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=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.
