Probing dark energy with future redshift surveys: a comparison of emission line and broad-band selection in the near-infrared

Future galaxy surveys will map the galaxy distribution in the redshift interval 0.5 < z < 2 using near-infrared cameras and spectrographs. The primary scientific goal of such surveys is to constrain the nature of the dark energy by measuring the large-scale structure of the Universe. This requires a tracer of the underlying dark matter which maximizes the useful volume of the survey. We investigate two potential survey selection methods: an emission-line sample based on the Hα line and a sample selected in the H band. We present predictions for the abundance and clustering of such galaxies, using two published versions of the GALFORM galaxy formation model. Our models predict that Hαselected galaxies tend to avoid massive dark matter haloes and instead trace the surrounding filamentary structure; H-band selected galaxies, on the other hand, are found in the highest mass haloes. This has implications for the measurement of the rate at which fluctuations grow due to gravitational instability. We use mock catalogues to compare the effective volumes sampled by a range of survey configurations. To give just two examples: a redshift survey down to H_AB = 22 samples an effective volume that is approximately five to 10 times larger than that probed by an Hα survey with log(F_Hα[ergs^-1cm^-2]) > -15.4 and a flux limit of at least log(F_Hα[ergs^-1cm^-2]) = -16 is required for an Hα sample to become competitive in effective volume.