Discovery of a 21 cm absorption system at z=2.327 with CHIME

TL;DR

This work presents a pilot spectrally blind search for 21 cm HI absorption with CHIME, targeting $0.78<z<2.55$ along 202 bright background sources. By processing four months of data and employing a matched-filter search, continuum corrections, and Gaussian decomposition constrained by the Bayesian Information Criterion, the study identifies three absorbers: two intervening systems previously known and one new associated absorber at $z=2.32743$ toward NVSS J164725+375218. The associated absorber exhibits a two-component profile with a total width of ${ m ~}\sim 69~\rm km\,s^{-1}$ and a small velocity offset relative to the host, consistent with circumnuclear disk gas, while the two re-detections validate CHIME's absorber pipeline. The results demonstrate CHIME's potential to expand the high-redshift HI absorber census and inform models of neutral gas in AGN environments, with plans to scale the survey to larger data volumes and broader sky coverage, including sources lacking optical redshifts.

Abstract

We report the detection of a new 21 cm absorption system associated with the radio source NVSS J164725+375218 at a redshift of z=2.327, identified through a pilot survey conducted by the Canadian Hydrogen Intensity Mapping Experiment (CHIME). This is the fifth detection of an associated system at z > 2. By analyzing a subset of available data, we conduct a spectrally blind survey for 21 cm absorption systems within the redshift range of 0.78 to 2.55 along 202 lines of sight toward known sources in the declination range of 35 to 60 degrees. We detect three 21 cm absorbers: two previously known intervening systems and one newly discovered associated system. By fitting the absorption profiles with models containing one to three Gaussian components and selecting the best model using the Bayesian information criterion, we estimate the optical depth, velocity-integrated optical depth, and the ratio between the HI column density and the spin temperature of the absorption systems. These results demonstrate CHIME's ability to discover new absorbers, even in a small subset of its full dataset.

Figures (5)

• Figure 1: Detection of a 21 cm absorption feature in the spectrum of the radio source NVSS J164725+375218 from October 2021 (orange) and April 2023 (blue) data. Frequencies in the top and bottom panels are presented in the observer and barycentric frames, respectively. The raw spectrum is normalized by the continuum value at the absorption frequency, with the continuum derived by fitting a polynomial of degree 6, excluding 400 around the absorption feature. Note that the barycentric correction involves interpolating data at the barycentric frequencies, which explains why the shapes of the curves in the two panels are not identical.
• Figure 2: Detection of Hi absorption at $z=2.32743$ after combining October 2021 and April 2023 data. The top axis shows the velocity of the absorbing gas relative to the redshift of the absorber (total redshift in Table \ref{['summary_table']}). The vertical black dotted line corresponds to the redshift of the host radio source. The vertical axis represents the normalized spectrum, which is the ratio of the spectrum to the estimated continuum level at the absorption line's peak.
• Figure 3: Detection of two known 21 cm absorbers. Both spectra utilize December 2021 and 2023 data. The top and bottom axes in each panel are velocity in the rest frame of the absorption system and frequency in the barycentric frame, respectively.
• Figure 4: Top panel: Absorption toward NVSS J031443+431405 at z = 2.290 overplotted with digitized data from yorkDiscovery21cmAbsorption2007 (red). Bottom panel: Absorption toward NVSS J235421+455304 at z = 0.780 overplotted with digitized data from darlingDetection21Centimeter2004 (red).
• Figure 5: Number of features with positive and negative amplitudes as a function of their peak-to-rms value. Positive amplitude features represent emission-like signals, which are not expected in our survey and thus serve as a measure of false positives. Negative amplitude features follow the same trend, except for three outliers. Gaussian fits to the negative and positive feature distributions are shown by black solid and black dashed lines, respectively. The Gaussian fit to the negative feature distribution confirms that the three absorption candidates that passed visual inspection are significant outliers to the distribution of false positives.