Bhaskar Agarwal

Ubiquitous seeding of supermassive black holes by direct collapse

We study for the first time the environment of massive black hole (BH) seeds (∼10 4−5 M� ) formed via the direct collapse of pristine gas clouds in massive haloes (≥10 7 M� )a tz > 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 Population III (Pop III) and Population II (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, Jlocal, can be up to ∼10 6 times the spatially averaged background in the simulated volume and exceeds the critical value, Jcrit, for the complete suppression of molecular cooling, in some cases by four 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 BHs (DCBHs) with decreasing redshift from 10 −3 Mpc −3 z −1 at z = 12 to 10 −2 Mpc −3 z −1 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 Jlocal > Jcrit, 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 over an order of magnitude at z = 7 after accounting for reionization feedback. Haloes hosting DCBHs are more clustered than similar massive counterparts that do not host DCBHs, especially at redshifts z 10. Also, the DCBHs that form at z > 10 are found to reside in highly clustered regions, whereas the DCBHs formed around z ∼ 6a re more common. We also show that planned surveys with James Webb Space Telescope should be able to detect the supermassive stellar precursors of DCBHs.

The First Billion Years project: birthplaces of direct collapse black holes

ABSTRACT We investigate the environment in which direct-collapse black holes may form by analysing acosmological, hydrodynamical simulation that is part of the First Billion Years project. Thissimulation includes the most relevant physical processes leading to direct collapse of haloes,most importantly,molecularhydrogendepletionbydissociation of H 2 and H − fromthe evolv-ing Lyman-Werner radiation field. We selected a sample of pristine atomic cooling haloes thathave never formed stars in their past, have not been polluted with heavy elements and arecooling predominantlyvia atomic hydrogenlines. Amongst them we identified six haloes thatcould potentially harbour massive seed black holes formed via direct collapse (with massesin the range of 10 4−6 M ⊙ ). These potential hosts of direct-collapse black holes form as satel-lites are found within 15 physical kpc of proto-galaxies, with stellar masses in the range≈10 5−7 M ⊙ and maximal star formation rates of ≈0.1M ⊙ yr −1 over the past 5Myr, andare exposed to the highest flux of Lyman-Werner radiation emitted from the neighbouringgalaxies. It is the proximity to these proto-galaxies that differentiates these haloes from restof the sample.Key words: insert keywords

Unravelling obese black holes in the first galaxies

We predict the existence and observational signatures of a new class of objects that assembled early, during the first billion years of cosmic time: Obese Black-hole Galaxies (OBGs). OBGs are objects in which the mass of the central black hole initially exceeds that of the stellar component of the host galaxy, and the luminosity from black-hole accretion dominates the starlight. From a cosmological simulation, we demonstrate that there are sites where star formation is initially inhibited and direct-collapse black holes (DCBHs) form due to the photo-dissociating effect of Lyman-Werner radiation on molecular hydrogen. We show that the formation of OBGs is inevitable, because the probability of finding the required extra-galactic environment and the right physical conditions in a halo conducive to DCBH formation is quite high in the early universe. We estimate an OBG number density of 0.009/Mpc^3 at z~8 and 0.03/Mpc^3 at z~6. Extrapolating from our simulation volume, we infer that the most luminous quasars detected at z~6 likely transited through an earlier OBG phase. We find that these primordial galaxies start off with an over-massive BH and acquire their stellar component from subsequent merging as well as in-situ star formation. In doing so, they inevitably go through an OBG phase dominated by the accretion luminosity at the Eddington rate or below, released from the growing BH. The OBG phase is characterised by an ultra-violet (UV) spectrum with slope of beta ~ -2.3 and the absence of a Balmer Break. OBGs should also be spatially unresolved, and are expected to be brighter than the majority of known high-redshift galaxies. OBGs could potentially be revealed via HST follow-up imaging of samples of brighter Lyman-break galaxies provided by wide-area ground-based surveys such as UltraVISTA, and should be easily uncovered and studied with instruments aboard JWST...(abridged)

Detecting direct collapse black holes : making the case for CR7

We propose that one of the sources in the recently detected system CR7 by Sobral et al. through spectrophotometric measurements at z=6.6 harbours a direct collapse black hole (DCBH). We argue that ...

Revised rate coefficients for H2 and H− destruction by realistic stellar spectra

Understanding the processes that can destroy H2 and H species is quintessential in governing the formation of the first stars, black holes and galaxies . In this study we compute the reaction rate coefficients for H 2 photo‐dissociation by Lyman‐Werner photons (11.2 13.6 eV), and H photo‐detachment by 0.76 eV photons emanating from self-consistent stellar populations that we model using publicly available stellar synthesis codes. So far studies that include chemical networks for the formation of molecular hydrogen take these processes into account by assuming that the source spectra can be approximated by a power-law dependency or a black-body spectrum at 10 4 or 10 5 K. We show that using spectra generated from realistic stellar population models can alter the reaction ra tes for photo-dissociation, kdi, and photo-detachment, kde, significantly. In particular, kde can be up to � 2 4 orders of magnitude lower in the case of realistic stellar spectra sugges ting that previous calculations have over-estimated the impact that radiation has on lowering H2 abundances. In contrast to burst modes of star formation, we find that models with continuous s tar formation predict increasing kde and kdi, which makes it necessary to include the star formation history of sources to derive self-consistent reaction rates, and that it is not enough to just calculate J21 for the background. For models with constant star formation rate the change in shape of the spectral energy distribution leads to a non-negligible late-ti me contribution to kde and kdi, and we present self-consistently derived cosmological reacti on rates based on star formation rates consistent with observations of the high redshift Universe.

New constraints on direct collapse black hole formation in the early Universe

Direct collapse black holes (DCBH) have been proposed as a solution to the challenge of assembling supermassive black holes by

The impact of reionization on the formation of supermassive black hole seeds

z>6

Unveiling the first black holes with JWST: multi-wavelength spectral predictions

to explain the bright quasars observed at this epoch. The formation of a DCBH seed with

Metallicity evolution of direct collapse black hole hosts: CR7 as a case study

rm M_{BH}sim10^{4-5} rm M_{odot}

The Chandra COSMOS Legacy Survey: Energy Spectrum of the Cosmic X-Ray Background and Constraints on Undetected Populations

requires a pristine atomic-cooling halo to be illuminated by an external radiation field that is sufficiently strong to entirely suppress H