Intensity mapping with SDSS/BOSS Lyman-α emission, quasars, and their Lyman-α forest

We investigate the large-scale structure of Lyα emission intensity in the Universe at redshifts |z| = 2–3.5 using cross-correlation techniques. Our Lyα emission samples are spectra of BOSS Luminous Red Galaxies from Data Release 12 with the best-fitting model galaxies subtracted. We cross-correlate the residual flux in these spectra with BOSS quasars, and detect a positive signal on scales |1~ 15\, h⁻¹\, Mpc|⁠. We identify and remove a source of contamination not previously accounted for, due to the effects of quasar clustering on cross-fibre light. Corrected, our quasar – Lyα emission cross-correlation is 50 per cent lower than that seen by Croft et al. for DR10, but still significant. Because only |~ 3\ per\ cent| of space is within |15 \, h⁻¹\, Mpc| of a quasar, the result does not fully explore the global large-scale structure of Lyα emission. To do this, we cross-correlate with the Lyα forest. We find no signal in this case. The 95 per cent upper limit on the global Lyα mean surface brightness from Lyα emission – Lyα forest cross-correlation is |μ α < 1.2 × 10⁻²² \, erg\, s⁻¹\, cm⁻² \, A ⁻¹\, arcsec⁻²|⁠. This null result rules out the scenario where the observed quasar – Lyα emission cross-correlation is primarily due to the large-scale structure of star-forming galaxies. Taken in combination, our results suggest that Lyα emitting galaxies contribute, but quasars dominate within |15\, h⁻¹\, Mpc|⁠. A simple model for Lyα emission from quasars based on hydrodynamic simulations reproduces both the observed forest – Lyα emission and quasar – Lyα emission signals. The latter is also consistent with extrapolation of observations of fluorescent emission from smaller scales |r< 1\, h⁻¹\, Mpc|⁠.