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.Comment: 24 pages, 5 figures, invited submission to a focus issue on galactic centers in Classical and Quantum Gravit

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.Comment: 15 pages, 5 figures. Accepted for publication in MNRA

Theory of the proximity effect in junctions with unconventional superconductors

We present a general theory of the proximity effect in junctions between diffusive normal metals (DN) and unconventional superconductors in the framework of the quasiclassical Green's function formalism. Various possible symmetry classes in a superconductor are considered which are consistent with the Pauli principle: even-frequency spin-singlet even-parity (ESE) state, even-frequency spin-triplet odd-parity (ETO) state, odd-frequency spin-triplet even-parity (OTE) state and odd-frequency spin-singlet odd-parity (OSO) state. For each of the above four cases symmetry and spectral properties of the induced pair amplitude in the DN are determined. It is shown that the pair amplitude in a DN belongs respectively to an ESE, OTE, OTE and ESE pairing state.Comment: 5 pages with one figur

Hydrologic activity during late Noachian and Early Hesperian downwarping of Borealis Basin, Mars

Pronounced global volcanism as well as fracturing and erosion along the highland/lowland boundary (HLB) during the Late Noachian (LN) and Early Hesperian (EH) led McGill and Dimitriou to conclude that the Borealis basin formed tectonically during this period. This scenario provides a basis for interpretation of the initiation and mode of formation of erosional and collapse features along the HLB. The interpretation, in turn, is integral to hypotheses regarding the development of ancient lakes (or an ocean) and their impact on the climate history of Mars. Hydrologic features of Mars are discussed along with their implications for paleolakes and climate history

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.Comment: 18 pages, 5 figures, accepted for publication in a focus issue on PTAs in Classical and Quantum Gravit

Many-body spin Berry phases emerging from the π\pi-flux state: antiferromagnetic/valence-bond-solid competition

We uncover new topology-related features of the $\pi$-flux saddle-point solution of the $D$=2+1 Heisenberg antiferromagnet. We note that symmetries of the spinons sustain a built-in competition between antiferromagnetic (AF) and valence-bond-solid (VBS) orders, the two tendencies central to recent developments on quantum criticality. An effective theory containing an analogue of the Wess-Zumino-Witten term is derived, which generates quantum phases related to AF monopoles with VBS cores, and reproduces Haldane's hedgehog Berry phases. The theory readily generalizes to $\pi$-flux states for all $D$.Comment: 4 pages, revise

Geologic history and channeling episodes of the Chryse Planitia region of Mars

The study of the Chryse Planitia region of Mars is based on geologic mapping on a 1:5,000,000 scale shaded relief map. The map area includes Chryse and southern Acidalia Planitiae; the circum Chryse channels and chaotic terrains; Xanthe, southern Tempe, and western Arabia Terrae; Lunae Planum; and northeastern Valles Marineris. The aim of the study is twofold: (1) to obtain relative ages of the outflow channels by performing and compiling detailed stratigraphic analyses; and (2) to correlate channeling episodes with causative mechanisms (such as volcanism and tectonism) and resulting effects (such as climate change). The geologic history given based on this mapping, includes the documentation of a previously unproposed channeling episode in the region as well as the presently favored hypotheses concerning the nature and origin of the channeling events. It is concluded that the history of the Chryse region suggests that two major periods of tectonic activity resulted in two episodes of channeling in the highlands surrounding Chryse Planitia

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.Comment: 9 pages, 5 figures, accepted for publication in MNRA

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.Comment: 19 pages, 8 figures, conference title : Frontier Research in Astrophysics - II (https://pos.sissa.it/269/