Predicting Hα emission-line galaxy counts for future galaxy redshift surveys

Knowledge of the number density of Ha emitting galaxies is vital for assessing the scientific impact of the Euclid and Wide Field Infrared Survey Telescope (WFIRST) missions. In this work, we present predictions from a galaxy formation model, GALACTICUS, for the cumulative number counts of Ha-emitting galaxies. We couple GALACTICUS to three different dust attenuation methods and examine the counts using each method. A chi(2) minimisation approach is used to compare the model predictions to observed galaxy counts and calibrate the dust parameters. We find that weak dust attenuation is required for the GALACTICUS counts to be broadly consistent with the observations, though the optimum dust parameters return large values for chi(2), suggesting that further calibration of GALACTICUS is necessary. The model predictions are also consistent with observed estimates for the optical depth and the Ha luminosity function. Finally, we present forecasts for the redshift distributions and number counts for two Euclid-like and one WFIRST-like surveys. For a Euclid-like survey with redshift range of 0.9 <= z <= 1.8 and H alpha + [N II] blended flux limit of 2 x 10(-16) erg s(-1) cm(-2), we predict a number density between 3900 and 4800 galaxies per square degree. For a WFIRST-like survey with redshift range of 1 <= z <= 2 and blended flux limit of 1 x 10(-16) erg s(-1) cm(-2), we predict a number density between 10400 and 15200 galaxies per square degree.