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

Abstract

We study how statistical properties of supermassive black holes depend on the frequency and conditions for massive seed formation in cosmological simulations of structure formation. We develop a novel method to re-calculate detailed growth histories and merger trees of black holes within the framework of the Illustris simulation for several seed formation models, including a physically motivated model where black hole seeds only form in progenitor galaxies that conform to the conditions for direct collapse black hole formation. While all seed models considered here are in a broad agreement with present observational constraints on black hole populations from optical, UV and X-ray studies, we find they lead to widely different black hole number densities and halo occupation fractions which are currently observationally unconstrained. In terms of future electromagnetic spectrum observations, the faint-end quasar luminosity function and the low mass-end black hole-host galaxy scaling relations are very sensitive to the specific massive seed prescription. Specifically, the direct collapse model exhibits a seeding efficiency which decreases rapidly with cosmic time and produces much fewer black holes in low mass galaxies, in contrast to the original Illustris simulation. We further find that the total black hole merger rate varies by more than one order of magnitude for different seed models, with the redshift evolution of the chirp mass changing as well. Supermassive black hole merger detections with LISA and International Pulsar Timing Array may hence provide the most direct means of constraining massive black hole seed formation in the early Universe.

On the detectability of massive black hole merger eventsby LISA

“…The Illustris black holes are limited to the high-end of LISA's peak sensitivity, and the majority of LISA detections would be expected to come from mergers below Illustris' black hole seed mass (see, e.g. Ricarte & Natarajan 2018;Dayal et al 2019;DeGraf & Sijacki 2020;Volonteri et al 2020). As such, the rates plotted should not be interpreted as predictions for all LISA detections, but rather to demonstrate the importance of including a time delay when predicting merger signals, at least for the high-mass end of the LISA-sensitivity range.…”

“…Volonteri & Natarajan 2009;Simon & Burke-Spolaor 2016;Shankar et al 2016), and how supermassive seeds form (e.g. Sesana et al 2007;Ricarte & Natarajan 2018;DeGraf & Sijacki 2020), while multimessenger studies combining GW and electromagnetic data should provide additional information about the galaxies in which mergers occur (e.g. Volonteri et al 2020;DeGraf et al 2021).…”

Resolving Complex Inner X-ray Structure of the Gravitationaly Lensed AGN MGB2016+112

“…The occurrence of binary and dual AGNs has important implications for assessing the time needed for the final stage of merging of the two supermassive black holes (SMBH). The eventual coalescence of such SMBH binaries is expected to be a major source of gravitational waves detected by the Laser Interferometer Space Antenna (LISA), or by the Pulsar Timing Array (PTA) if M>10 7 M (Sesana et al 2009;Dal Canton et al 2019;DeGraf & Sijacki 2020).…”

Dynamical Friction Modeling of Massive Black Holes in Cosmological Simulations and Effects on Merger Rate Predictions

“…LISA will provide a unique way of probing the high-redshift universe and understanding the early formation of the SMBHs, especially when combined with the soon-to-come observations of the electromagnetic (EM) counterparts (Natarajan et al 2017;DeGraf & Sijacki 2020). For instance, they will potentially allow us to distinguish between different BH seeding mechanisms at high-redshift (Ricarte & Natarajan 2018), to obtain information on the dynamical evolution of massive black holes (Bonetti et al 2019), and to gain information about the gas properties within the accretion disc (Derdzinski et al 2019).…”