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 reference

We investigate the properties of the first galaxies at z 10 with highly resolved numerical simulations, starting from cosmological initial conditions and taking into account all relevant primordial chemistry and cooling. A first galaxy is characterized by the onset of atomic hydrogen cooling, once the virial temperature exceeds 10 4 K, and its ability to retain photoheated gas. We follow the complex accretion and star formation history of a 5 × 10 7 M system by means of a detailed merger tree and derive an upper limit on the number of Population III (Pop III) stars formed prior to its assembly. We investigate the thermal and chemical evolution of infalling gas and find that partial ionization at temperatures 10 4 K catalyses the formation of H 2 and hydrogen deuteride, allowing the gas to cool to the temperature of the cosmic microwave background. Depending on the strength of radiative and chemical feedback, primordial star formation might be dominated by intermediate-mass Pop III stars formed during the assembly of the first galaxies. Accretion on to the nascent galaxy begins with hot accretion, where gas is accreted directly from the intergalactic medium and shock heated to the virial temperature, but is quickly accompanied by a phase of cold accretion, where the gas cools in filaments before flowing into the parent halo with high velocities. The latter drives supersonic turbulence at the centre of the galaxy and could lead to very efficient chemical mixing. The onset of turbulence in the first galaxies thus likely marks the transition to Pop II star formation.

We find that at redshifts z > 10, HD line cooling allows strongly-shocked primordial gas to cool to the temperature of the cosmic microwave background (CMB). This temperature is the minimum value attainable via radiative cooling. Provided that the abundance of HD, normalized to the total number density, exceeds a critical level of ~ 10^{-8}, the CMB temperature floor is reached in a time which is short compared to the Hubble time. We estimate the characteristic masses of stars formed out of shocked primordial gas in the wake of the first supernovae, and resulting from the mergers of dark matter haloes during hierarchical structure formation to be ~ 10 M_{solar}. In addition, we show that cooling by HD enables the primordial gas in relic H II regions to cool to temperatures considerably lower than those reached via H_2 cooling alone. We confirm that HD cooling is unimportant in cases where the primordial gas does not go through an ionized phase, as in the formation process of the very first stars in z ~ 20 minihaloes of mass ~ 10^{6} M_{solar}.Comment: 10 pages, 10 figures, accepted for publication in MNRAS with minor revisions, new table adde

We perform three-dimensional smoothed particle hydrodynamics simulations in a realistic cosmological setting to investigate the expansion, feedback, and chemical enrichment properties of a 200 M pair-instability supernova in the high-redshift universe. We find that the SN remnant propagates for a Hubble time at z ' 20 to a final massweighted mean shock radius of 2.5 kpc (proper), roughly half the size of the H ii region, and in this process sweeps up a total gas mass of 2:5 ; 10 5 M . The morphology of the shock becomes highly anisotropic once it leaves the host halo and encounters filaments and neighboring minihalos, while the bulk of the shock propagates into the voids of the intergalactic medium. The SN entirely disrupts the host halo and terminates further star formation for at least 200 Myr, while in our specific case it exerts positive mechanical feedback on neighboring minihalos by shock-compressing their cores. In contrast, we do not observe secondary star formation in the dense shell via gravitational fragmentation, due to the previous photoheating by the progenitor star. We find that cooling by metal lines is unimportant for the entire evolution of the SN remnant, while the metal-enriched, interior bubble expands adiabatically into the cavities created by the shock, and ultimately into the voids with a maximum extent similar to the final mass-weighted mean shock radius. Finally, we conclude that dark matter halos of at least M vir k 10 8 M must be assembled to recollect all components of the swept-up gas.

We calculate the rate of in‐fall of stellar matter on an accretion disc during the collapse of a rapidly rotating massive star and estimate the luminosity of the relativistic jet that results from accretion on to the central black hole. We find that the jet luminosity remains high for about 102 s, at a level comparable to the typical luminosity observed in gamma‐ray bursts (GRBs). The luminosity then decreases rapidly with time for about ∼103 s, roughly as ∼t−3; the duration depends on the size and rotation speed of the stellar core. The rapid decrease of the jet power explains the steeply declining X‐ray flux observed at the end of most long‐duration GRBs. Observations with the Swift satellite show that, following the steep decline, many GRBs exhibit a plateau in the X‐ray light curve (XLC) that lasts for about 104 s. We suggest that this puzzling feature is due to continued accretion in the central engine. A plateau in the jet luminosity can arise when the viscosity parameter α is small, ∼10−2 or less. A plateau is also produced by continued fall‐back of matter – either from an extended stellar envelope or from material that failed to escape with the supernova ejecta. In a few GRBs, the XLC is observed to drop suddenly at the end of the plateau phase, while in others the XLC declines more slowly as ∼t−1−t−2. These features arise naturally in the accretion model depending on the radius and mean specific angular momentum of the stellar envelope. The total energy in the disc‐wind accompanying accretion is found to be about 1052 erg. This is comparable to the energy observed in supernovae associated with GRBs, suggesting that the wind might be the primary agent responsible for the explosion. The accretion model thus provides a coherent explanation for the diverse and puzzling features observed in the early XLC of GRBs. It might be possible to use this model to invert gamma‐ray and X‐ray observations of GRBs and thereby infer basic properties of the core and envelope of the GRB progenitor star.

We investigate the conditions under which the first low-mass stars formed in the Universe by confronting theoretical predictions governing the transition from massive Population III to lowmass Population II stars with recent observational C and/or O abundance data of metal-poor Galactic stars. We introduce a new 'observer-friendly' function, the transition discriminant D trans , which provides empirical constraints as well as a powerful comparison between the currently available data of metal-poor halo stars and theoretical predictions of the formation of the first low-mass stars ( 1 M ). Specifically, we compare the empirical stellar results with the theory that fine-structure lines of C and O dominate the transition from Population III to Population II in the early Universe. We find the currently available data for halo stars as well as for dwarf spheroidal (dSph) galaxies and globular clusters to be consistent with this theory. An explanation for the observed lack of metal-poor stars in dSph galaxies and globular clusters is also suggested. Finally, we predict that any star to be found with [Fe/H] −4 should have enhanced C and/or O abundances. The high C and O abundances of the two most iron-poor stars are in line with our prediction.

We investigate the evolution of the primordial gas surrounding the first massive black holes formed by the collapse of Population III stars at redshifts z > 20. Carrying out three-dimensional hydrodynamical simulations using GADGET, we study the dynamical, thermal and chemical evolution of the first relic H II regions. We also carry out simulations of the mergers of relic H II regions with neighboring neutral minihaloes, which contain high density primordial gas that can accrete onto a Pop III remnant black hole. We find that there may have been a significant time delay, of order ~10^8 yr, between black hole formation and the onset of efficient accretion. The build-up of supermassive black holes, believed to power the z > 6 quasars observed in the Sloan Digital Sky Survey, therefore faces a crucial early bottleneck. More massive seed black holes may thus be required, such as those formed by the direct collapse of a primordial gas cloud facilitated by atomic line cooling. The high optical depth to Lyman-Werner (LW) photons that results from the high fraction of H_2 molecules that form in relic H II regions, combined with the continued formation of H_2 inside the dynamically expanding relic H II region, leads to shielding of the molecules inside these regions at least until a critical background LW flux of \~10^{-24} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1}, is established. Furthermore, we find that a high fraction of HD molecules, X_{HD} > 10^{-7}, is formed, potentially enabling the formation of Pop II.5 stars during later stages of structure formation when the relic H II region gas is assembled into a sufficiently deep potential well to gravitationally confine the gas again.Comment: 14 pages, 10 figures, MNRAS accepted; one figure and references adde

The first galaxies form under the influence of radiative feedback from the first generations of stars. This feedback acts to heat and ionize the gas within the H ii regions surrounding the first stars, as well as to photodissociate hydrogen molecules within the larger Lyman-Werner ( LW ) bubbles that surround these sources. Using a ray-tracing method in three-dimensional cosmological simulations, we self-consistently track the formation of, and radiative feedback from, individual stars in the formation of a protogalaxy. We compute in detail the H ii regions of each of these sources, as well as the regions affected by their molecule-dissociating radiation. We follow the thermal, chemical, and dynamical evolution of the primordial gas as it becomes incorporated into the protogalaxy. While the IGM is, in general, optically thick to LW photons only over physical distances of k30 kpc at redshifts z P 20, the high molecule fraction that is built up in relic H ii regions and their increasing volume-filling factor renders even the local IGM optically thick to LW photons over physical distances of a few kiloparsecs. We find that Population III relic black holes may begin accreting efficiently after $60 Myr from the time of their formation, when the photoheated relic H ii region gas can cool and recollapse into the 10 6 M minihalo which hosts the black hole. Population II.5 stars, postulated to have masses of the order of 10 M , can also likely form from this recollapsing relic H ii region gas. Overall, we find that the local radiative feedback from Population III stars suppresses the star formation rate only slightly.