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 $0.9\lesssim z\lesssim 1.9$ (Halpha) and $1.5\lesssim z\lesssim 2.7$ ([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 deg${}^2$ and includes 1277 emission line galaxies. We detect 1091 ($\sim$3270 deg${}^{-2}$) Halpha+[NII]-emitting galaxies in the range $0.9\le z\le 1.6$ and 162 ($\sim$440 deg${}^{-2}$) [OIII]$\lambda$5007-emitters over $1.5\le z\le 2.3$ with line fluxes $\ge 2\times 10^{-16}$ erg s${}^{-1}$ cm${}^{-2}$. The median of the Halpha+[NII] equivalent width distribution is $\sim$250Å, and the effective radii of the continuum and Halpha+[NII] emission are correlated with a median of $\sim$0.38" and significant scatter ($\sigma \sim$0.2"$-$0.35"). Finally, we explore the prevalence of redshift misidentification in future Euclid samples, finding potential contamination rates of $\sim$14-20% and $\sim$6% down to $2\times 10^{-16}$ and $6\times 10^{-17}$ erg s${}^{-1}$ cm${}^{-2}$, respectively, though with increased wavelength coverage these percentages drop to nearly zero.

Accepted for publication in ApJ. 27 pages, 17 figures, 3 tables

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