The mutually complementary Euclid and Roman galaxy redshift surveys will use H \alpha - and [ \ion O3 ] -selected emission line galaxies as tracers of the large scale structure at 0.9 \lesssim z \lesssim 1.9 ( H \alpha ) and 1.5 \lesssim z \lesssim 2.7 ( [ \ion O3 ] ) .
It is essential to have a reliable and sufficiently precise knowledge of the expected numbers of H \alpha -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 } ) H \alpha + [ \ion N2 ] -emitting galaxies in the range 0.9 \leq z \leq 1.6 and 162 ( \sim 440 deg ^ { -2 } ) [ \ion O3 ] \lambda 5007 -emitters over 1.5 \leq z \leq 2.3 with line fluxes \geq 2 \times 10 ^ { -16 } erg s ^ { -1 } cm ^ { -2 } .
The median of the H \alpha + [ \ion N2 ] equivalent width distribution is \sim 250 Å , and the effective radii of the continuum and H \alpha + [ \ion N2 ] emission are correlated with a median of \sim 0 \farcs 38 and significant scatter ( \sigma \sim 0 \farcs 2 - 0 \farcs 35 ) .
Finally , we explore the prevalence of redshift mis-identification 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 .