Primordial black holes as supermassive black holes seeds (2411.03448v1)

Abstract: The presence of supermassive black holes (SMBHs, $M_{\bullet}\sim 10^{6-10}~M_{\odot}$) in the first cosmic Gyr ($z\gtrsim 6$) challenges current models of BH formation and evolution. We propose a novel mechanism for the formation of early SMBH seeds based on primordial black holes (PBHs). We assume a non-Gaussian primordial power spectrum as expected in inflationary models; these scenarios predict that PBHs are initially clustered and preferentially formed in the high-$\sigma$ fluctuations of the large-scale density field, out of which dark matter (DM) halos are originated. Our model accounts for (i) PBH accretion and feedback, (ii) DM halo growth, and (iii) gas dynamical friction. PBHs lose angular momentum due to gas dynamical friction, sink into a dense core, where BH binaries form and undergo a runaway merger, eventually leading to the formation of a single, massive seed. This mechanism starts at $z\sim 20-40$ in rare halos ($M_h\sim 10^7\ M_\odot$ corresponding to $\sim 5-7\sigma$ fluctuations), and provides massive ($\sim 10^{4-5}~ M_{\odot}$) seeds by $z\sim 10-30$. We derive a physically-motivated seeding prescription that provides the mass of the seed, $ M_{\rm seed}(z)=3.1\times 10^{5}\ { M_{\odot}}[(1+z)/10]^{-1.2}$, and seeded halo, $ M_{h}(z)=2\times 10^{9}\ {M_{\odot}}[(1+z)/10]^{-2}e^{-0.05z}$ as a function of redshift. This seeding mechanism requires that only a small fraction of DM is constituted by PBHs, namely $f_{\rm PBH}\sim 3 \times 10^{-6}$. We find that $z\sim 6-7$ quasars can be explained with $6\times 10^4 M_{\rm \odot}$ seeds planted at $z\sim 32$, and growing at sub-Eddington rates, $\langle\lambda_{\rm E}\rangle\sim 0.55$. The same scenario reproduces the BH mass of GNz11 at $z=10.6$, while UHZ1 ($z=10.1$) and GHZ9 ($z=10$) data favour instead slightly later ($z\sim 20-25$), more massive ($10^5~M_{\rm \odot}$) seeds. [Abridged]

• The paper proposes a model where primordial black holes (PBHs) clustered by non-Gaussian inflation serve as seeds for supermassive black holes (SMBHs) in the early universe.
• The model combines astrophysical processes like PBH accretion, feedback, dark matter halo growth, and dynamical friction, leading to a runaway merger process that forms massive seeds.
• Quantitative prescriptions for seed masses (e.g., M_seed(z)=3.1 x 10^5 M sun [(1+z)/10]^-1.2) and host halo masses are provided, explaining observed high-redshift SMBHs and predicting detectable gravitational waves.

Primordial Black Holes as Supermassive Black Hole Seeds: A Novel Seeding Mechanism

The paper by Ziparo et al. proposes a compelling model for the formation of supermassive black holes (SMBHs) in the early universe, attributing their origins to primordial black holes (PBHs). This model is founded on a variant of the inflationary scenario, characterized by the generation of a non-Gaussian primordial power spectrum. Such a scenario results in the initial clustering of PBHs within the high-density peaks of the large-scale density field. These nascent PBHs are then thought to catalyze the formation of SMBHs by undergoing a rapid accretion and merger process within early dark matter (DM) halos.

Key Components of the Proposed Model

1. PBH Origin and Distribution: In the non-Gaussian scenarios of inflation, PBHs are initially clustered, preferring the high-sigma fluctuations that eventually give rise to DM halos. This initial condition sets the stage for the subsequent dynamics and interactions that lead to the formation of SMBHs.
2. Astrophysical Processes: The model intricately combines several astrophysical mechanisms, specifically PBH accretion, feedback processes, DM halo growth, and gas dynamical friction. These processes collectively facilitate the sinking of PBHs to the halo center and their coalescence into a single massive black hole.
3. Runaway Merger Process: Due to gas dynamical friction, PBHs gradually lose angular momentum, accumulating within compact cores where PBH binaries form and progressively merge. This runaway merger process is crucial for creating a singular, massive seed that could grow into an SMBH.
4. Evolutionary Timeline: The formation of massive BH seeds, as advanced by the authors, commences in halos of order $10^7 M_\odot$ at redshifts $z\sim 20-40$. This provides seeds with masses in the range of $10^{4-5} M_{\odot}$, a promising starting point for the observed SMBHs at high redshifts ($z\sim 6-10$).

Numerical Findings and Predictions

The paper provides quantitative prescriptions for the seed masses and the corresponding host halo masses as functions of redshift, given by $$M_{\rm seed}(z)=3.1 \times 10^{5} M_{\odot} [(1+z)/10]^{-1.2}$$ and $$M_{h,\rm seed}(z)=2 \times 10^{9} M_{\odot}[(1+z)/10]^{-2} e^{-0.05z}$$, respectively. This model can account for the observed masses of SMBHs at $z \sim 6-10$, using an average sub-Eddington accretion rate of $\langle \lambda_{\rm E} \rangle \sim 0.55$. Particularly, SMBHs observed in GNz11 at $z=10.6$, along with high-redshift quasars such as UHZ1 ($z=10.1$) and GHZ9 ($z=10$), are successfully explained by seeds from slightly later epochs and with slightly more massive profiles.

Implications and Future Directions

The findings propose intriguing implications for cosmology and the understanding of high-redshift SMBH formation. The proposed seeding model offers an alternative to conventional seed formation scenarios, such as those based on PopIII star remnants, nuclear star clusters (NSCs), or direct collapse black holes (DCBHs). By embedding itself in a robust theoretical framework of primordial non-Gaussianities, the PBH model could provide insights into inflationary physics and gravitational wave observations. The expected gravitational radiation during the runaway PBH mergers could be observable with future gravitational wave detectors, like the Einstein Telescope, yielding direct constraints on inflationary scenarios and PBH properties.

In summary, this paper provides a sophisticated framework for SMBH seed formation through a novel PBH-based mechanism, grounded in early universe physics. The model's synthesis of detailed astrophysical processes, along with its alignment with observations of distant quasars, highlights its potential as a significant contribution to the understanding of the cosmic history of structure formation. Further developments in observational cosmology and gravitational wave astronomy could further validate or refine this theory.