3-cm Fine Structure Masers: A Unique Signature of Supermassive Black Hole Formation via Direct Collapse in the Early Universe

TL;DR

This paper proposes that 3-cm fine structure maser emission, caused by Lyman alpha pumping in primordial gas clouds, could serve as a direct observational signature of supermassive black hole formation via direct collapse in the early universe.

Contribution

It introduces a novel mechanism linking Lyman alpha pumping to 3-cm maser emission as a signature of direct collapse black holes, providing a potential observational probe.

Findings

Masers can be amplified up to 10^5 times the CMB intensity.

The 3-cm line profile is broad and asymmetric, with fluxes detectable by SKA1-MID.

Detection of this line would directly support the DCBH formation scenario.

Abstract

The direct collapse black hole (DCBH) scenario describes the isothermal collapse of a pristine gas cloud directly into a massive, M_BH=10^4-10^6 M_sun black hole. In this paper we show that large HI column densities of primordial gas at T~10^4 K with low molecular abundance - which represent key aspects of the DCBH scenario - provide optimal conditions for pumping of the 2p-level of atomic hydrogen by trapped Lyman alpha (Lya) photons. This Lya pumping mechanism gives rise to inverted level population of the 2s_1/2-2p_3/2 transition, and therefore to stimulated fine structure emission at 3.04 cm (rest-frame). We show that simplified models of the DCBH scenario amplify the CMB by up to a factor of 10^5, above which the maser saturates. Hyperfine splitting of the 3-cm transition gives rise to a characteristic broad (FWHM ~ tens of MHz in the observers frame) asymmetric line profile. This signal subtends an angular scale of ~ 1-10 mas, which translates to a flux of ~ 0.3-3 microJy, which is detectable with ultra-deep surveys being planned with SKA1-MID. While challenging, as the signal is visible for a fraction of the collapse time of the cloud, the matching required physical conditions imply that a detection of the redshifted 3-cm emission line would provide direct evidence for the DCBH scenario.