The hierarchical assembly of galaxies and black holes in the first billion years: predictions for the era of gravitational wave astronomy

Abstract

In this work we include black hole (BH) seeding, growth and feedback into our semianalytic galaxy formation model, Delphi. Our model now fully tracks the, accretionand merger-driven, hierarchical assembly of the dark matter halo, gas, stellar and BH masses of high-redshift (z > ∼ 5) galaxies. We explore a number of physical scenarios that include: (i) two types of BH seeds (stellar and those from Direct Collapse BH; DCBH); (ii) the impact of reionization; and (iii) the impact of instantaneous versus delayed galaxy mergers on the baryonic growth. Using a minimal set of mass-and zindependent free parameters associated with star formation and BH growth, and their associated feedback, and including suppressed BH growth in lower-mass galaxies, we show that our model successfully reproduces all available data sets for early galaxies and quasars. While both reionization and delayed galaxy mergers have no sensible impact on the evolving ultra-violet luminosity function, the impact of the former dominates in determining the stellar mass density for observed galaxies as well as the BH mass function. We then use this model to predict the LISA detectability of merger events at high-z. As expected, mergers of stellar BHs dominate the merger rates for all scenarios and our model predicts an expected upper limit of about 20 mergers using instantaneous merging and no reionization feedback over the 4-year mission duration. Including the impact of delayed mergers and reionization feedback provides about 12 events over the same observational time-scale.

Unveiling early black hole growth with multifrequency gravitational wave observations

“…The event rates in the LISA band are comparable to those obtained in other studies (e.g. Ricarte & Natarajan 2018;Bonetti et al 2019;Dayal et al 2019).…”

“…This has been investigated in a number of studies so far, under different assumptions and approaches regarding the merger timescales (e.g. Enoki et al 2005;Sesana et al 2011;Klein et al 2016;Tamanini et al 2016;Ricarte & Natarajan 2018;Bonetti et al 2019;Dayal et al 2019;Katz et al 2020;Volonteri et al 2020, and references therein).…”

“…Using the SAM Delphi, Dayal et al (2019) find that binaries with total masses of 10 3.5 − 10 5 M ⊙ are detectable, with a S/N > 7, in the redshift range z ∼ 5−13, with the large fraction being mergers of light seeds (called "Type 1" mergers). This is consistent with our predictions shown in Fig.…”

Black hole mergers from dwarf to massive galaxies with the NewHorizon and Horizon-AGN simulations

“…Recently Barausse et al (2020) include in a semi-analytical additional kpc-scale delays before formation of binaries, and show that the merger rate is then greatly reduced. Furthermore, the highest merger rate in semi-analytical models is predicted when including the mergers of the remnants of Population III stars in very high redshift galaxies (Sesana et al 2007;Klein et al 2016;Ricarte & Natarajan 2018;Dayal et al 2019), which we do not treat in our simulations (see Katz et al 2020, for a careful comparison between simulations and semi-analytical models).…”

Euclidpreparation

“…The challenge posed to the standard model of SMBH formation by Eddington-limited growth from stellar-mass seed black holes (e.g. Volonteri 2010), has led to investigation of the formation of massive (M > 10 4 M ) black-hole seeds through direct collapse (Bromm & Loeb 2003;Begelman et al 2006;Ferrara et al 2014;Dayal et al 2019), or rapid growth via periods of superor even hyper-Eddington accretion from lower-mass seeds (e.g. Ohsuga et al 2005;Inayoshi et al 2016).…”