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

The direct collapse black hole (DCBH) scenario describes the isothermal collapse of a pristine gas cloud directly into a massive, {M}_{BH} = 10⁴-10⁶{M}_⊙ black hole. In this paper we show that large H I 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 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 λ =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 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.