A study of selection methods for Hα-emitting galaxies at z ∼ 1.3 for the Subaru/FMOS galaxy redshift survey for cosmology (FastSound)

The efficient selection of high-redshift emission galaxies is important for future large galaxy redshift surveys for cosmology. Here we describe the target selection methods for the FastSound project, a redshift survey for Hα-emitting galaxies at z = 1.2–1.5 using Subaru/FMOS to measure the linear growth rate fσ8 via redshift space distortion and constrain the theory of gravity. To select ∼ 400 target galaxies in the 0.2 deg2 FMOS field of view from photometric data of CFHTLS-Wide (u*g ′ r ′ i ′ z ′), we test several different methods based on color–color diagrams or photometric redshift estimates from spectral energy distribution (SED) fitting. We also test the improvement in selection efficiency that can be achieved by adding near-infrared data from the UKIDSS DXS (J). The success rates of Hα detection with FMOS averaged over two observed fields using these methods are 11.3% (color–color, optical), 13.6% (color–color, optical + NIR), 17.3% (photo-z, optical), and 15.1% (photo-z, optical + NIR). Selection from photometric redshifts tends to give a better efficiency than color-based methods, although there is no significant improvement by adding J-band data within the statistical scatter. We also investigate the main limiting factors for the success rate, by using the sample of the HiZELS Hα emitters that were selected by narrow-band imaging. Although the number density of total Hα emitters having higher Hα fluxes than the FMOS sensitivity is comparable with the FMOS fiber density, the limited accuracy of photometric redshift and Hα flux estimations have comparable effects on the success rate of ≲ 20% obtained from SED fitting.