[Paper Review] Detecting the Birth of Supermassive Black Holes Formed from Heavy Seeds

Fabio Pacucci, Vivienne Baldassare+12 more|arXiv (Cornell University)|Mar 18, 2019

Galaxies: Formation, Evolution, Phenomena50 references4 citations

TL;DR

This white paper proposes that upcoming observatories—JWST, Athena, and Lynx—can detect heavy black hole seeds (10⁴–10⁶ M☉) formed via direct gas collapse in the early Universe (z ≳ 10). By modeling infrared and X-ray fluxes under Compton-thin and thick conditions, it predicts 1–10 deg⁻² detectable sources with JWST and 5–50 deg⁻² with Lynx, while LISA could detect 2–20 mergers of 10⁵ M☉ seeds over 4 years.

ABSTRACT

In this white paper we explore the capabilities required to identify and study supermassive black holes formed from heavy seeds ($\mathrm{M_{\bullet}} \sim 10^4 - 10^6 \, \mathrm{M_{\odot}}$) in the early Universe. To obtain an unequivocal detection of heavy seeds we need to probe mass scales of $\sim 10^{5-6} \, \mathrm{M_{\odot}}$ at redshift $z \gtrsim 10$. From this theoretical perspective, we review the observational requirements and how they compare with planned/proposed instruments, in the infrared, X-ray and gravitational waves realms. In conclusion, detecting heavy black hole seeds at $z \gtrsim 10$ in the next decade will be challenging but, according to current theoretical models, feasible with upcoming/proposed facilities. Their detection will be fundamental to understand the early history of the Universe, as well as its evolution until now. Shedding light on the dawn of black holes will certainly be one of the key tasks that the astronomical community will focus on in the next decade.

Motivation & Objective

• To identify the observational capabilities required to detect heavy black hole seeds formed via direct collapse in the early Universe (z ≳ 10).
• To assess the detectability of these seeds using future infrared (JWST) and X-ray (Athena, Lynx) observatories.
• To evaluate the synergy between electromagnetic and gravitational wave observations (e.g., LISA) in probing the formation and evolution of early black hole seeds.
• To quantify expected source counts for detectable DCBHs under varying theoretical number densities and flux sensitivity limits.

Proposed method

• Modeling the spectral energy distribution and X-ray emission of direct collapse black holes (DCBHs) at z ≳ 10, accounting for Compton-thin and Compton-thick absorption scenarios.
• Estimating flux density requirements for detection: 10⁻¹⁶ erg s⁻¹ cm⁻² for infrared (JWST) and 10⁻¹⁶ erg s⁻¹ cm⁻² (Athena) or 10⁻¹⁹ erg s⁻¹ cm⁻² (Lynx) for X-rays.
• Using theoretical number densities of DCBHs (10⁻¹⁰–10⁻¹ Mpc⁻³ at z ≈ 10) to predict observable source counts per square degree.
• Projecting detection rates for JWST based on its 3×10⁻¹⁷ erg s⁻¹ cm⁻² flux limit for a 10 ks exposure at ~1 μm.
• Assessing LISA’s sensitivity to gravitational wave signals from mergers of 10⁵ M☉ seeds at z ≳ 8, with signal-to-noise ratios ~200.
• Evaluating the complementary role of infrared and X-ray observations in constraining emission mechanisms and host environment column densities.

Experimental results

Research questions

• RQ1 What flux sensitivity is required for JWST to detect heavy black hole seeds at z ≳ 10 in the infrared domain?
• RQ2 How many DCBHs can be detected per square degree with JWST and Lynx under intermediate DCBH number density assumptions?
• RQ3 What X-ray flux limits are necessary to detect Compton-thick DCBHs, and which missions can achieve them?
• RQ4 How do gravitational wave detections by LISA constrain the population of early black hole seeds and their merger rates?
• RQ5 What is the expected synergy between electromagnetic (JWST, Athena, Lynx) and gravitational wave (LISA) observations in studying early black hole formation?

Key findings

• JWST is expected to detect 1–10 DCBHs per square degree at z ≳ 10, assuming intermediate DCBH number densities and a flux limit of ~3×10⁻¹⁷ erg s⁻¹ cm⁻².
• Lynx could detect 5–50 DCBHs per square degree at z ≳ 10, with higher sensitivity enabling detection of lower-mass seeds (~10⁵ M☉) over longer timescales.
• Athena’s flux sensitivity is sufficient to detect Compton-thin DCBHs at z ≳ 10, requiring a flux density ≥10⁻¹⁶ erg s⁻¹ cm⁻².
• For Compton-thick DCBHs (NH ≳ 1.5×10²⁴ cm⁻²), flux limits of ~10⁻¹⁹ erg s⁻¹ cm⁻²—achievable with Lynx—are necessary for detection.
• LISA is predicted to detect 2–20 mergers of 10⁵ M☉ black hole seeds over 4 years of operation, with signal-to-noise ratios ~200.
• The combined use of infrared (JWST) and X-ray (Athena, Lynx) observations is essential to fully characterize the emission mechanisms and column densities of early DCBHs.