CHORD HI-Galaxy Survey Forecasts: Searching for nearby dark galaxies and high redshift giants
Population studies of gas-rich galaxies across the full range of Neutral Hydrogen (HI) masses that galaxies are known to exhibit ($10^5 \lesssim M_{\mathrm{HI}} \lesssim 10^{11} M_{\odot}$) remain limited by the need to conduct high-sensitivity, wide-band surveys across significant sky areas. The Canadian Hydrogen Observatory and Radio-transient Detector (CHORD) is a next-generation radio telescope that will significantly expand the census of HI-galaxies to date from untargeted drift-scan surveys at declinations $+20^{\circ} < δ< +80^{\circ}$. We draw survey realizations from a known HI mass function (HIMF) to forecast HI detections in fiducial 1-year and 5-year CHORD surveys. The 5-year survey source counts is expected exceed currently available catalogs by roughly an order of magnitude, notwithstanding the potential impacts of radio frequency interference (RFI) and spectroscopic source confusion that we also estimate. We predict that CHORD will push the low-mass HI galaxy census to $M_{\mathrm{HI}} \sim 10^{5.5} M_{\odot}$, over an order of magnitude lower than has been previously achieved. At the high mass end of the HIMF, CHORD is expected to detect $\sim10^{3}$ massive gas-rich giants ($M_{\mathrm{HI}} \gtrsim 10^{10.5}\, M_{\odot}$) at $0.3 \lesssim z \lesssim 0.5$, which will explore the evolution of this population relative to local universe estimates. CHORD HI surveys will therefore improve our understanding of the neutral gas reservoirs at the low-mass and high-mass extremes of the galaxy population.