High-redshift cosmology with oxygen lines from Hα surveys

A new generation of cosmological experiments will spectroscopically detect the Hα line from emission-line galaxies at optical/near-infrared frequencies. Other emission lines will also be present, which may come from the same Hα sample or constitute a new galaxy sample altogether. Our goal is to assess the value, for cosmological investigation, of galaxies at z ≳ 2 present in Hα galaxy surveys and identifiable by the highly redshifted ultraviolet and optical lines – namely the O ii line and the O iii doublet in combination with the Hβ line. We use state-of-the-art luminosity functions to estimate the number density of O iii + Hβ and O ii ELGs. We study the constraining power of these high-redshift galaxy samples on cosmological parameters such as the baryon acoustic oscillation (BAO) amplitude, H(z), DA(z), fσ8(z), and bσ8(z) for different survey designs. We present a strong science case for extracting the O iii + Hβ sample, which we consider as an independent probe of the Universe in the redshift range 2−3. Moreover, we show that the O ii sample can be used to measure the BAO and growth of structure above z = 3; albeit it may be shot-noise dominated, it will none the less provide valuable tomographic information. We discuss the scientific potential of a sample of galaxies, which, so far, has been mainly considered as a contaminant in Hα galaxy surveys. Our findings indicate that planed Hα surveys should include the extraction of these oxygen-line samples in their pipeline, to enhance their scientific impact on cosmology.