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 |$1\sim 15\, h^{-1}\, {\rm Mpc}$|⁠. 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 |${\sim }3{{\ \rm per\ cent}}$| of space is within |$15 \, h^{-1}\, {\rm Mpc}$| 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 |$\langle \mu _{\alpha } \rangle \lt 1.2 \times 10^{-22} \, {\rm erg}\, {\rm s}^{-1}\, {\rm cm}^{-2} \, {\mathring{\rm A} }^{-1}\, {\rm arcsec}^{-2}$|⁠. 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 |$15\, h^{-1}\, {\rm Mpc}$|⁠. 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 |$r\lt 1\, h^{-1}\, {\rm Mpc}$|⁠.