Direct Collapse Pre-supermassive Black Hole Objects as Ly$α$ Emitters

Yang Luo, Isaac Shlosman

First listed 2025-06-23 | Last updated 2025-07-13

Abstract

The Direct Collapse scenario to form the supermassive black hole (SMBH) seeds offers the most promising way to explain the origin of quasars at $z>7$. Assuming atomic primordial gas, can Ly$α$ photons escape from the central regions of the collapse and serve as a diagnostic for the detection of these pre-SMBH objects? Models of spherical collapse have found these photons to be trapped and destroyed. We use Ly$α$ radiation transfer within the inflow-outflow geometry, based on earlier zoom-in cosmological modeling involving radiation transfer and magnetic forces. Adopting geometry that includes ongoing disk and spherical accretion, and formation of a biconical outflow funnel, we obtain the formation of a dense radiatively driven expanding shell. The Ly$α$ transfer is performed using a Monte Carlo algorithm, accounting for the destruction of Ly$α$ photons and the emergence of two-photon emission. We find that a substantial fraction of Ly$α$ photons can escape through the funnel and calculate the line profiles, the line peak velocity shift, asymmetry, and cuspiness, by varying basic model parameters. The escaping Ly$α$ emission is anisotropic and sensitive to the overall inflow-outflow geometry. The escaping fraction of Ly$α$ radiation exceeds 95% from a $z=10$ pre-SMBH object -- in principle detectable by the JWST NIRSpec in the MOS mode, during $\sim 10^4$ seconds for a $10σ$ signal-to-noise ratio. Moreover, comparisons with line shapes from high-$z$ galaxies and quasars allow us to separate them from pre-SMBH objects based on the line shape: the pre-SMBH lines show a profound asymmetry and extended red tail.

Short digest

The authors post-process a direct-collapse pre-SMBH inflow–outflow geometry with a Monte Carlo Lyα transfer that includes photon destruction and two-photon emission, finding that a radiatively driven shell plus a biconical polar funnel enables substantial escape. The emergent Lyα is highly anisotropic and line shapes depend on funnel opening angle and outer column density, yielding strong asymmetry with an extended red tail and measurable peak shifts/cuspiness. The escaping fraction can exceed 95% for a z=10 pre-SMBH object, with only the red wing detectable. Such sources are in principle reachable with NIRSpec/MOS in ~10^4 s at 10σ, and their line profiles provide a diagnostic to separate them from high-z galaxies and quasars.

Key figures to inspect

• Figure 1: Use the schematic to locate the radiatively driven shell and the polar funnel relative to the disky and spherical inflows; this sets the sightlines where Lyα escape and strong anisotropy are expected.
• Figure 2: Compare the Monte Carlo escape fractions against the Neufeld (1990) curves across column density and ionization fraction to see where f_esc > 0.95 occurs and how neutral vs. ionized gas controls the outcome.
• Figure 3: Track how changing the funnel opening angle and the outer optical depth reshapes the profile—peak velocity shift, cuspiness, and the strength/extent of the red tail—highlighting signatures distinctive of pre-SMBH funnels.
• Figure 4: Read off the fraction of intrinsic flux emerging in the red wing and the predicted integrated two-photon flux to gauge detectability and the balance between red-wing line emission and two-photon continuum.