Tracking Hydrogen Through the Cosmic Ages

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The recently commenced Canadian Hydrogen Observatory and Radio-transient Detector (CHORD) provides an unprecedented opportunity to piece together an uninterrupted story of how hydrogen evolved through cosmic time, encompassing nearby observations of nearby hydrogen-rich galaxies that can be resolved and studied in detail as well as surveys of the distant universe where the collective radio emission from these galaxies is examined statistically ("intensity mapping") to probe the large-scale distribution of matter---in principle. In practice, there are two hurdles to realizing this promise. First, there does not exist a unified theoretical framework for exploiting the complementarity of the two types of observations. Second, even with such a framework, our ability to extract science will only be as good as our ability to deal with instrumental systematics in CHORD. In this proposal we address both challenges. On the theoretical front we will establish an analysis framework capable of self-consistently modelling both individual nearby galaxies and the intensity mapping of distant populations. Using this, we will then answer key questions of how empirical constraints on galaxy properties at low redshifts (such as how hydrogen mass traces total mass in a galaxy) can be used to place priors on high-redshift intensity mapping models. With such calibrated models we will use end-to-end simulations of CHORD observations to critically examine the way in which individual galaxy observations and intensity mapping can complement each other. We will produce a series of definitive forecasts that include systematics in a way that is faithful to what will be seen in CHORD. Additionally, we confront our framework with the ultimate test of real data obtained from CHORD's early commissioning observations.