The observational signatures of supermassive black hole seeds

Abstract

The origin and properties of the initial black hole seeds that grow to produce the observed population of accreting sources remain to be determined. It is a challenge to uniquely disentangle signatures of seeding from fueling and dynamics that shapes the assembly history of growing black holes. To address this, we use a semi-analytic model developed to track the growth of supermassive black holes adopting multiple prescriptions for accretion. In contrast with earlier treatments, we explore the interplay between seeding models and two accretion modes. We find that signatures of the initial seeding do survive in the following observational probes: the black hole occupation fraction; contribution to the unresolved X-ray background; low-luminosity and high-redshift luminosity functions; and in gravitational wave event signatures. We find that the behaviour of the low-mass end of the M • − σ relation is dominated by uncertainties in the adopted accretion prescriptions and does not offer clear discrimination between seeding models. We make concrete predictions for future surveys, particularly for the Lynx X-ray surveyor and LISA (The Laser Interferometer Space Antenna) mission, which will each provide different and yet strong constraints on the seed population. Black hole coalescences detected by LISA and high-redshift quasar luminosity functions observed by Lynx will offer the sharpest seeding discriminants. Although the signatures of the black hole seeding mechanism that persist remain linked to our understanding of black hole accretion and dynamics, we offer new insights on how these upcoming multi-wavelength observations could be leveraged to effectively disentangle them.

Cosmological simulations of massive black hole seeds: predictions for next-generation electromagnetic and gravitational wave observations

“…In particular, a recent paper by Dayal et al (2018) used a semi-analytic model to predict high redshift merger rates, and found that for z > 5 mergers, LISA detections of mergers between a DCBH and a stellar mass seed would outnumber the DCBH-DCBH merger detections by two orders of magnitude or more. Furthermore, similar to Ricarte & Natarajan (2018), Dayal et al (2018) found that the merger rate would peak at very low masses, with LISA-detectable mergers (based on SNR > 7) peaking at MBH(1 + z) ∼ 10 4 − 10 5 M , suggesting that the predictions we make for the rate of high-mass DCBH-DCBH mergers represents only a subset of all LISA detections.…”

The clustering of undetected high-redshift black holes and their signatures in cosmic backgrounds

“…The ability for z = 0 measurements to constrain z > 6 predictions underscores the power of our self-consistent modelling. As shown in Ricarte & Natarajan (2018b), our heavy seeding prescription is consistent with current constraints based on X-ray emission, while the light seeding prescription optimistically assigns almost every low-mass galaxy a MBH seed.…”

Tracing black hole and galaxy co-evolution in the Romulus simulations

“…The physics of seeding, the abundance of seeds and their initial mass function is one of the key unsolved problems in black hole physics today (see Ricarte & Natarajan 2018b, for a recent detailed discussion). Leveraging the resolution of the Romulus simulations, SMBHs are seeded using recipes based on the local gas properties, rather than simply imposing a halo mass threshold as is often implemented (e.g., Springel et al 2005;Vogelsberger et al 2013;Schaye et al 2015).…”