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 optical tracers, this feature is detectable independently of the distance to the absorber. 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. In this thesis, I present the CHIME Absorber Project: a blind survey for 21-cm absorption conducted with CHIME, a transit radio telescope operating at 400-800 MHz, corresponding to the redshift range 0.78 < z < 2.55.
I describe the design and implementation of an offline processing pipeline that processes raw high-spectral-resolution data from the CHIME Absorber backend, corrects for instrumental systematics, and applies a search algorithm to the resulting spectra. Candidate absorption features identified by the algorithm then undergo visual inspection for confirmation. This pipeline represents the primary technical contribution of the thesis.
I then present a pilot spectrally blind survey using four months of CHIME Absorber data, which yielded a new 21-cm absorber at z = 2.327, only the fifth detection of an absorber associated with a quasar at z > 2. The survey also recovered two previously known intervening absorbers, validating the pipeline.
Finally, I describe a follow-up search that applied the pipeline to absorption candidates identified in CHIME intensity mapping data, confirming three new absorbers, all at z > 1. Together, these four new absorbers represent a roughly 10 per cent increase in the known sample at z > 1, obtained from a small fraction of the available data. This demonstrates the potential of CHIME to substantially expand the census of high-redshift 21-cm absorbers through a fully blind survey.
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2026-07-23T13:00:002026-07-23T14:00:00Search for 21-cm absorption systems with CHIMEEvent Information:
Abstract:
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 optical tracers, this feature is detectable independently of the distance to the absorber. 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. In this thesis, I present the CHIME Absorber Project: a blind survey for 21-cm absorption conducted with CHIME, a transit radio telescope operating at 400-800 MHz, corresponding to the redshift range 0.78 < z < 2.55.
I describe the design and implementation of an offline processing pipeline that processes raw high-spectral-resolution data from the CHIME Absorber backend, corrects for instrumental systematics, and applies a search algorithm to the resulting spectra. Candidate absorption features identified by the algorithm then undergo visual inspection for confirmation. This pipeline represents the primary technical contribution of the thesis.I then present a pilot spectrally blind survey using four months of CHIME Absorber data, which yielded a new 21-cm absorber at z = 2.327, only the fifth detection of an absorber associated with a quasar at z > 2. The survey also recovered two previously known intervening absorbers, validating the pipeline.Finally, I describe a follow-up search that applied the pipeline to absorption candidates identified in CHIME intensity mapping data, confirming three new absorbers, all at z > 1. Together, these four new absorbers represent a roughly 10 per cent increase in the known sample at z > 1, obtained from a small fraction of the available data. This demonstrates the potential of CHIME to substantially expand the census of high-redshift 21-cm absorbers through a fully blind survey.
Event Location:
HENN 309