Lyman-alpha forest-CMB cross-correlation and the search for the ionised baryons at high redshift

The intergalactic neutral hydrogen which is responsible for the Lyα forest of quasar absorption lines is a tracer of much larger amounts of ionised hydrogen. The ionised component has yet to be detected directly, but is expected to scatter CMB photons via the Sunyaev-Zel’dovich effect. We use hydrodynamic simulations of a LambdaCDM universe to create mock quasar spectra and CMB sky maps from the same volume of space. We find that the high redshift Lyα forest gas causes temperature fluctuations of the order of 1 μK rms in the CMB on arcmin scales. The kinetic and thermal SunyaevZel’dovich effects have a similar magnitude at redshift three, with the thermal effect becoming relatively weaker as expected at higher redshift. The CMB signal associated with lines of sight having HI column densities > 10cm is only marginally stronger than that for lower column density sightlines. There is a much more significant dependence of rms temperature fluctuation on mean Lyα absorbed flux, however, suggesting that the CMB signal effectively arises in lower density material. We investigate the extent to which it is possible to cross-correlate information from the Lyα forest and the microwave background to detect the Sunyaev-Zel’dovich effect at redshifts 2 − 4. In so doing, we are enable to set direct limits on the density of diffuse ionised intergalactic baryons. We carry out a preliminary comparison at a mean redshift z = 3 of 3488 quasar spectra from SDSS Data Release 3 and the WMAP first year data. Assuming that the baryons are clustered as in a LambdaCDM cosmology, and have the same mean temperature, the cross-correlation yields a weak limit on the cosmic density of ionised baryons Ωb,I. As a fraction of the critical density, we find Ωb,I < 0.8 at 95% confidence. With data from upcoming CMB telescopes, we anticipate that a direct detection of the high redshift ionised IGM will soon be possible, providing an important consistency check on cosmological models.