Search for 21cm absorption systems with CHIME

The 21 cm hyperfine transition of neutral hydrogen is a powerful probe of cold gas across cosmic time. When a radio source lies behind a cloud of neutral hydrogen, the cold gas imprints a characteristic absorption feature in the spectrum of the background source at the redshifted 21 cm frequency. Unlike 21 cm emission line, the absorption feature is detectable independently of the distance to the absorber, making 21 cm absorption equally sensitive at low and high redshift. Despite this advantage, the known sample of extragalactic 21 cm absorbers at redshifts z > 1 has remained small: only 42 systems were known prior to this work, reflecting the limitations of targeted surveys and the scarcity of suitable radio facilities at the relevant frequencies. This thesis presents the CHIME Absorber Project: a blind survey for 21 cm absorption conducted with the Canadian Hydrogen Intensity Mapping Experiment (CHIME), a transit radio telescope operating at 400–800 MHz, corresponding to the redshift range of 0.78 < z < 2.55 for the 21 cm line. An offline processing pipeline was developed to process raw archived data and return a list of absorber candidates for visual inspection. The design and implementation of this pipeline represent the primary technical contribution of this thesis. A pilot spectrally blind survey along 202 lines of sight, using four months of CHIME Absorber data, yielded a new associated 21 cm absorber at z = 2.327 toward NVSS J164725-+375218, only the fifth detection of an associated absorber at z > 2. Two previously known intervening absorbers were also recovered, validating the pipeline. Moreover, a follow-up search using the Absorber pipeline, targeting absorption candidates identified in CHIME intensity mapping data, confirmed three new 21 cm absorbers at z = 1.1235, z = 1.1577, and z = 1.1747. The discovery of these four new 21 cm absorbers represents a roughly 10 per cent increase in the known sample at z > 1, obtained from a small fraction of the available data, demonstrating the potential of CHIME to substantially expand the census of high-redshift 21 cm absorbers through a fully blind survey.