Matched Filtering for the Canadian Hydrogen Observatory and Radio-Transient Detector Galaxy Search

TL;DR

This paper develops a matched filtering strategy for the CHORD radio telescope to detect galaxies via the 21 cm line, addressing spatial aliasing issues and proposing an optimal observation strategy.

Contribution

It introduces a novel spatial signal extraction method using matched filters and a prediction tool for alias mitigation in the CHORD survey.

Findings

Aliasing is distinguishable over time with data combination.

Offset scanning improves alias resolution.

Optimal re-pointing in declination reduces degeneracy.

Abstract

We present the spatial part of the point source signal extraction strategy for the upcoming CHORD galaxy survey. CHORD, the Canadian Hydrogen Observatory and Radio-transient Detector, is an under-construction drift-scanning compact interferometric radio telescope. CHORD comprises 512 six meter dishes and observes in the 300 to 1500 MHz frequency range. One of its science goals is producing a catalogue of galaxies detected by the neutral hydrogen (HI) 21 cm emission line. CHORD's highly redundant dish layout creates the problem of spatial aliasing, the effect where the same signal could be feasibly produced from sources at multiple locations on the sky. The search will be done with a matched filter in the visibility plane. This paper presents the search strategy and a prediction tool that can quickly estimate the matched filter response at a given sky position, allowing a prediction of alias locations and severity. This tool confirms that although aliases are impossible to distinguish in a single snapshot, they become possible to distinguish when combining data over a period of time. It predicts that aliases will be harder to distinguish for observations closer to the celestial equator, but that scanning with offset adjacent strips can remove this degeneracy. It predicts that the optimal strategy for a single offset to disambiguate aliases is to re-point the array in declination by about two degrees. A future paper will combine these findings with realistic noise estimates and galaxy population statistics to make forecasts of the population of galaxies that CHORD will detect.