Empirical Hα emitter count predictions for dark energy surveys

Future galaxy redshift surveys aim to measure cosmological quantities from the galaxy power spectrum. A prime example is the detection of baryonic acoustic oscillations, providing a standard ruler to measure the dark energy equation of state, w(z), to high precision. The strongest practical limitation for these experiments is how quickly accurate redshifts can be measured for sufficient galaxies to map the large-scale structure. A promising strategy is to target emission-line (i.e. star-forming) galaxies at high redshift (z ~ 0.5-2) not only is the space density of this population increasing out to z ~ 2, but also emission lines provide an efficient method of redshift determination. Motivated by the prospect of future dark energy surveys targeting Hα emitters at near-infrared wavelengths (i.e. z > 0.5), we use the latest empirical data to model the evolution of the Hα luminosity function out to z ~ 2 and thus provide predictions for the abundance of Hα emitters for practical limiting fluxes. We caution that the estimates presented in this work must be tempered by an efficiency factor, ɛ, giving the redshift success rate from these potential targets. For a range of practical efficiencies and limiting fluxes, we provide an estimate of , where is the 3D galaxy number density and P_0.2 is the galaxy power spectrum evaluated at k = 0.2hMpc^-1. Ideal surveys must provide in order to balance shot-noise and cosmic variance errors. We show that a realistic emission-line survey (ɛ = 0.5) could achieve out to z ~ 1.5 with a limiting flux of 10^-16ergs^-1cm^-2. If the limiting flux is a factor of 5 brighter, then this goal can only be achieved out to z ~ 0.5, highlighting the importance of survey depth and efficiency in cosmological redshift surveys.