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

We investigate the large-scale structure of Ly alpha emission intensity in the Universe at redshifts z = 2-3.5 using cross-correlation techniques. Our Ly alpha 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 similar to 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 - Ly alpha emission cross-correlation is 50 per cent lower than that seen by Croft et al. for DR10, but still significant. Because only similar to 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 Ly alpha emission. To do this, we cross-correlate with the Ly alpha forest. We find no signal in this case. The 95 per cent upper limit on the global Ly alpha mean surface brightness from Ly alpha emission - Ly alpha forest cross-correlation is < 1.2 x 10(-22) erg s(-1) cm(-2) angstrom(-1) arcsec(-2). This null result rules out the scenario where the observed quasar - Ly alpha emission cross-correlation is primarily due to the large-scale structure of star-forming galaxies. Taken in combination, our results suggest that Ly alpha emitting galaxies contribute, but quasars dominate within 15 h(-1) Mpc. A simple model for Ly alpha emission from quasars based on hydrodynamic simulations reproduces both the observed forest - Ly alpha emission and quasar - Ly alpha emission signals. The latter is also consistent with extrapolation of observations of fluorescent emission from smaller scales r < 1 h(-1) Mpc.