The brief era of direct collapse black hole formation (1402.5675v1)

Abstract: It has been proposed that the first, intermediate-mass ($\approx 10^{5-6}~M_\odot$) black holes might form through direct collapse of unpolluted gas in atomic-cooling halos exposed to a strong Lyman-Werner (LW) or near-infrared (NIR) radiation. As these systems are expected to be Compton-thick, photons above 13.6 eV are largely absorbed and re-processed into lower energy bands. It follows that direct collapse black holes (DCBHs) are very bright in the LW/NIR bands, typically outshining small high-redshift galaxies by more than 10 times. Once the first DCBHs form, they then trigger a runaway process of further DCBH formation, producing a sudden rise in their cosmic mass density. The universe enters the "DCBH era" at $z \approx 20$ when a large fraction of atomic-cooling halos are experiencing DCBH formation. By combining the clustering properties of the radiation sources with Monte Carlo simulations we show that in this scenario the DCBH mass density rises from $\sim 5$~$M_\odot$ Mpc$^{-3}$ at $z\sim 30$ to the peak value $\sim5\times10^5 M_\odot$ Mpc$^{-3}$ at $z \sim 14$ in our fiducial model. However, the abundance of \textit{active} (accreting) DCBHs drops after $z \sim 14$, as gas in the potential formation sites (unpolluted halos with virial temperature slightly above $10^4$~K) is photoevaporated. This effect almost completely suppresses DCBH formation after $z\sim 13$. The DCBH formation era lasts only $\approx 150$ Myr, but it might crucially provide the seeds of the supermassive black holes (SMBHs) powering $z\sim6$ quasars.

Formation of Direct Collapse Black Holes in the Early Universe

The paper "The brief era of direct collapse black hole formation" explores the formation of intermediate-mass black holes through the mechanism of direct collapse in atomic-cooling halos during the early stages of the universe. The primary focus is on the formation era of direct collapse black holes (DCBHs) and the conditions necessary for their creation.

Key Findings

Formation Mechanism: The paper posits that DCBHs, with masses in the range of $10^{5-6} M_\odot$, can form from pristine gas undergoing direct collapse in halos that cool via atomic processes. These halos are capable of sustaining DCBH formation when exposed to strong Lyman-Werner (LW) or near-infrared (NIR) radiation. Such conditions suppress molecular hydrogen ($\rm H_2$) formation, preventing gas fragmentation and facilitating black hole formation.

Cosmic Era of DCBHs: The research indicates that the universe undergoes a "DCBH era" commencing at redshift $z \approx 20$ and lasting for about 150 million years. This period is characterized by a significant increase in DCBH mass density, peaking at redshift $z \sim 14$ with values around $5 \times 10^5 M_\odot \text{ Mpc}^{-3}$.

Feedback Mechanisms:

• Positive Feedback: Once initial DCBHs form, their intense LW emissions can induce neighboring halos to form additional DCBHs, potentially leading to a runaway formation process.
• Negative Feedback: Metal enrichment from nearby galaxies and progenitor halos can impede DCBH formation by providing conditions favorable for gas fragmentation and star formation. Additionally, photoevaporation due to ionizing radiation can strip gas from potential DCBH formation sites, contributing to the cessation of the DCBH era.

Impact on Supermassive Black Holes (SMBHs): The seeds from DCBHs formed during this brief era are potentially crucial for the growth of SMBHs observed at redshifts around $z \sim 6$, providing a plausible pathway for their rapid accretion and growth in the universe's early stages.

Implications and Future Considerations

Theoretical Implications: The paper advances our understanding of early universe black hole formation, specifically the conditions necessary for DCBH creation. It highlights the intricate interplay between cosmic radiation fields, halo clustering, and metal enrichment that governs DCBH formation rates.

Practical Implications: Understanding the formation and evolution of DCBHs can provide insights into the observed NIR background fluctuations and the fate of intermediate-mass black holes in the local universe. The research suggests a potential abundance of undetected relic intermediate-mass black holes that could be targets for future observational studies.

Future Developments: The paper sets the stage for more detailed models exploring varying radiation intensities, feedback mechanisms, and the role of gas dynamics in DCBH formation. Improving photoevaporation models and integration with larger cosmological simulations could refine predictions about DCBH abundance and SMBH formation.

In summary, through a comprehensive investigation of radiation feedback and halo dynamics, the paper underscores the transient nature yet significant cosmological role of DCBHs in shaping early universe structure and seeding SMBHs.