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Although there is strong evidence that many long GRBs are associated with the collapse of a massive star, tantalizing results in recent years have upended the direct association of all long GRBs with massive stars. In particular, kilonova signals in some long GRB light curves as well as a suggested uptick in the rate density of long GRBs at low redshifts (deviating significantly from the star formation rate) suggest that compact object mergers may be a non-negligible fraction of the long GRB population. Here we investigate the contribution of white dwarf-black hole mergers to the long GRB population. We present evidence for the deviation of the long GRB rate density from the star formation rate at low redshifts, and provide analytic and numerical arguments for why a white dwarf-black hole merger system may be a viable progenitor to explain this deviation. We show the range of parameter space in which the durations, energetics, and rates of these systems can account for a significant sub-population of low-redshift long GRBs., Comment: Accepted for publication in MNRAS

The supermassive black holes (SMBHs) that power active galactic nuclei found at $z > 6$ were formed during the epoch of reionization. Because reionization is an inhomogeneous process, the physical properties of SMBH host galaxy environments will vary spatially during reionization. We construct a semi-analytic model to estimate the impact of reionization on SMBH growth. Using a series of merger trees, reionization models, and black hole growth models, we find that early reionization can reduce an SMBH's mass by up to [50, 70, 90] % within dark matter halos of mass [$10^{12}$, $10^{11}$, $10^{10}$] M$_{\odot}$ by $z$ = 6. Our findings also suggest that the redshift range in which black hole growth is impacted by reionization strongly depends on whether the Eddington accretion rate can be exceeded. If so, we find that black hole masses are significantly suppressed principally during the early phases of reionization ($z$ > 10), while they are more readily suppressed across the full redshift range if super-Eddington growth is not allowed. We find that the global average impact of reionization may be to reduce the masses of black holes residing in < 10$^{11}$ M$_{\odot}$ halos by a factor of > 2. The census of supermassive black holes being uncovered by the James Webb Space Telescope may offer a means to test the basic prediction that more massive black holes reside in cosmological volumes that are reionized at later times., Comment: 12 pages, 8 figures. Accepted to ApJ

For gamma-ray bursts (GRBs) with durations greater than two seconds (so-called long GRBs), the intrinsic prompt gamma-ray emission appears, on average, to last longer for bursts at lower redshifts. We explore the nature of this duration-redshift anti-correlation, describing systems and conditions in which this cosmological evolution could arise. In particular, we explore its dependence on metallicity of a massive star progenitor, as we can securely count on the average stellar metallicity to increase with decreasing redshift. Although higher metallicity/lower redshift stars lose mass and angular momentum through line-driven winds, in some cases these stars are able to form more extended accretion disks when they collapse, potentially leading to longer duration GRBs. We also examine how this duration-redshift trend may show up in interacting binary models composed of a massive star and compact object companion, recently suggested to be the progenitors of radio bright GRBs. Under certain conditions, mass loss and equation of state effects from higher metallicity, lower redshift massive stars can decrease the binary separation. This can then lead to spin-up of the massive star and allow for a longer duration GRB upon the massive star's collapse. Finally, the duration-redshift trend may also be supported by a relatively larger population of small-separation binaries born in situ at low redshift., Comment: Accepted to ApJ

IBM quantum computers are used to simulate the dynamics of small systems of interacting quantum spins. For time-independent systems with fewer than three spins, we compute the exact time evolution at arbitrary times and measure spin expectation values and energy. It is demonstrated that even in such small systems, one can observe the connection between conservation laws and symmetries in the model. Larger systems require approximating the time-evolution operator, and we investigate the case of $N=3$ spins explicitly. While it is shown to be unfeasible to use such devices to probe such larger systems without more advanced algorithms or reliable error correction, we demonstrate that the quantum circuit simulator is an easy-to-use method for studying spin dynamics in systems with $N\sim\mathcal{O}(10)$) spins. The computations presented provide an interesting experimental component to the standard treatment of quantum spin in an undergraduate quantum mechanics course., Comment: 13 pages, 9 figures

A recent experiment testing the necessity of complex numbers in the standard formulation of quantum theory is recreated using IBM quantum computers. To motivate the experiment, we present a basic construction for real-valued quantum theory. The real-valued description is shown to predict correlations identical to those of complex-valued quantum mechanics for two types of Bell tests based on the Clauser-Horne-Shimony-Holt (CHSH) inequality. A slight modification to one test, however, results in different predictions for the real- and complex-valued constructions. While noisier devices are incapable of delivering convincing results, it is shown that certain devices possess sufficiently small error rates to falsify real-valued formulations of quantum theory for composite states. The results obtained with quantum computers are consistent with published experiments. This work demonstrates the feasibility of using freely-available quantum devices to explore foundational features of quantum mechanics with minimal technical expertise. Accordingly, this treatment could inspire novel projects for undergraduate students taking a course on quantum mechanics., Comment: 12 pages, 4 figures

The immense power of gamma-ray bursts (GRBs) make them ideal probes of the early universe. By using absorption lines in the afterglows of high-redshift GRBs, astronomers can study the evolution of metals in the early universe. With an understanding of the nature of GRB progenitors, the rate and properties of GRBs observed at high redshift can probe the star formation history and the initial mass function of stars at high redshift. This paper presents a detailed study of the metallicity- and mass-dependence of the properties of long-duration GRBs under the black-hole accretion disk paradigm to predict the evolution of these properties with redshift. These models are calibrated on the current GRB observations and then used to make predictions for new observations and new missions (e.g. the proposed Gamow mission) studying high-redshift GRBs., Comment: 21 pages, 20 figures, submitted to ApJ

Gamma-ray burst (GRB) data suggest that the jets from GRBs in the high redshift universe are more narrowly collimated than those at lower redshifts. This implies that we detect relatively fewer long GRB progenitor systems (i.e. massive stars) at high redshifts, because a greater fraction of GRBs have their jets pointed away from us. As a result, estimates of the star formation rate (from the GRB rate) at high redshifts may be diminished if this effect is not taken into account. In this paper, we estimate the star formation rate (SFR) using the observed GRB rate, accounting for an evolving jet opening angle. We find that the SFR in the early universe (z > 3) can be up to an order of magnitude higher than the canonical estimates, depending on the severity of beaming angle evolution and the fraction of stars that make long gamma-ray bursts. Additionally, we find an excess in the SFR at low redshifts, although this lessens when accounting for evolution of the beaming angle. Finally, under the assumption that GRBs do in fact trace canonical forms of the cosmic SFR, we constrain the resulting fraction of stars that must produce GRBs, again accounting for jet beaming-angle evolution. We find this assumption suggests a high fraction of stars in the early universe producing GRBs - a result that may, in fact, support our initial assertion that GRBs do not trace canonical estimates of the SFR., Comment: Final version accepted to MNRAS

The direct formation of a massive black hole is a potential seeding mechanism of the earliest observed supermassive black holes. We investigate how the existence of a massive black hole seed impacts the ionization and thermal state of its pre-galactic host halo and subsequent star formation. We show that its X-ray radiation ionizes and heats the medium, enhancing $\rm{H}_2$ formation in shielded regions, within the nuclear region in the span of a million years. The enhanced molecular cooling triggers the formation of a $\sim 10^4~{\rm M}_\odot$ metal-free stellar cluster at a star formation efficiency of $\sim 0.1\%$ in a single event. Star formation occurs near the edges of the H II region that is partially ionized by X-rays, thus the initial size depends on the black hole properties and surrounding environment. The simulated metal-free galaxy has an initial half-light radius of $\sim 10$ pc but expands to $\sim 50$ pc after 10 million years because of the outward velocities of their birth clouds. Supernova feedback then quenches any further star formation for tens of millions of years, allowing the massive black hole to dominate the spectrum once the massive metal-free stars die., Comment: Accepted for publication in ApJ Letters, 7 pages, 5 figures

The existence of quasi-long range order is demonstrated in nonequilibrium steady states in isotropic $XY$ spin chains including of two types of additional terms that each generate a gap in the energy spectrum. The system is driven out of equilibrium by initializing a domain-wall magnetization profile through application of an external magnetic field and switching off the magnetic field at the same time the energy gap is activated. An energy gap is produced by either applying a staggered magnetic field in the $z$ direction or introducing a modulation to the $XY$ coupling. The magnetization, spin current, and spin-spin correlation functions are computed analytically in the thermodynamic limit at long times after the quench. For both types of systems, we find the persistence of power-law correlations despite the ground-state correlation functions exhibiting exponential decay., Comment: 19 pages, 9 figures, published version

We present a framework for investigating effective dynamics of SU(3) color charge. Two- and three-body effective interaction terms inspired by the Heisenberg spin model are considered. In particular, a toy model for a three-source "baryon" is constructed and investigated analytically and numerically for various choices of interactions. VPython is used to visualize the nontrivial color charge dynamics. The treatment should be accessible to undergraduate students who have taken a first course in quantum mechanics, and suggestions for independent student projects are proposed., Comment: 14 pages, 6 figures

An approximate solution is presented for simple harmonic motion in the presence of damping by a force which is a general power-law function of the velocity. The approximation is shown to be quite robust, allowing for a simple way to investigate amplitude decay in the presence of general types of weak, nonlinear damping., Comment: 6 pages, 3 figures

High redshift quasars emit copious X-ray photons which heat the intergalactic medium to temperatures up to $\sim$ 10$^6$ K. At such high temperatures the primordial gas will not form stars until it is assembled into dark matter haloes with masses of up to $\sim$ 10$^{11}$ M$_{\odot}$, at which point the hot gas collapses and cools under the influence of gravity. Once this occurs, there is a massive reservoir of primordial gas from which stars can form, potentially setting the stage for the brightest Population (Pop) III starbursts in the early Universe. Supporting this scenario, recent observations of quasars at z $\sim$ 6 have revealed a lack of accompanying Lyman $\alpha$ emitting galaxies, consistent with suppression of primordial star formation in haloes with masses below $\sim$ 10$^{10}$ M$_{\odot}$. Here we model the chemical and thermal evolution of the primordial gas as it collapses into such a massive halo irradiated by a nearby quasar in the run-up to a massive Pop III starburst. We find that within $\sim$ 100 kpc of the highest redshift quasars discovered to date the Lyman-Werner flux produced in the quasar host galaxy may be high enough to stimulate the formation of a direct collapse black hole (DCBH). A survey with single pointings of the NIRCam instrument at individual known high-z quasars may be a promising strategy for finding Pop III stars and DCBHs with the James Webb Space Telescope., Comment: 9 pages, 9 figures, 2 tables; ApJ in press

Supermassive black holes (SMBHs) are the central engines of luminous quasars and are found in most massive galaxies today. But the recent discoveries of ULAS J1120+0641, a $2 \times 10^9$ M$_{\odot}$ BH at $z =$ 7.1, and ULAS J1342+0928, a $8.0 \times 10^{8}$ M$_{\odot}$ BH at $z =$ 7.5, now push the era of quasar formation up to just 690 Myr after the Big Bang. Here we report new cosmological simulations of SMBHs with X-rays fully coupled to primordial chemistry and hydrodynamics that show that J1120 and J1342 can form from direct collapse black holes (DCBHs) if their growth is fed by cold, dense accretion streams, like those thought to fuel rapid star formation in some galaxies at later epochs. Our models reproduce all of the observed properties of J1120: its mass, luminosity, and H II region as well as star formation rates and metallicities in its host galaxy. They also reproduce the dynamical mass of the innermost 1.5 kpc of its emission region recently measured by ALMA and J-band magnitudes that are in good agreement with those found by the VISTA Hemisphere Survey., Comment: 7 pages, 7 figures. Accepted by ApJ

In this study we focus on the $z\sim6.6$ Lyman-$\alpha$ CR7 consisting of clump A that is host to a potential direct collapse black hole (DCBH), and two metal enriched star forming clumps B and C. In contrast to claims that signatures of metals rule out the existence of DCBHs, we show that metal pollution of A from star forming clumps clumps B and C is inevitable, and that A can form a DCBH well before its metallicity exceeds the critical threshold of $10^{-5}-10^{-6}\ \rm Z_{\odot}$. Assuming metal mixing happens instantaneously, we derive the metallicity of A based on the star formation history of B and C. We find that treating a final accreting black hole of $10^6-10^7\ \rm M_{\odot}$ in A for nebular emission already pushes its $H_{160}$ - [3.6] and [3.6]-[4.5] colours into the 3$\sigma$ limit of observations. Hence, we show that the presence of metals in DCBH hosts is inevitable, and that it is the coevolution of the LW radiation field and metals originating from neighbouring galaxies that governs DCBH formation in a neighbouring {initially} pristine atomic cooling haloes., Comment: 6 pages, accepted for publication in MNRAS

We assess the impact of trapped Lyman alpha cooling radiation on the formation of direct collapse black holes (DCBHs). We apply a one-zone chemical and thermal evolution model, accounting for the photodetachment of H- ions, precursors to the key coolant H2, by Lyman alpha photons produced during the collapse of a cloud of primordial gas in an atomic cooling halo at high redshift. We find that photodetachment of H- by trapped Lyman alpha photons may lower the level of the H2-dissociating background radiation field required for DCBH formation substantially, dropping the critical flux by up to a factor of a few. This translates into a potentially large increase in the expected number density of DCBHs in the early Universe, and supports the view that DCBHs may be the seeds for the BHs residing in the centers of a significant fraction of galaxies today. We find that detachment of H- by Lyman alpha has the strongest impact on the critical flux for the relatively high background radiation temperatures expected to characterize the emission from young, hot stars in the early Universe. This lends support to the DCBH origin of the highest redshift quasars., Comment: 7 pages, 6 figures, 1 table; recommended by editor for publication in A&A

We present the first ab initio cosmological simulations of a CR7-like object which approximately reproduce the observed line widths and strengths. In our model, CR7 is powered by a massive ($3.23 \times 10^7$ $M_\odot$) black hole (BH) the accretion rate of which varies between $\simeq$ 0.25 and $\simeq$ 0.9 times the Eddington rate on timescales as short as 10$^3$ yr. Our model takes into account multi-dimensional effects, X-ray feedback, secondary ionizations and primordial chemistry. We estimate Ly-$\alpha$ line widths by post-processing simulation output with Monte Carlo radiative transfer and calculate emissivity contributions from radiative recombination and collisional excitation. We find the luminosities in the Lyman-$\alpha$ and He II 1640 angstrom lines to be $5.0\times10^{44}$ and $2.4\times10^{43}$ erg s$^{-1}$, respectively, in agreement with the observed values of $>$ $8.3\times10^{43}$ and $2.0\times10^{43}$ erg s$^{-1}$. We also find that the black hole heats the halo and renders it unable to produce stars as required to keep the halo metal free. These results demonstrate the viability of the BH hypothesis for CR7 in a cosmological context. Assuming the BH mass and accretion rate that we find, we estimate the synchrotron luminosity of CR7 to be $P \simeq 10^{40} - 10^{41}$ erg s$^{-1}$, which is sufficiently luminous to be observed in $\mu$Jy observations and would discriminate this scenario from one where the luminosity is driven by Population III stars., Comment: 5 pages, 4 figures, Accepted by ApJ Letters

Out-of-equilibrium behavior is explored in the one-dimensional anisotropic $XY$ model. Initially preparing the system in the isotropic $XX$ model with a linearly varying magnetic field to create a domain-wall magnetization profile, dynamics is generated by rapidly changing the exchange interaction anisotropy and external magnetic field. Relaxation to a nonequilibrium steady state is studied analytically at the critical transverse Ising point, where correlation functions may be computed in closed form. For arbitrary values of anisotropy and external field, an effective generalized Gibbs' ensemble is shown to accurately describe observables in the long-time limit. Additionally, we find spatial oscillations in the exponentially decaying, transverse spin-spin correlation functions with wavelength set by the magnetization jump across the initial domain wall. This wavelength depends only weakly on anisotropy and magnetic field in contrast to the current, which is highly dependent on these parameters., Comment: typos corrected, references added

With detections of quasars powered by increasingly massive black holes (BHs) at increasingly early times in cosmic history over the past decade, there has been correspondingly rapid progress made on the theory of early BH formation and growth. Here we review the emerging picture of how the first massive BHs formed from the primordial gas and then grew to supermassive scales. We discuss the initial conditions for the formation of the progenitors of these seed BHs, the factors dictating the initial masses with which they form, and their initial stages of growth via accretion, which may occur at super-Eddington rates. Finally, we briefly discuss how these results connect to large-scale simulations of the growth of supermassive BHs over the course of the first billion years following the Big Bang., Comment: 13 pages, 9 figures, invited review accepted for publication in PASA

We propose that one of the sources in the recently detected system CR7 by Sobral et al. (2015) through spectro-photometric measurements at $z = 6.6$ harbors a direct collapse blackhole (DCBH). We argue that the LW radiation field required for direct collapse in source A is provided by sources B and C. By tracing the LW production history and star formation rate over cosmic time for the halo hosting CR7 in a $\Lambda$CDM universe, we demonstrate that a DCBH could have formed at $z\sim 20$. The spectrum of source A is well fit by nebular emission from primordial gas around a BH with MBH $\sim 4.4 \times 10^6 \ \rm M_{\odot}$ accreting at a 40 % of the Eddington rate, which strongly supports our interpretation of the data. Combining these lines of evidence, we argue that CR7 might well be the first DCBH candidate., Comment: 8 pages, 4 figures. Accepted for publication in MNRAS

The analogy of classical repulsive interactions emerging from the exchange of mediating particles is revisited with a quantitative approach. Simulations are presented for a particular toy model which are accessible to undergraduate students at any level in the physics curriculum. Analytic treatment of the various regimes shows rigorously how effective force laws can emerge from an underlying microscopic model and should be accessible to advanced undergraduate physics majors. The analysis presented uses the concept of emergence as motivation for students to gain experience building and testing simplified models for complex physical processes., Comment: 16 pages, 4 figures

Cosmological simulations of Population (Pop) III star formation suggest that the primordial initial mass function may have extended to sub-solar masses. If Pop III stars with masses < 0.8 M_Sun did form, then they should still be present in the Galaxy today as either main sequence or red giant stars. Despite broad searches, however, no primordial stars have yet been identified. It has long been recognized that the initial metal-free nature of primordial stars could be masked due to accretion of metal-enriched material from the interstellar medium (ISM). Here we point out that while gas accretion from the ISM may readily occur, the accretion of dust from the ISM can be prevented due to the pressure of the radiation emitted from low-mass stars. This implies a possible unique chemical signature for stars polluted only via accretion, namely an enhancement in gas phase elements relative to those in the dust phase. Using Pop III stellar models, we outline the conditions in which this signature could be exhibited, and we derive the expected signature for the case of accretion from the local ISM. Intriguingly, due to the large fraction of iron depleted into dust relative to that of carbon and other elements, this signature is similar to that observed in many of the so-called carbon-enhanced metal-poor (CEMP) stars. We therefore suggest that some fraction of the observed CEMP stars may, in fact, be accretion-polluted Pop III stars., Comment: 9 pages, 9 figures, 1 table; accepted for publication in MNRAS

Direct collapse black holes (DCBHs) formed from the collapse of atomically-cooled primordial gas in the early Universe are strong candidates for the seeds of supermassive BHs. DCBHs are thought to form in atomic cooling haloes in the presence of a strong molecule-dissociating, Lyman-Werner (LW) radiation field. Given that star forming galaxies are likely to be the source of the LW radiation in this scenario, ionizing radiation from these galaxies may accompany the LW radiation. We present cosmological simulations resolving the collapse of primordial gas into an atomic cooling halo, including the effects of both LW and ionizing radiation. We find that in cases where the gas is not self-shielded from the ionizing radiation, the collapse can be delayed by ~ 25 Myr. When the ionized gas does collapse, the free electrons that are present catalyze H2 formation. In turn, H2 cooling becomes efficient in the center of the halo, and DCBH formation is prevented. We emphasize, however, that in many cases the gas collapsing into atomic cooling haloes at high redshift is self-shielding to ionizing radiation. Therefore, it is only in a fraction of such haloes in which DCBH formation is prevented due to reionization., Comment: 9 pages, 8 figures; published in MNRAS; Figure 1 slightly degraded to reduce size; acknowledgement added

We investigate the environment in which direct-collapse black holes may form by analysing a cosmological, hydrodynamical simulation that is part of the First Billion Years project. This simulation includes the most relevant physical processes leading to direct collapse of haloes, most importantly, molecular hydrogen depletion by dissociation of $H_2$ and $H^-$ from the evolving Lyman-Werner radiation field. We selected a sample of pristine atomic cooling haloes that have never formed stars in their past, have not been polluted with heavy elements and are cooling predominantly via atomic hydrogen lines. Amongst them we identified six haloes that could potentially harbour massive seed black holes formed via direct collapse (with masses in the range of $10^{4-6} M_{sun}$). These potential hosts of direct-collapse black holes form as satellites and are found within 15 physical kpc of proto-galaxies, with stellar masses in the range $10^{5-7} M_{sun}$ and maximal star formation rates of 0.1 Msun/yr over the past 5 Myr, and are exposed to the highest flux of Lyman-Werner radiation emitted from the neighbouring galaxies. It is the proximity to these proto-galaxies that differentiates these haloes from rest of the sample., Comment: 10 pages, 8 figures, accepted MNRAS

Population III supernovae have been of growing interest of late for their potential to directly probe the properties of the first stars, particularly the most energetic events that are visible near the edge of the observable universe. But until now, hypernovae, the unusually energetic Type Ib/c supernovae that are sometimes associated with gamma-ray bursts, have been overlooked as cosmic beacons at the highest redshifts. In this, the latest of a series of studies on Population III supernovae, we present numerical simulations of 25 - 50 M$_{\odot}$ hypernovae and their light curves done with the Los Alamos RAGE and SPECTRUM codes. We find that they will be visible at z = 10 - 15 to the James Webb Space Telescope (JWST) and z = 4 - 5 to the Wide-Field Infrared Survey Telescope (WFIRST), tracing star formation rates in the first galaxies and at the end of cosmological reionization. If, however, the hypernova crashes into a dense shell ejected by its progenitor, it is expected that a superluminous event will occur that may be seen at z ~ 20, in the first generation of stars., Comment: 15 pages, 9 figures, accepted by ApJ

Supernovae are important probes of the properties of stars at high redshifts because they can be detected at early epochs and their masses can be inferred from their light curves. Finding the first cosmic explosions in the universe will only be possible with the James Webb Space Telescope, the Wide-Field Infrared Survey Telescope and the next generation of extremely large telescopes. But strong gravitational lensing by massive clusters, like those in the Cluster Lensing and Supernova Survey with Hubble (CLASH), could reveal such events now by magnifying their flux by factors of 10 or more. We find that CLASH will likely discover at least 2 - 3 core-collapse supernovae at 5 < z < 12 and perhaps as many as ten. Future surveys of cluster lenses similar in scope to CLASH by the James Webb Space Telescope might find hundreds of these events out to z ~ 15 - 17. Besides revealing the masses of early stars, these ancient supernovae will also constrain cosmic star formation rates in the era of first galaxy formation., Comment: 6 pages, 4 figures, submitted to ApJL

One of the leading contenders for the origin of supermassive black holes at $z \gtrsim$ 7 is catastrophic baryon collapse in atomically-cooled halos at $z \sim$ 15. In this scenario, a few protogalaxies form in the presence of strong Lyman-Werner UV backgrounds that quench H$_2$ formation in their constituent halos, preventing them from forming stars or blowing heavy elements into the intergalactic medium prior to formation. At masses of 10$^ 8$ \Ms\ and virial temperatures of 10$^4$ K, gas in these halos rapidly cools by H lines, in some cases forming 10$^4$ - 10$^6$ \Ms\ Pop III stars and, a short time later, the seeds of supermassive black holes. Instead of collapsing directly to black holes some of these stars died in the most energetic thermonuclear explosions in the universe. We have modeled the explosions of such stars in the dense cores of line-cooled protogalaxies in the presence of cosmological flows. In stark contrast to the explosions in diffuse regions in previous simulations, these SNe briefly engulf the protogalaxy but then collapse back into its dark matter potential. Fallback drives turbulence that efficiently distributes metals throughout the interior of the halo and fuels the rapid growth of nascent black holes at its center. The accompanying starburst and x-ray emission from these line-cooled galaxies easily distinguish them from more slowly evolving neighbors and might reveal the birthplaces of supermassive black holes on the sky., Comment: 9 pages, 8 figures, accepted by ApJ

Supermassive primordial stars are expected to form in a small fraction of massive protogalaxies in the early universe, and are generally conceived of as the progenitors of the seeds of supermassive black holes (BHs). Supermassive stars with masses of ~55,000 M_Sun, however, have been found to explode and completely disrupt in a supernova (SN) with an energy of up to ~10^55 erg instead of collapsing to a BH. Such events, ~10,000 times more energetic than typical SNe today, would be among the biggest explosions in the history of the universe. Here we present a simulation of such a SN in two stages. Using the RAGE radiation hydrodynamics code we first evolve the explosion from an early stage through the breakout of the shock from the surface of the star until the blast wave has propagated out to several parsecs from the explosion site, which lies deep within an atomic cooling dark matter (DM) halo at z ~ 15. Then, using the GADGET cosmological hydrodynamics code we evolve the explosion out to several kiloparsecs from the explosion site, far into the low-density intergalactic medium. The host DM halo, with a total mass of 4 x 10^7 M_Sun, much more massive than typical primordial star-forming halos, is completely evacuated of high density gas after < 10 Myr, although dense metal-enriched gas recollapses into the halo, where it will likely form second-generation stars with metallicities of ~ 0.05 Z_Sun after > 70 Myr. The chemical signature of supermassive star explosions may be found in such long-lived second-generation stars today., Comment: 9 pages, 7 figures; accepted for publication in ApJ; a short movie of the explosion can be found at http://www.as.utexas.edu/~jljohnson/SMS_SN.mp4

We estimate the maximum temperature at which planets can form via gravitational instability (GI) in the outskirts of early circumstellar disks. We show that due to the temperature floor set by the cosmic microwave background, there is a maximum distance from their host stars beyond which gas giants cannot form via GI, which decreases with their present-day age. Furthermore, we show that planet formation via GI is not possible at metallicities < 10^-4 Z_Sun, due to the reduced cooling efficiency of low-metallicity gas. This critical metallicity for planet formation via GI implies a minimum distance from their host stars of ~ 6 AU within which planets cannot form via GI; at higher metallicity, this minimum distance can be significantly greater, out to several tens of AU. We show that these maximum and minimum distances significantly constrain the number of observed planets to date that are likely to have formed via GI at their present locations. That said, the critical metallicity we find for GI is well below that for core accretion to operate; thus, the first planets may have formed via GI, although only within a narrow region of their host circumstellar disks., Comment: 6 pages, 3 figures; version accepted for publication in MNRAS

The first stars are the key to the formation of primitive galaxies, early cosmological reionization and chemical enrichment, and the origin of supermassive black holes. Unfortunately, in spite of their extreme luminosities, individual Population III stars will likely remain beyond the reach of direct observation for decades to come. However, their properties could be revealed by their supernova explosions, which may soon be detected by a new generation of NIR observatories such as JWST and WFIRST. We present light curves and spectra for Pop III pair-instability supernovae calculated with the Los Alamos radiation hydrodynamics code RAGE. Our numerical simulations account for the interaction of the blast with realistic circumstellar envelopes, the opacity of the envelope, and Lyman absorption by the neutral IGM at high redshift, all of which are crucial to computing the NIR signatures of the first cosmic explosions. We find that JWST will detect pair-instability supernovae out to z > 30, WFIRST will detect them in all-sky surveys out to z ~ 15 - 20 and LSST and Pan-STARRS will find them at z ~ 7 - 8. The discovery of these ancient explosions will probe the first stellar populations and reveal the existence of primitive galaxies that might not otherwise have been detected., Comment: 17 pages, 12 figures, accepted by ApJ

Recent observations of quasars powered by supermassive black holes (SMBHs) out to z > 7 constrain both the initial seed masses and the growth of the most massive black holes (BHs) in the early universe. Here we elucidate the implications of the radiative feedback from early generations of stars and from BH accretion for popular models for the formation and growth of seed BHs. We show that by properly accounting for (1) the limited role of mergers in growing seed BHs as inferred from cosmological simulations of early star formation and radiative feedback, (2) the sub-Eddington accretion rates of BHs expected at the earliest times, and (3) the large radiative efficiencies (e_rad) of the most massive BHs inferred from observations of active galactic nuclei at high redshift (e_rad > 0.1), we are led to the conclusion that the initial BH seeds may have been as massive as > 10^5 M_Sun. This presents a strong challenge to the Population III seed model, which calls for seed masses of ~ 100 M_Sun and, even with constant Eddington-limited accretion, requires e_rad < 0.09 to explain the highest-z SMBHs in today's standard LambdaCDM cosmological model. It is, however, consistent with the prediction of the direct collapse scenario of SMBH seed formation, in which a supermassive primordial star forms in a region of the universe with a high molecule-dissociating background radiation field, and collapses directly into a 10^4--10^6 M_Sun seed BH. These results corroborate recent cosmological simulations and observational campaigns which suggest that these massive BHs were the seeds of a large fraction of the SMBHs residing in the centers of galaxies today., Comment: 9 pages, 2 figures; accepted for publication in ApJ

With the first metal enrichment by Population (Pop) III supernovae (SNe), the formation of the first metal-enriched, Pop II stars becomes possible. In turn, Pop III star formation and early metal enrichment are slowed by the high energy radiation emitted by Pop II stars. Thus, through the SNe and radiation they produce, Populations II and III coevolve in the early Universe, one regulated by the other. We present large (4 Mpc)^3, high resolution cosmological simulations in which we self-consistently model early metal enrichment and the stellar radiation responsible for the destruction of the coolants (H2 and HD) required for Pop III star formation. We find that the molecule-dissociating stellar radiation produced both locally and over cosmological distances reduces the Pop III star formation rate at z > 10 by up to an order of magnitude compared to the case in which this radiation is not included. However, we find that the effect of LW feedback is to enhance the amount of Pop II star formation. We attribute this to the reduced rate at which gas is blown out of dark matter haloes by SNe in the simulation with LW feedback, which results in larger reservoirs for metal-enriched star formation. Even accounting for metal enrichment, molecule-dissociating radiation and the strong suppression of low-mass galaxy formation due to reionization at z < 10, we find that Pop III stars are still formed at a rate of ~ 10^-5 M_sun yr^-1 Mpc^-3 down to z ~ 6. This suggests that the majority of primordial pair-instability SNe that may be uncovered in future surveys will be found at z < 10. We also find that the molecule-dissociating radiation emitted from Pop II stars may destroy H2 molecules at a high enough rate to suppress gas cooling and allow for the formation of supermassive primordial stars which collapse to form ~ 100,000 solar mass black holes., Comment: 17 pages, 11 figures; accepted for publication in MNRAS; some figures downgraded, high resolution version available at http://www.as.utexas.edu/~jljohnson/FiBY_III.pdf

One of the most promising explanations for the origin of the billion solar mass black holes (BHs) inferred to power quasars at redshifts z > 6 is that supermassive stars (SMSs) with masses > 10,000 solar masses collapse to form the seed BHs from which they grow. Here we review recent theoretical advances which provide support for this scenario. Firstly, given sufficiently high accretion rates of gas into the cores of primordial protogalaxies, it appears that neither the high energy radiation emitted from the stellar surface nor the limited lifetime of SMSs can prevent their growth to masses of up to > 100,000 solar masses. Secondly, recent cosmological simulations suggest that the high fluxes of molecule-dissociating radiation which may be required in order to achieve such high accretion rates may be more common in the early universe than previously thought. We conclude that the majority of supermassive BHs may originate from SMSs at high redshifts., Comment: 4 pages, 3 figures; proceedings of "First Stars IV", held in Kyoto, Japan, May 21-25, 2012

We study for the first time the environment of massive black hole (BH) seeds (~10^4-5 Msun) formed via the direct collapse of pristine gas clouds in massive haloes (>10^7 Msun) at z>6. Our model is based on the evolution of dark matter haloes within a cosmological N-body simulation, combined with prescriptions for the formation of BH along with both Pop III and Pop II stars. We calculate the spatially-varying intensity of Lyman Werner (LW) radiation from stars and identify the massive pristine haloes in which it is high enough to shut down molecular hydrogen cooling. In contrast to previous BH seeding models with a spatially constant LW background, we find that the intensity of LW radiation due to local sources, J_local, can be up to 10^6 times the spatially averaged background in the simulated volume and exceeds the critical value, J_crit, for the complete suppression of molecular cooling, in some cases by 4 orders of magnitude. Even after accounting for possible metal pollution in a halo from previous episodes of star formation, we find a steady rise in the formation rate of direct collapse (DC) BHs with decreasing redshift from 10^{-3}/Mpc^3/z at z=12 to 10^{-2}/Mpc^3/z at z=6. The onset of Pop II star formation at z~16 simultaneously marks the onset of the epoch of DCBH formation, as the increased level of LW radiation from Pop II stars is able to elevate the local levels of the LW intensity to J_local > J_crit while Pop III stars fail to do so at any time. The number density of DCBHs is sensitive to the number of LW photons and can vary by an order of magnitude at z=6 after accounting for reionisation feedback. Haloes hosting DCBHs are more clustered than similar massive counterparts that do not host DCBHs, especially at redshifts z>10. We also show that planned surveys with JWST should be able to detect the supermassive stellar precursors of DCBHs., Comment: 19 pages, 17 figures, v2 accepted for publication in MNRAS, minor additions in text and updates in references

A rapidly growing body of observational results suggests that planet formation takes place preferentially at high metallicity. In the core accretion model of planet formation this is expected because heavy elements are needed to form the dust grains which settle into the midplane of the protoplanetary disk and coagulate to form the planetesimals from which planetary cores are assembled. As well, there is observational evidence that the lifetimes of circumstellar disks are shorter at lower metallicities, likely due to greater susceptibility to photoevaporation. Here we estimate the minimum metallicity for planet formation, by comparing the timescale for dust grain growth and settling to that for disk photoevaporation. For a wide range of circumstellar disk models and dust grain properties, we find that the critical metallicity above which planets can form is a function of the distance r at which the planet orbits its host star. With the iron abundance relative to that of the Sun [Fe/H] as a proxy for the metallicity, we estimate a lower limit for the critical abundance for planet formation of [Fe/H]_crit ~ -1.5 + log(r/1 AU), where an astronomical unit (AU) is the distance between the Earth and the Sun. This prediction is in agreement with the available observational data, and carries implications for the properties of the first planets and for the emergence of life in the early Universe. In particular, it implies that the first Earth-like planets likely formed from circumstellar disks with metallicities Z > 0.1 Z_Sun. If planets are found to orbit stars with metallicities below the critical metallicity, this may be a strong challenge to the core accretion model., Comment: 12 pages, 5 figures; accepted for publication in ApJ

The collapse of baryons into extremely massive stars with masses exceeding 10^4 M_Sun in a small fraction of protogalaxies at z > 10 is a promising candidate for the origin of supermassive black holes, some of which grow to a billion solar masses by z ~ 7. We determine the maximum masses such stars can attain by accreting primordial gas. We find that at relatively low accretion rates the strong ionizing radiation of these stars limits their masses to M_* ~ 10^3 M_Sun (dM_acc/dt / 10^-3 M_Sun yr^-1)^8/7, where dM_acc/dt is the rate at which the star gains mass. However, at the higher central infall rates usually found in numerical simulations of protogalactic collapse (>~ 0.1 M_Sun yr^-1), the lifetime of the star instead limits its final mass to >~ 10^6 M_Sun. Furthermore, for the spherical accretion rates at which the star can grow, its ionizing radiation is confined deep within the protogalaxy, so the evolution of the star is decoupled from that of its host galaxy. Lyman alpha emission from the surrounding H II region is trapped in these heavy accretion flows and likely reprocessed into strong Balmer series emission, which may be observable by the James Webb Space Telescope. This, along with strong He II 1640 Angstrom and continuum emission, are likely to be the key observational signatures of the progenitors of supermassive black holes at high redshift., Comment: 14 pages, 4 figures; now accepted for publication in ApJ

The properties of the first galaxies are shaped in large part by the first generations of stars, which emit high energy radiation and unleash both large amounts of mechanical energy and the first heavy elements when they explode as supernovae. We survey the theory of the formation of the first galaxies in this context, focusing on the results of cosmological simulations to illustrate a number of the key processes that define their properties. We first discuss the evolution of the primordial gas as it is incorporated into the earliest galaxies under the influence of the high energy radiation emitted by the earliest stars; we then turn to consider how the injection of heavy elements by the first supernovae transforms the evolution of the primordial gas and alters the character of the first galaxies. Finally, we discuss the prospects for the detection of the first galaxies by future observational missions, in particular focusing on the possibility that primordial star-forming galaxies may be uncovered., Comment: 31 pages, 19 figures; chapter to appear in 'The First Galaxies - Theoretical Predictions and Observational Clues'; corrected typo in equation (32); some figures downgraded

While stars are widely discussed as the source of the high energy photons which reionized the universe, an additional source of ionizing photons that must also contribute to reionization in this scenario is the supernovae (SNe) which mark the end of the life of massive stars. Here we estimate the relative contributions of SNe and stars to reionization. While the rate at which ionizing photons are produced in SN shocks is well below that at which they are produced by stars, the harder spectra of radiation emitted from SNe leads to an enhanced escape fraction of SN generated photons relative to that of stellar photons. In particular, along a given line of sight out of a galaxy, we find that for neutral hydrogen column densities N_H >~10^18 cm^-2 the contribution to reionization from SNe is greater than that from stars. Drawing on the results of simulations presented in the literature, we find that the overall (line of sight-averaged) SNe shock-generated ionizing photon escape fraction is larger than the stellar photon escape fraction by a factor of \simeq 4 to \simeq 7, depending on the metallicity of the stellar population. Overall, our results suggest that the effect of SNe is an enhancement of up to ~ 10 percent in the fraction of hydrogen reionized by stellar sources. We briefly discuss the implications of our results for the population of galaxies responsible for reionization., Comment: 12 pages, 4 figures; ApJ in press

We calculate the amount of angular momentum that thermal photons carry out of a viscous black hole accretion disc, due to the strong Doppler shift imparted to them by the high orbital velocity of the radiating disc material. While the emission of radiation can not drive accretion on its own, we find that it does result in a loss of specific angular momentum, thereby contributing to an otherwise viscosity-driven accretion flow. In particular, we show that the fraction of the angular momentum that is lost to thermal emission at a radius r in a standard, multi-color disc is ~ 0.4r_s/r, where r_s is the Schwarzschild radius of the black hole. We briefly highlight the key similarities between this effect and the closely related Poynting-Robertson effect., Comment: 6 pages, 2 figures; to appear in AN

Motivated by recent theoretical work suggesting that a substantial fraction of Population (Pop) III stars may have had masses low enough for them to survive to the present day, we consider the role that the accretion of metal-enriched gas may have had in altering their surface composition, thereby disguising them as Pop II stars. We demonstrate that if weak, Solar-like winds are launched from low-mass Pop III stars formed in the progenitors of the dark matter halo of the Galaxy, then such stars are likely to avoid significant enrichment via accretion of material from the interstellar medium. We find that at early times accretion is easily prevented if the stars are ejected from the central regions of the haloes in which they form, either by dynamical interactions with more massive Pop III stars, or by violent relaxation during halo mergers. While accretion may still take place during passage through sufficiently dense molecular clouds at later times, we find that the probability of such a passage is generally low (< 0.1), assuming that stars have velocities of order the maximum circular velocity of their host haloes and accounting for the orbital decay of merging haloes. In turn, due to the higher gas density required for accretion onto stars with higher velocities, we find an even lower probability of accretion (~ 0.01) for the subset of Pop III stars formed at z > 10, which are more quickly incorporated into massive haloes than stars formed at lower redshift. While there is no a priori reason to assume that low-mass Pop III stars do not have Solar-like winds, without them surface enrichment via accretion is likely to be inevitable. We briefly discuss the implications that our results hold for stellar archaeology., Comment: 9 pages; 2 figures; MNRAS accepted

We study cosmological simulations of early structure formation, including non-equilibrium molecular chemistry, metal pollution from stellar evolution, transition from population III (popIII) to population II (popII) star formation, regulated by a given critical metallicity, and feedback effects. We investigate the properties of early metal spreading from the different stellar populations and its interplay with primordial molecular gas. We find that, independently of the details about popIII modeling, after the onset of star formation, regions enriched below the critical level are mostly found in isolated environments, while popII star formation regions are much more clumped. Typical star forming haloes show average SN driven outflow rates of up to 10^{-4} Msun/yr in enriched gas, initially leaving the original star formation regions almost devoid of metals. The polluted material, which is gravitationally incorporated in over-dense environments on timescales of 10^7 yr, is mostly coming from external, nearby star forming sites ("gravitational enrichment"). In parallel, the pristine-gas inflow rates are between 10^{-3} - 10^{-1} Msun/yr. However, thermal feedback from SN generates turbulence and destroys molecules within the pristine gas, and only the polluted material, incorporated via gravitational enrichment, can continue to cool by atomic metal fine-structure transitions on time scales short enough to end the initial popIII regime within less than 10^8 yr., Comment: Accepted on the 31/1/2011

One possible scenario for the formation of massive black holes (BHs) in the early Universe is from the direct collapse of primordial gas in atomic-cooling dark matter haloes in which the gas is unable to cool efficiently via molecular transitions. We study the formation of such BHs, as well as the accretion of gas onto these objects and the high energy radiation emitted in the accretion process, by carrying out cosmological radiation hydrodynamics simulations. In the absence of radiative feedback, we find an upper limit to the accretion rate onto the central object which forms from the initial collapse of hot (~ 10^4 K) gas of the order of 0.1 MSun per year. This is high enough for the formation of a supermassive star, the immediate precursor of a BH, with a mass of the order of 10^5 MSun. Assuming that a fraction of this mass goes into a BH, we track the subsequent accretion of gas onto the BH self-consistently with the high energy radiation emitted from the accretion disk. Using a ray-tracing algorithm to follow the propagation of ionizing radiation, we model in detail the evolution of the photoionized region which forms around the accreting BH. We find that BHs with masses of the order of 10^4 MSun initially accrete at close to the Eddington limit, but that the accretion rate drops to of order 1 percent of the Eddington limit after ~ 10^6 yr, due to the expansion of the gas near the BH in response to strong photoheating and radiation pressure. One signature of the accretion of gas onto BHs formed by direct collapse, as opposed to massive Pop III star formation, is an extremely high ratio of the luminosity emitted in He II 1640 to that emitted in H_alpha (or Ly_alpha); this could be detected by the James Webb Space Telescope. Finally, we briefly discuss implications for the coevolution of BHs and their host galaxies., Comment: 16 pages; 17 figures, slightly reduced quality; MNRAS in press

Detection of the radiation emitted from some of the earliest galaxies will be made possible in the next decade, with the launch of the James Webb Space Telescope (JWST). A significant fraction of these galaxies may host Population (Pop) III star clusters. The detection of the recombination radiation emitted by such clusters would provide an important new constraint on the initial mass function (IMF) of primordial stars. Here I review the expected recombination line signature of Pop III stars, and present the results of cosmological radiation hydrodynamics simulations of the initial stages of Pop III starbursts in a first galaxy at z ~ 12, from which the time-dependent luminosities and equivalent widths of IMF-sensitive recombination lines are calculated. While it may be unfeasible to detect the emission from Pop III star clusters in the first galaxies at z > 10, even with next generation telescopes, Pop III star clusters which form at lower redshifts (i.e. at z < 6) may be detectable in deep surveys by the JWST., Comment: 6 pages, 5 figures; Proceedings of 'The First Stars and Galaxies: Challenges for the Next Decade", Austin, TX, March 8-11, 2010

We consider the effects of supernovae (SNe) on accretion and star formation in a massive gaseous disk in a large primeval galaxy. The gaseous disk we envisage, roughly 1 kiloparsec (kpc) in size with >~ 10^8 M_Sun of gas, could have formed as a result of galaxy mergers where tidal interactions removed angular momentum from gas at larger radius and thereby concentrated it within the central ~ 1 kpc region. We find that SNe lead to accretion in the disk at a rate of roughly 0.1 - 1 M_Sun per year and induce star formation at a rate of ~ 10 - 100 M_Sun per year which contributes to the formation of a bulge; a part of the stellar velocity dispersion is due to the speed of SNa shells from which stars are formed and a part due to the repeated action of the stochastic gravitational field of the network of SNa remnants on stars. The rate of SNe in the inner kpc is shown to be self- regulating, and it cycles through phases of low and high activity. The supernova-assisted accretion transports gas from about one kpc to within a few pc of the center. If this accretion were to continue down to the central black hole (BH) then the resulting ratio of BH mass to the stellar mass in the bulge would be of order ~ 10^-2 - 10^-3, in line with the observed Magorrian relation., Comment: 16 pages, 1 figure; MNRAS in press

In reionized regions of the Universe, gas can only collapse to form stars in dark matter (DM) haloes which grow to be sufficiently massive. If star formation is prevented in the minihalo progenitors of such DM haloes at redshifts z >~ 20, then these haloes will not be self-enriched with metals and so may host Population (Pop) III star formation. We estimate an upper limit for the abundance of Pop III star clusters which thus form in the reionized Universe, as a function of redshift. Depending on the minimum DM halo mass for star formation, between of the order of one and of the order of a thousand Pop III star clusters per square degree may be observable at 2 <~ z <~ 7. Thus, there may be a sufficient number density of Pop III star clusters for detection in surveys such as the Deep-Wide Survey (DWS) to be conducted by the James Webb Space Telescope (JWST). We predict that Pop III clusters formed after reionization are most likely to be found at z >~ 3 and within ~ 40 arcsec (~ 1 Mpc comoving) of DM haloes with masses of ~ 10^11 M_Sun, the descendants of the haloes at z ~ 20 which host the first galaxies that begin reionization. If the stellar initial mass function (IMF) is top-heavy the clusters may have sufficiently high luminosities in both Ly_alpha and He II lambda1640 to be detected and for constraints to be placed on the Pop III IMF. While a small fraction of DM haloes with masses as high as ~10^9 M_Sun at redshifts z <~ 4 are not enriched due to star formation in their progenitors, external metal enrichment due to galactic winds is likely to preclude Pop III star formation in a large fraction of otherwise unenriched haloes, perhaps even preventing star formation in pristine haloes altogether after reionization is complete., Comment: 13 pages, 6 figures, accepted for publication in MNRAS

We use three-dimensional smoothed particle hydrodynamics simulations together with a dynamical ray-tracing scheme to investigate the build-up of the first H ii regions around massive Population III stars in minihaloes. We trace the highly anisotropic breakout of the ionising radiation into the intergalactic medium, allowing us to predict the resulting recombination radiation with greatly increased realism. Our simulations, together with Press-Schechter type arguments, allow us to predict the Population III contribution to the radio background at 100 MHz via bremsstrahlung and 21 cm emission. We find a global bremsstrahlung signal of around 1 mK, and a combined 21 cm signature which is an order of magnitude larger. Both might be within reach of the planned Square Kilometer Array experiment, although detection of the free-free emission is only marginal. The imprint of the first stars on the cosmic radio background might provide us with one of the few diagnostics to test the otherwise elusive minihalo star formation site., Comment: 13 pages, 7 figures, published in MNRAS

Detection of the radiation emitted from the first galaxies at z > 10 will be made possible in the next decade, with the launch of the James Webb Space Telescope (JWST). We carry out cosmological radiation hydrodynamics simulations of Population III (Pop III) starbursts in a 10^8 M_Sun dwarf galaxy at z = 12.5. For different star formation efficiencies and stellar initial mass functions (IMFs), we calculate the luminosities and equivalent widths (EWs) of the recombination lines H_alpha, Ly_alpha, and He II 1640, under the simple assumption that the stellar population does not evolve over the first ~3 Myr of the starburst. Although only < 40 percent of the gas in the central 100 pc of the galaxy is photoionized, we find that photoheating by massive stars causes a strong dynamical response, which results in a weak correlation between luminosity emitted in hydrogen recombination lines and the total mass in stars. However, owing to the low escape fraction of He II-ionizing photons, the luminosity emitted in He II 1640 is much more strongly correlated with the total stellar mass. The ratio of the luminosity in He II 1640 to that in Ly_alpha or H_alpha is found to be a good indicator of the IMF in many cases. The ratio of observable fluxes is F_1640/F_Halpha ~ 1 for clusters of 100 M_Sun Pop III stars and F_1640/F_Halpha ~ 0.1 for clusters of 25 M_Sun Pop III stars. The EW of the He II 1640 emission line is the most reliable IMF indicator, its value varying between ~ 20 and ~ 200 angstrom for a massive and very massive Pop III IMF, respectively. Even the bright, initial stages of Pop III starbursts in the first dwarf galaxies will likely not be directly detectable by the JWST. Instead, the JWST may discover only more massive, and hence more chemically evolved, galaxies which host normal, Pop I/II, star formation., Comment: 13 pages, 10 figures; accepted for publication in MNRAS