PREDICTING THE REDSHIFT 2 H α LUMINOSITY FUNCTION USING [O iii] EMISSION LINE GALAXIES

Upcoming space-based surveys such as Euclid and WFIRST-AFTA plan to measure baryonic acoustic oscillations in order to study dark energy. These surveys will use IR slitless grism spectroscopy to measure redshifts of a large number of galaxies over a significant redshift range. In this paper, we use the Wide Field Camera 3 Infrared Spectroscopic Parallel Survey (WISP) to estimate the expected number of H α emitters observable by these future surveys. WISP is an ongoing Hubble Space Telescope slitless spectroscopic survey, covering the 0.8–1.65 μ m wavelength range and allowing the detection of H α emitters up to z ∼ 1.5 and [O iii ] emitters to z ∼ 2.3. We derive the H α –[O iii ] bivariate line luminosity function (LLF) for WISP galaxies at z ∼ 1 using a maximum likelihood estimator that properly accounts for uncertainties in line luminosity measurements and we demonstrate how it can be used to derive the H α luminosity function by exclusively fitting [O iii ] data. Using the [O iii ] LLF and assuming that the relation between H α and [O iii ] luminosity does not change significantly over the redshift range, we predict the H α number counts at —the upper end of the redshift range of interest for future surveys. For the redshift range we expect ∼3000 galaxies deg −2 for a flux limit of 3 × 10 −16 erg s −1 cm −2 (the proposed depth of the Euclid galaxy redshift survey) and ∼20,000 galaxies deg −2 for a flux limit of ∼10 −16 erg s −1 cm −2 (the baseline depth of the WFIRST galaxy redshift survey).