3-cm FINE STRUCTURE MASERS: A UNIQUE SIGNATURE OF SUPERMASSIVE BLACK HOLE FORMATION VIA DIRECT COLLAPSE IN THE EARLY UNIVERSE

摘要

The direct collapse black hole (DCBH) scenario describes the isothermal collapse of a pristine gas cloud directly into a massive, 104–106 black hole. In this paper we show that large H i column densities of primordial gas at 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 transition, and therefore also gives rise to stimulated fine structure emission at (rest-frame). We show that simplified models of the DCBH scenario amplify the CMB by up to a factor of , 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.