PREDICTING THE REDSHIFT 2 H alpha 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 alpha emitters observable by these future surveys. WISP is an ongoing Hubble Space Telescope slitless spectroscopic survey, covering the 0.8-1.65 mu m wavelength range and allowing the detection of H alpha emitters up to z similar to 1.5 and [O III] emitters to z similar to 2.3. We derive the H alpha-[O III] bivariate line luminosity function (LLF) for WISP galaxies at z similar to 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 alpha luminosity function by exclusively fitting [O III] data. Using the z similar to 2 [O III] LLF and assuming that the relation between H alpha and [O III] luminosity does not change significantly over the redshift range, we predict the H alpha number counts at z similar to 2-the upper end of the redshift range of interest for future surveys. For the redshift range 0.7 < z < 2, we expect similar to 3000 galaxies deg(-2) for a flux limit of 3 x 10(-16) erg s(-1) cm(-2) (the proposed depth of the Euclid galaxy redshift survey) and similar to 20,000 galaxies deg(-2) for a flux limit of similar to 10(-16) erg s(-1) cm(-2) (the baseline depth of the WFIRST galaxy redshift survey).