Forecasts for Broadband Intensity Mapping of the Ultraviolet-Optical Background with CASTOR and SPHEREx

Abstract

Broadband tomography statistically extracts the redshift distribution of frequency dependent emission from the cross correlation of intensity maps with a reference catalog of galaxy tracers. We make forecasts for the performance of future all-sky UV experiments doing broadband tomography. We consider the Cosmological Advanced Survey Telescope for Optical-UV Research (CASTOR) and the Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx). The dominant uncertainty is from variability in the photometric zero point, which scales with limiting magnitude and so mirror size. With this scaling, and assuming a galaxy number density characteristic of future spectroscopic datasets, we find that CASTOR measures the UV background SED $2-10$ times better than existing data. The applicable redshift range will expand from the current $z < 1$ to $z \approx 0-3$ with CASTOR and $z =5-9$ with SPHEREx. We show that CASTOR can provide competitive constraints on the EBL monopole to those available from galaxy number counts and direct measurement techniques. At high redshift especially, these results will help understand galaxy formation and reionization. Our modelling code and chains are publicly available.