High-redshift cosmology with oxygen lines from H$\alpha$ surveys
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(2020)cite arxiv:2001.04473Comment: 9 pages, 6 figures, 2 tables; comments welcome.

A new generation of cosmological experiments will spectroscopically detect the H$\alpha$ line from emission-line galaxies (ELGs) at optical/near-infrared frequencies. Other emission lines will also be present, which may come from the same H$\alpha$ sample or constitute a new galaxy sample altogether. Our goal is to assess the value, for cosmological investigation, of galaxies at $z\gtrsim2$ present in H$\alpha$ galaxy surveys and identifiable by the highly redshifted ultra-violet and optical lines - namely the OII line and the OIII doublet in combination with the H$\beta$ line. We use state-of-the-art models of luminosity functions of astrophysical spectral lines to estimate the volumetric number density of OIII+H$\beta$ and OII ELGs. We focus on a wavelength range which will be covered by planned cosmological surveys. We study the constraining power of these high-redshift galaxy samples on cosmological parameters such as the amplitude of baryon acoustic oscillations, $H(z)$, $D_A(z)$, $f\sigma_8(z)$, and $b\sigma_8(z)$ for different survey designs. We present a strong science case for extracting the OIII+H$\beta$ sample, which we consider as an independent probe of the Universe in the redshift range 2 to 3. Moreover, we show that the OII sample can be used to measure the baryon acoustic oscillations and the growth of structures above $z=3$; albeit it may be shot-noise dominated, it will nonetheless provide valuable tomographic information. Summarising, we discuss the scientific potential of a sample of galaxies which, so far, has been mainly considered as a contaminant in H$\alpha$ galaxy surveys. Our findings indicate that planed H$\alpha$ surveys should include the extraction of these oxygen-line samples in their pipeline, to enhance their scientific impact on cosmology.
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