Seeds of supermassive black holes in general relativistic and alternative cosmologies: Implications of massive seeds

TL;DR

The paper investigates how massive seeds for high-redshift supermassive black holes can form under different cosmological backgrounds, including GR-based models ($\Lambda$CDM, $\omega$CDM, DDE) and a braneworld scenario. It combines Eddington-limited and super-Eddington accretion with variable black hole spin to derive seed masses required to reach $M_{\rm BH}\sim 10^8-10^9 M_\odot$ by $z_f=10$, starting from $z_i=30$, and shows seeds of $\gtrsim 10^4 M_\odot$ suffice in most cases, with lighter seeds possible under strong super-Eddington growth for high spins. The study also explores primordial black hole seeds, evaluating $f_{PBH}$ and $n_{PBH}$ under seed- and Poisson-effects, and examines the resulting gas accretion and $M_{\rm BH}/M_*$ in PBH-seeded halos, finding broad compatibility with observed high-redshift galaxy properties. Overall, cosmologies beyond $\Lambda$CDM yield only modest shifts in seed masses, while PBH-based seeding remains a plausible pathway for forming massive galaxies and predicting gravitational-wave signatures accessible to current detectors.

Abstract

Presence of supermassive black holes (SMBHs) with mass $(10^{6}-10^{9}) M_{\odot}$ at $z = 10$ has been recently revealed by James Webb Space Telescope (JWST) observations. In this study we generate seeds for the above range of SMBHs in various background cosmologies. We consider cosmic timescales required for black hole growth provided by three general relativistic cosmological models ($Λ$CDM, $ω$CDM and Dynamical Dark Energy(DDE) and the braneworld cosmology. The growth of SMBHs is studied through Eddington limited and super-Eddington accretion, where the accretion starts at z=30. It is found that growth of SMBHs by z=10 within Eddington limited accretion is possible through massive seeds $(M\geq10^{4}M_{\odot})$ in all cosmologies. Super Eddington accretion onto spinning black holes with mass of few tens of solar masses can result in SMBHs by z=10 in all cosmologies. The viable cosmologies considered here are found to be unable to strongly distinguish between the seed black hole masses. The seeds generated in this work are assumed to be of primordial origin in order to satisfy the criteria of formation of high redshift massive galaxies. The fraction of primordial black holes (PBHs) contributing to dark matter ($f_{PBH}$) and their corresponding number densities for the mass range ($10^{5}-10^{8}$) $M_{\odot}$ are calculated in both seed effect and Poisson effect. In seed effect, PBHs of mass $\geq 10^{7} M_{\odot}$ contributes $\leq 10^{-2}$ to the dark matter fraction. The evolution of gas mass inside a PBH seeded dark matter halo is studied. The ratio of black hole to stellar mass is also evaluated for star formation efficiency in the range (0.1-1) and found to be ($10^{-3}-1$) for $M_{BH}=10^{8} M_{\odot}$ and ($10^{-2}-10$) for $M_{BH}=10^{9} M_{\odot}$.

Figures (11)

• Figure 1: Mass of black hole as a function of redshift for the 34 black holes confirmed by JWST as detailed in Table 1 dayal2024exploring. The data points corresponds to asymmetric error bars based on broad Balmer lines, high-ionization lines or X-ray counterparts.
• Figure 2: Evolution of the expansion rate H(z) within the redshift slice (0-30) in various cosmologies
• Figure 3: Fractional difference of cosmic age relative to the $\Lambda$CDM
• Figure 4: Eddington fraction ($f$) vs black hole spin $(\chi)$ for different choices of the accretion ratio
• Figure 5: Seed masses required to form $(10^{8}-10^{9}) M_{\odot}$ black holes within the redshift slice $(z_{i}-z_{f})$ allowed for growth within Eddington limit in various cosmologies.