Not-quite-primordial black holes

Abstract

We propose a new mechanism for the formation of seeds of supermassive black holes at early cosmic epochs. Our scenario explores density fluctuations that are enhanced relative to $\Lambda$CDM expectations, but with amplitudes that are not large enough to form primordial black holes, and that can still lead to collapsed dark matter halos at very early times. For halos forming prior to $1+z\approx 200$, the Cosmic Microwave Background (CMB) is energetic enough to suppress the formation of molecular hydrogen, hence preventing cooling and fragmentation, as a consequence of which baryons falling into the potential well of the halo may undergo ``direct collapse" into a black hole. We show using a few illustrative models how this mechanism may account for the abundance of high-redshift black holes inferred from observations by the \textit{James Webb Space Telescope} while remaining consistent with limits from CMB spectral distortions. Limits on the primordial power spectrum are also derived by requiring that the universe not re-ionize too early.

Comments: 11 pages, 5 figures