3-cm Fine structure masers: a unique signature of supermassive black hole formation via direct collapse in the early universe

Abstract

The direct collapse black hole (DCBH) scenario describes the isothermal collapse of a pristine gas cloud directly into a massive, ${M}_{\mathrm{BH}}\;=$ 104–106${M}_{\odot }$ black hole. In this paper we show that large H i column densities of primordial gas at $T\sim {10}^{4}$ K with low molecular abundance—which represent key aspects of the DCBH scenario—provide optimal conditions for the pumping of the 2p-level of atomic hydrogen by trapped Lyα photons. This Lyα pumping mechanism gives rise to an inverted level population of the $2{s}_{1/2}-2{p}_{3/2}$ transition, and therefore also gives rise to stimulated fine structure emission at $\lambda =3.04\;\mathrm{cm}$ (rest-frame). We show that simplified models of the DCBH scenario amplify the CMB by up to a factor of $\sim {10}^{5}$, 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 μJy, 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 could provide direct evidence for the DCBH scenario.