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_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.

17 pages, 10 figures, Accepted for publication in ApJ

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• The Assembly of the First Massive Black Holes
• Titans of the Early Universe: The Prato Statement on the Origin of the First Supermassive Black Holes
• RadioAstron Observations of the Quasar 3C273: a Challenge to the Brightness Temperature Limit
• First Identification of Direct Collapse Black Hole Candidates in the Early Universe in CANDELS/GOODS-S
• Lyman Alpha Signatures from Direct Collapse Black Holes
• Radiative effects during the assembly of direct collapse black holes
• Boosting Lya and HeII 1640A Line Fluxes from Pop III Galaxies: Stochastic IMF Sampling and Departures from Case-B
• Enhanced direct collapse due to Lyman alpha feedback
• Lyman-$\alpha$ feedback prevails at Cosmic Dawn: Implications for the first galaxies, stars, and star clusters
• Detection of Tidal Disruption Events around Direct Collapse Black Holes at High Redshifts with the James Webb Space Telescope
• Radio signals from early direct collapse black holes
• The mysterious $6565\mathring{A}$ absorption feature of the Galactic halo