HST Grism-derived Forecasts for Future Galaxy Redshift Surveys
Abstract
The mutually complementary Euclid and Roman galaxy redshift surveys will use Halpha- and [OIII]-selected emission line galaxies as tracers of the large scale structure at 0.9 z 1.9 (Halpha) and 1.5 z 2.7 ([OIII]). It is essential to have a reliable and sufficiently precise knowledge of the expected numbers of Halpha-emitting galaxies in the survey volume in order to optimize these redshift surveys for the study of dark energy. Additionally, these future samples of emission-line galaxies will, like all slitless spectroscopy surveys, be affected by a complex selection function that depends on galaxy size and luminosity, line equivalent width, and redshift errors arising from the misidentification of single emission-line galaxies. Focusing on the specifics of the Euclid survey, we combine two slitless spectroscopic WFC3-IR datasets -- 3D-HST+AGHAST and the WISP survey -- to construct a Euclid-like sample that covers an area of 0.56 deg2 and includes 1277 emission line galaxies. We detect 1091 (3270 deg-2) Halpha+[NII]-emitting galaxies in the range 0.9≤ z ≤ 1.6 and 162 (440 deg-2) [OIII]λ5007-emitters over 1.5≤ z ≤ 2.3 with line fluxes ≥ 2 × 10-16 erg s-1 cm-2. The median of the Halpha+[NII] equivalent width distribution is 250A, and the effective radii of the continuum and Halpha+[NII] emission are correlated with a median of 0.38" and significant scatter (σ 0.2"-0.35"). Finally, we explore the prevalence of redshift misidentification in future Euclid samples, finding potential contamination rates of 14-20% and 6% down to 2× 10-16 and 6 × 10-17 erg s-1 cm-2, respectively, though with increased wavelength coverage these percentages drop to nearly zero.