Primordial seeds of supermassive black holes

TL;DR

SMBHs in galactic centers and the existence of high-redshift quasars motivate a primordial seed mechanism. The authors study a double-inflation framework that can produce a sharply peaked density-perturbation spectrum, yielding a narrow PBH mass function with $M_{\rm BH}$ around $10^{4-5} M_\odot$ to seed SMBHs while satisfying observational bounds. In a concrete smooth hybrid plus new inflation model, a peak at $k_p$ leads to PBHs that can evolve into SMBHs, but CMB $\mu$-distortion constraints require $M_{\rm BH} \lesssim 10^5 M_\odot$ and predict a negative running $d n_s/d\ln k \sim -0.01$ to $-0.02$, with potential signatures in 21-cm observations. Overall, the work provides a plausible, testable primordial SMBH seeding mechanism grounded in multi-stage inflation and consistent with current cosmological data.

Abstract

Supermassive black holes exist in the centers of galaxies, including Milky Way, but there is no compelling theory of their formation. Furthermore, observations of quasars imply that supermassive black holes have already existed at some very high redshifts, suggesting the possibility of their primordial origin. In a class of well-motivated models, inflationary epoch could include two or more periods of inflation dominated by different scalar fields. The transition between such periods of inflation could enhance the spectrum of density perturbations on some specific scale, which could lead to formation of primordial black holes with a very narrow range of masses of the order of 10^5 solar masses. These primordial black holes could have provided the requisite seeds for the observed population of supermassive black holes.

Figures (3)

• Figure 1: The devolved mass distribution of PBHs (on the left) that produces an acceptable evolution of mass and number density of black holes in galactic halos (right), according to Bean and Magueijo astro-ph/0204486. The data points are from Ref. 1992MNRAS.259..421C. The two distributions are for the limiting halo mass of $10^9M_\odot$ (dashed line) and $10^{10} M_\odot$ (solid line); see discussion in Ref. astro-ph/0204486.
• Figure 2: Power spectra of curvature perturbations for $\mu \simeq 2 \times 10^{-3}, v=\mu/4, \kappa = 0.05$ and $g=2\times 10^{-5}$. The solid, dashed and dashed dotted curves are for $M= 0.52, 0.59$ and $0.48$, and their peaks are responsible for formation of PBHs with mass $\simeq 5\times 10^{5}M_{\odot}, 5\times 10^{6}M_{\odot}$ and $5\times 10^{4}M_{\odot}$, respectively.
• Figure 3: Spectral index and the running parameter predicted in the smooth hybrid new inflation model for $M_{\rm BH} = 10^3M_{\odot}$ and $10^5M_{\odot}$. We also show the constraint from the combined analysis of WMAP and SPT data arXiv:1105.3182