binary.

astro-ph.CO↗