Non-gravitational contributions to the clustering of Lyα selected galaxies: implications for cosmological surveys

We show that the dependence of Ly alpha absorption on environment leads to significant non-gravitational features in the redshift space power spectrum of Ly alpha selected galaxies. We derive a physically motivated fitting formula that can be included in clustering analyses, and use this to discuss the predicted features in the Ly alpha galaxy power spectrum based on detailed models in which Ly alpha absorption is influenced by gas infall and/or by strong galactic outflows. We show that power-spectrum measurements could be used to study the astrophysics of the galaxy-intergalactic medium (IGM) connection, and to measure the properties of outflows from star-forming galaxies. Applying the modified redshift space power spectrum to a Ly alpha survey with parameters corresponding to the planned Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), we find that the dependence of observed Lya flux on velocity gradient and ionizing background may compromise the ability of Ly alpha selected galaxy redshift surveys to constrain cosmology using information from the full power spectrum. This is because the effects of fluctuating ionizing background and velocity gradients affect the shape of the observed power spectrum in ways that are similar to the shape of the primordial power spectrum and redshift space distortions, respectively. We use the Alcock-Paczynski test to show that without prior knowledge of the details of Ly alpha absorption in the IGM, the precision of line of sight and transverse distance measurements for HETDEX will be similar to 1.3-1.7 per cent, decreased by a factor of similar to 1.5-2 relative to the best case precision of similar to 0.8 per cent available in a traditional galaxy redshift survey. We specify the precision with which modelling of Ly alpha radiative transfer must be understood in order for HETDEX to achieve distance measurements that are better than 1 per cent.