The collapse of supermassive primordial stars in hot, atomically-cooled halos may have given birth to the first quasars at z ∼ 15 -20. Recent numerical simulations of these rapidly accreting stars reveal that they are cool, red hypergiants shrouded by dense envelopes of pristine atomically-cooled gas at 6,000 -8,000 K, with luminosities L 10 10 L ⊙ . Could such luminous but cool objects be detected as the first stage of quasar formation in future near infrared (NIR) surveys? We have now calculated the spectra of supermassive primordial stars in their birth envelopes with the Cloudy code. We find that some of these stars will be visible to the James Webb Space Telescope (JWST) at z 20 and that with modest gravitational lensing Euclid and the Wide-Field Infrared Space Telescope (WFIRST) could detect them out to z ∼ 10 -12. Rather than obscuring the star, its accretion envelope enhances its visibility in the NIR today by reprocessing its short-wavelength flux into photons that are just redward of the Lyman limit in the rest frame of the star.
Pristine, atomically-cooled haloes are leading contenders for the sites of primordial quasar formation because atomic cooling triggers rapid baryon collapse that can create 10 4 -10 5 M black hole seeds. However, until now no numerical simulations with a wide range of halo spins and assembly histories have followed the collapse for the times required to form a black hole. We have now performed cosmological simulations of baryon collapse in atomically-cooled haloes for times that are sufficient for supermassive stars to form and die as direct-collapse black holes (DCBHs). Our simulations reveal that fragmentation of the accretion disk at the center of the halo after ∼ 500 kyr is nearly ubiquitous and in most cases leads to the formation of binary or multiple supermassive stellar systems. They also confirm that rapid baryon collapse proceeds for the times required for these stars to collapse to DCBHs. Our discovery raises the exciting possibility of detecting gravitational waves from DCBH mergers with LISA and tidal disruption events in the near infrared with the James Webb Space Telescope and ground-based telescopes in the coming decade.
Primordial supermassive stars (SMSs) formed in atomic-cooling halos at z ∼ 15–20 are leading candidates for the seeds of the first quasars. Past numerical studies of the evolution of SMSs have typically assumed constant accretion rates rather than the highly variable flows in which they form. We model the evolution of SMSs in the cosmological flows that create them using the Kepler stellar evolution and implicit hydrodynamics code. We find that they reach masses of 1 − 2 × 105 M⊙ before undergoing direct collapse to black holes (DCBHs) during or at the end of their main-sequence hydrogen burning, at 1–1.5 Myr, regardless of halo mass, spin, or merger history. We also find that realistic, highly variable accretion histories allow for a much greater diversity of supermassive stellar structures, including in some cases largely thermally relaxed objects, which may provide a significant source of radiative feedback. Our models indicate that the accretion histories predicted for purely atomic-cooling halos may impose a narrow spectrum of masses on the seeds of the first massive quasars; however, further studies incorporating realistic feedback will be essential in order to confirm whether or not this holds true in all cases. Our results also indicate that multiple SMSs at disparate stages of evolution can form in these halos, raising the possibility of SMS binaries and supermassive X-ray binaries, as well as DCBH mergers that could be detected by LISA.
Pristine, atomically-cooled haloes may be the sites of primordial quasar formation because atomic cooling triggers rapid baryon collapse that can create 104–105 M⊙ black hole seeds. However, no numerical simulation has ever followed the collapse of these haloes for the times required to form supermassive stars and direct-collapse black holes (DCBHs). We have now modeled baryon collapse in atomically-cooled haloes with a wide range of spin parameters and assembly histories for times that are sufficient for DCBH formation. Fragmentation of accretion disks after ∼500 kyr is nearly ubiquitous in these haloes and in most cases leads to the formation of binary or multiple supermassive stellar systems. They also confirm that rapid baryon collapse proceeds for the times required for these stars to form DCBHs. Our simulations suggest that binary or even multiple DCBH formation was the rule rather than the exception in the primordial Universe.
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