Very high redshift quasars and the rapid emergence of super-massive black holes

Abstract

The observation of quasars at very high redshift such as Pōniuā’ena is a challenge for models of super-massive black hole (SMBH) formation. This work presents a study of SMBH formation via known physical processes in star-burst clusters formed at the onset of the formation of their hosting galaxy. While at the early stages hyper-massive star-burst clusters reach the luminosities of quasars, once their massive stars die, the ensuing gas accretion from the still forming host galaxy compresses its stellar black hole (BH) component to a compact state overcoming heating from the BH–BH binaries such that the cluster collapses, forming a massive SMBH-seed within about a hundred Myr. Within this scenario the SMBH–spheroid correlation emerges near-to-exactly. The highest-redshift quasars may thus be hyper-massive star-burst clusters or young ultra-compact dwarf galaxies (UCDs), being the precursors of the SMBHs that form therein within about 200 Myr of the first stars. For spheroid masses ≲ 109.6 M⊙ a SMBH cannot form and instead only the accumulated nuclear cluster remains. The number evolution of the quasar phases with redshift is calculated and the possible problem of missing quasars at very high redshift is raised. SMBH-bearing UCDs and the formation of spheroids are discussed critically in view of the high redshift observations. A possible tension is found between the high star-formation rates (SFRs) implied by downsizing and the observed SFRs, which may be alleviated within the IGIMF theory and if the downsizing times are somewhat longer.

The Lifetimes of Star Clusters Born with a Top-heavy IMF

“…As a result of the removal of stars from the outer parts of a cluster by the tidal field of the host galaxy and spiraling of stellar-mass BHs toward the center of their clusters due to dynamical friction, the BHs that formed through stellar evolution of the most massive stars segregate into the cluster's core and form a subsystem of BHs (or "dark star clusters," as named for the first time by Banerjee & Kroupa 2011). The formation of such a BH subsystem is already well studied in star clusters with a canonical IMF (Banerjee & Kroupa 2011;Breen & Heggie 2013) and is also of interest in the context of galactic nuclei (Kroupa et al 2020). In the future, we will explore the effect of the top-heaviness of the IMF, as well as different values of the compact remnant retention fraction, on the formation and the lifetime of dark star clusters.…”

“…The formation of a central compact remnant subcluster might be relevant for the finding of the central peak in dispersion velocity of some GCs. The relevance of a top-heavy IMF for the formation of supermassive BHs has been discussed by Kroupa et al (2020). Figure 11 compares the present-day global MF slope (in the mass range 0.3 − 0.8M of the computed models, with the eyeball fit to the observed data of Galactic GCs (shown as a red line) from De Marchi et al (2007) and Paust et al (2010).…”

Rates of compact object coalescences

“…Given the many orders of magnitude uncertainties, we resist the temptation to review the merger rates of binaries involving IMBHs. Similarly, we do not specifically discuss hierarchical mergers of stellar-mass black holes or IMBHs as a channel for forming supermassive back holes at high redshifts (e.g., Volonteri 2010;Kroupa et al 2020). The heaviest merging binaries observed through gravitational waves could represent the tail end of this process of hierarchical massive black-hole formation associated with mergers of ultra-dwarf galaxies (Palmese and Conselice 2021), and many more such mergers may be observable with third-generation gravitational-wave detectors (Gair et al 2009).…”

Quasar clustering at redshift 6

“…Particularly pressing issues over the last years are the formation channels and efficiencies of massive BHs, and their growth rates with cosmic time (e.g., Johnson et al 2013;Valiante et al 2017Valiante et al , 2018Inayoshi et al 2020;Kroupa et al 2020). Quantifying the number of QSO pairs at high redshift is one observational avenue to constrain a combination of parameters of BH formation and BH growth rate.…”