Line intensity mapping using atomic hydrogen (HI) has the potential to efficiently map large volumes of the universe if the signal can be successfully separated from overwhelmingly bright radio foreground emission. This motivates cross-correlations, to ascertain the cosmological nature of measured HI fluctuations, and to study their connections with galaxies and the underlying matter density field. However, these same foregrounds render the cross-correlation with projected fields such as the lensing of the cosmic microwave background (CMB) difficult. Indeed, the correlated Fourier modes vary slowly along the line of sight, and are thus most contaminated by the smooth-spectrum radio continuum foregrounds. In this paper, we implement a method that avoids this issue by attempting to measure the non-linear gravitational coupling of the small-scale 21cm power from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) with large-scale Planck CMB lensing. This measurement is a position-dependent power spectrum, i.e. a squeezed integrated bispectrum. Using 94 nights of CHIME data between 1.0 < z < 1.3 and aggressive foreground filtering, we find that the expected signal is five times smaller than the current noise. We forecast that incorporating the additional nights of CHIME data already collected would enable a signal-to-noise ratio of 3, without any further improvements in filtering for foreground cleaning.

After reionization, neutral hydrogen (${\rm H\, \small {I}}$) traces the large-scale structure (LSS) of the Universe, enabling ${\rm H\, \small {I}}$ intensity mapping (IM) to capture the LSS in 3D and constrain key cosmological parameters. We present a new framework utilizing higher-order cross-correlations to study ${\rm H\, \small {I}}$ clustering around galaxies, tested using real-space data from the IllustrisTNG300 simulation. This approach computes the joint distributions of k-nearest neighbor (kNN) optical galaxies and the ${\rm H\, \small {I}}$ brightness temperature field smoothed at relevant scales (the kNN-field framework), providing sensitivity to all higher-order cross-correlations, unlike two-point statistics. To simulate ${\rm H\, \small {I}}$ data from actual surveys, we add random thermal noise and apply a simple foreground cleaning model, filtering out Fourier modes of the brightness temperature field with k∥ < kmin, ∥. Under current levels of thermal noise and foreground cleaning, typical of a Canadian Hydrogen Intensity Mapping Experiment (CHIME)-like survey, the ${\rm H\, \small {I}}$-galaxy cross-correlation signal in our simulations, using the kNN-field framework, is detectable at >30σ across r = [3, 12] h−1 Mpc. In contrast, the detectability of the standard two-point correlation function (2PCF) over the same scales depends strongly on the foreground filter: a sharp k∥ filter can spuriously boost detection to 8σ due to position-space ringing, whereas a less sharp filter yields no detection. Nonetheless, we conclude that kNN-field cross-correlations are robustly detectable across a broad range of foreground filtering and thermal noise conditions, suggesting their potential for enhanced constraining power over 2PCFs.

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 0.78–2.55 along 202 lines of sight toward known sources in the declination range of 35° to 60°. 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 H i 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.

Hybrid foreground residual subtraction (HyFoReS) is a new family of algorithms designed to remove systematics-induced foreground contamination for 21-cm intensity mapping data. Previously, the algorithm was shown to be effective in mitigating beam perturbations in sky maps from the Canadian Hydrogen Intensity Mapping Experiment (CHIME). In this study, we apply HyFoReS to CHIME simulations and test the algorithm's ability to mitigate antenna gain-type systematics in polarized visibilities. Simulating a two-cylinder telescope similar to the CHIME pathfinder, we find that HyFoReS reduces foreground bias caused by bandpass perturbations to a level below the thermal noise, provided that the rms value of the perturbations is on the order of 10-4 or lower. When tested with complex antenna-dependent gain errors, HyFoReS can reduce residual foreground bias in the power spectrum by up to 3 orders of magnitude. While noise bias and second-order perturbations are currently the limiting factors for the algorithm, we have demonstrated that HyFoReS can suppress gain-induced foreground leakage in polarized data from 21-cm telescopes, aiding in the detection of the 21-cm autopower spectrum for hydrogen intensity mapping experiments.

Observations with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) have been used to measure the 21 cm intensity mapping auto power spectrum, at z∼ 1, over a frequency range from 608.2 MHz to 707.8 MHz at wavenumbers 0.4 h Mpc^-1≲ k ≲ 1.5 h Mpc^-1. In this paper, we present the results of two different approaches to interpreting this measurement. In the first approach, we use a parametric power spectrum model to constrain an amplitude parameter, defined as 𝒜^2_ HI≡ 10^6 Ω_ HI^2(b^2_ HI+⟨ f μ^2⟩)^2, where Ω_ HI is the cosmological density parameter for atomic hydrogen (HI), b_ HI is the linear bias for HI, and ⟨ f μ^2⟩ incorporates the dominant large-scale impact of redshift-space distortions on the angle-averaged power spectrum. Imposing an additional prior on either Ω_ HI or b_ HI, based on values in the literature, allows us to break the pairwise degeneracy between those two parameters. In the second approach, we compare CHIME's measurement with predictions for the power spectrum of HI from the IllustrisTNG simulations, finding that the measurement disagrees with the TNG100 run at 3.1σ and the TNG300 run at 4.0σ. This disagreement is most likely attributable to the strength of nonlinear redshift-space clustering of HI in the simulations, rather than the total abundance of HI, and invites further investigation of the physical processes in the simulations that determine the behavior of HI at nonlinear scales. These results exemplify the ability of 21 cm intensity mapping to provide astrophysical information using measurements at nonlinear scales.

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.

Small-scale physics in the intergalactic medium (IGM) plays a crucial role in shaping the progress of cosmic reionization and several high-redshift observables that probe this period. Several recent studies have characterized the complex, dynamical response of the IGM to reionization down to kilo-parsec scales, including its effect on observables such as the Lyα forest. However, there has been no concentrated attempt to simulate and characterize these effects across the full parameter space of realistic large-scale IGM environments during reionization. To meet this need, we introduce the SAGUARO simulation suite, sub-titled “Simulating IGM Evolution and Environments At High Resolution”. SAGUARO is a suite of over two hundred high-resolution, coupled radiative-hydrodynamics simulations of IGM gas dynamics during and after reionization. The suite spans a grid of photoionization rates, redshifts of reionization, and box-scale densities. We also simulate other physical effects, such as X-ray pre-heating, recombination radiation, baryon-dark matter free-streaming, and alternative dark matter cosmologies. Our suite includes box sizes of 2 and 0.25 h^-1Mpc, extending to volumes large enough to begin capturing halos above the atomic cooling limit and resolutions high enough to fully resolve the IGM Jeans scale in the cold, neutral universe. We present a detailed description of the setup and first results from SAGUARO, descriptions of the IGM gas dynamics and thermal structure, opacity, self-shielding properties, the effect of the IGM on the reionization photon budget, and the halo mass function, and Lyα transmission properties. SAGUARO will help facilitate detailed studies of small-scale IGM structure and its effects that will help inform the next generation of reionization simulations and data interpretation.

Population studies of gas-rich galaxies across the full range of Neutral Hydrogen (HI) masses that galaxies are known to exhibit (10^5 ≲ M_HI≲ 10^11 M_⊙) remain limited by the need to conduct high-sensitivity, wide-band surveys across significant sky areas. The Canadian Hydrogen Observatory and Radio-transient Detector (CHORD) is a next-generation radio telescope that will significantly expand the census of HI-galaxies to date from untargeted drift-scan surveys at declinations +20^∘ < δ< +80^∘. We draw survey realizations from a known HI mass function (HIMF) to forecast HI detections in fiducial 1-year and 5-year CHORD surveys. The 5-year survey source counts is expected exceed currently available catalogs by roughly an order of magnitude, notwithstanding the potential impacts of radio frequency interference (RFI) and spectroscopic source confusion that we also estimate. We predict that CHORD will push the low-mass HI galaxy census to M_HI∼ 10^5.5 M_⊙, over an order of magnitude lower than has been previously achieved. At the high mass end of the HIMF, CHORD is expected to detect ∼10^3 massive gas-rich giants (M_HI≳ 10^10.5 M_⊙) at 0.3 ≲ z ≲ 0.5, which will explore the evolution of this population relative to local universe estimates. CHORD HI surveys will therefore improve our understanding of the neutral gas reservoirs at the low-mass and high-mass extremes of the galaxy population.

Full-shape analyses of 21 cm intensity maps with the effective field theory of large-scale structure will require priors on HI bias parameters, and the standard choice of broad uninformative priors can lead to cosmological constraints that are unnecessarily conservative. We present a simulation-based framework that replaces these broad priors with informative priors based on learning the conditional distribution p(_ EFT|_ HOD) between effective-field-theory-based bias parameters and the parameters of a model for HI clustering in the nonlinear regime. Specifically, we train a conditional normalizing flow on field-level measurements of the lowest-order local bias parameters (b_1,b_2,b_3) and the tidal bias b_𝒢_2 by applying a simple HI halo occupation distribution (HOD) to the Hidden Valley simulations. We find that the resulting HOD-to-bias mapping is highly structured, displaying a strong dependence on the power of halo mass in the HOD model. Propagating CHORD-like telescope sensitivity forecasts for the 21 cm power spectrum on nonlinear scales through this mapping produces non-Gaussian, correlated priors on the bias parameters that are substantially tighter than conventional flat priors across z=1–3, with the improvement most dramatic at high redshift. By repeating our analysis using halo catalogs from the IllustrisTNG simulations, we find non-negligible differences from the Hidden Valley results, indicating that future applications of simulation-based HI priors will need to carefully account for the dependence of these priors on the simulations used to construct them. Our framework provides an initial step toward informative EFT priors for current and forthcoming HI intensity mapping surveys, including CHIME, CHORD, and MeerKLASS.

The 21-cm line from neutral hydrogen has long been recognised as a promising tracer of the large-scale structure of the Universe. The line is weak however, making individual galaxy detections quite inefficient, especially at higher redshifts. The technique of 21-cm intensity mapping has been pioneered over the last two decades to address this limitation. Instead of detecting individual galaxies, the brightness temperature field from the combined 21-cm emission of many unresolved galaxies is mapped as a function of angle and frequency, resulting in 3D tracer maps of the large-scale structure. In this chapter, we review the major pioneering efforts to develop this observable into a competitive cosmological tool, paying particular attention to the status of pathfinder observations that have paved the way for a large and highly sensitive 21-cm intensity mapping survey with the SKA-Mid telescope.

Line intensity mapping using atomic hydrogen (H I ) has the potential to efficiently map large volumes of the Universe if the signal can be successfully separated from overwhelmingly bright radio foreground emission. This motivates cross correlations, to ascertain the cosmological nature of measured H I fluctuations, and to study their connections with galaxies and the underlying matter density field. However, these same foregrounds render the cross correlation with projected fields such as the lensing of the cosmic microwave background (CMB) difficult. Indeed, the correlated Fourier modes vary slowly along the line of sight and are thus most contaminated by the smooth-spectrum radio continuum foregrounds. In this paper, we implement a method that avoids this issue by attempting to measure the nonlinear gravitational coupling of the small-scale 21 cm power from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) with large-scale Planck CMB lensing. This measurement is a position-dependent power spectrum, i.e., a squeezed integrated bispectrum. Using 94 nights of CHIME data between 1.0 < z < 1.3 and aggressive foreground filtering, we find that the expected signal is 5 times smaller than the current noise. We forecast that incorporating the additional nights of CHIME data already collected would enable a signal-to-noise ratio of 3, without any further improvements in filtering for foreground cleaning.

The Canadian Hydrogen Observatory and Radio-transient Detector (CHORD) is a next-generation wideband radio interferometer currently being constructed and commissioned at the Dominion Radio Astrophysical Observatory in British Columbia, Canada. CHORD is designed for precision 21 cm cosmology, fast radio transient discovery, spectral line galaxy surveys, and pulsar science using a highly redundant large-N, small-diameter drift-scan array architecture. The telescope consists of a 512-element core array of 6 m dishes operating from 300–1500 MHz in drift-scan mode, together with two 64-dish outrigger stations located at the Hat Creek Radio Observatory and the Green Bank Observatory for long-baseline transient localization. The instrument supports multiple simultaneous digital backends for interferometric correlation, FRB detection, pulsar beamforming, and high spectral resolution surveys. CHORD is designed with an emphasis on precision beam control and stable instrumental response, incorporating lessons learned from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) while providing a substantial increase in sensitivity. Initial performance has been evaluated using a three-dish engineering array, and a 64-dish pathfinder array is currently being commissioned. The full array will be commissioned in 2028.

Extragalactic carbon monoxide (CO) line emission will likely be an important signal in current and future Cosmic Microwave Background (CMB) surveys on small scales. However, great uncertainty surrounds our current understanding of CO emission. We investigate the implications of this modeling uncertainty on CMB surveys. Using a range of star formation rate and luminosity relations, we generate a suite of CO simulations across cosmic time, together with the broadband cosmic infrared background (CIB). From these, we quantify the power spectrum signatures of CO that we would observe in a CMB experiment at 90, 150, and 220 GHz. We find that the resulting range of CO auto-spectra spans up to two orders of magnitude and that while CO on its own is unlikely to be detectable in current CMB experiments, its cross-correlation with the CIB will be a significant CMB foreground in future surveys. We then forecast the bias on CMB foregrounds that would result if CO were neglected in a CMB power spectrum analysis, finding shifts that can be comparable to some of the uncertainties on CMB foreground constraints from recent surveys, particularly for the thermal and kinetic Sunyaev-Zel'dovich effects and radio sources, and many times greater than the expected uncertainties expected for future data. Finally, we assess how the broad range of multifrequency CO×CIB spectra we obtain is captured by a reduced parameter set by performing a principal component analysis, finding that three amplitude parameters suffice for a CMB-S4-like survey. Our results demonstrate the importance for future CMB experiments to account for a wide range of CO modeling, and that high-precision CMB experiments may help constrain extragalactic CO models.

Hybrid Foreground Residual Subtraction (HyFoReS) is a new family of algorithms designed to remove systematics-induced foreground contamination for 21-cm intensity mapping data. Previously, the algorithm was shown to be effective in mitigating beam perturbations in sky maps from the Canadian Hydrogen Intensity Mapping Experiment (CHIME). In this study, we apply HyFoReS to CHIME simulations and test the algorithm's ability to mitigate antenna gain-type systematics in polarized visibilities. Simulating a two-cylinder telescope similar to the CHIME pathfinder, we find that HyFoReS reduces foreground bias caused by bandpass perturbations to a level below the thermal noise, provided that the RMS value of the perturbations is on the order of 10^-4 or lower. When tested with complex antenna-dependent gain errors, HyFoReS can reduce residual foreground bias in the power spectrum by up to three orders of magnitude. While noise bias and second-order perturbations are currently the limiting factors for the algorithm, we have demonstrated that HyFoReS can suppress gain-induced foreground leakage in polarized data from 21-cm telescopes, aiding in the detection of the 21-cm auto-power spectrum for hydrogen intensity mapping experiments.

We present the first detection of the cosmological 21 cm intensity mapping signal in auto-correlation at z 1 with the Canadian Hydrogen Intensity Mapping Experiment (CHIME). Using 94 nights of observation, we have measured the 21 cm auto-power spectrum over a frequency range from 608.2 MHz to 707.8 MHz (z = 1.34 to 1.01) at 0.4 h Mpc^-1 < k < 1.5 h Mpc^-1, with a detection significance of 12.5 sigma. Our analysis employs significant improvements to the CHIME data processing pipeline compared to previous work, including novel radio frequency interference (RFI) detection and masking algorithms, achromatic beamforming techniques, and foreground filtering before time averaging to minimize spectral leakage. We establish the robustness and reliability of our detection through a comprehensive suite of validation tests. We also measure the 21 cm signal in two independent sub-bands centered at z 1.08 and z 1.24 with detection significance of 8.7 sigma and 9.2 sigma, respectively. We briefly discuss the theoretical interpretation of these measurements in terms of a power spectrum model, deferring the details to a companion paper. This auto-power spectrum detection demonstrates CHIME's capability to probe large-scale structure through 21 cm intensity mapping without reliance on external galaxy surveys.

The main challenge of 21-cm cosmology experiments is astrophysical foregrounds that are difficult to separate from the signal due to telescope systematics. An earlier study has shown that foreground residuals induced by antenna gain errors can be estimated and subtracted using the hybrid foreground residual subtraction (HyFoReS) technique, which relies on cross-correlating linearly filtered data. In this paper, we apply a similar technique to the CHIME stacking analysis to subtract beam-induced foreground contamination. Using a linear high-pass delay filter for foreground suppression, the CHIME collaboration reported a 11.1 sigma detection in the 21-cm signal stacked on eBOSS quasar locations, despite foreground residual contamination mainly due to the instrument chromatic transfer function. Following the HyFoReS formalism, we cross-correlate the foreground-dominated data at low delay with the contaminated signal at high delay to estimate residual foregrounds and subtract them from the signal. We find that HyFoReS can improve the signal-to-noise ratio of the stacked 21-cm signal by 10%-20% after the delay foreground filter, although some of the improvement can also be achieved with an alternative flagging technique. We have shown that it is possible to use HyFoReS to reduce beam-induced foreground contamination, benefiting the analysis of the HI auto power spectrum with CHIME and enabling the of modes.

We present the first results of the holographic beam-mapping program for the Canadian Hydrogen Intensity Mapping Experiment (CHIME). We describe the implementation of a holographic technique as adapted for CHIME, and introduce the processing pipeline which prepares the raw holographic timestreams for analysis of beam features. We use data from six bright sources across the full 400-800 MHz observing band of CHIME to provide measurements of the copolar and cross-polar beam response in both amplitude and phase for all 1024 dual-polarized feeds in the array. In addition, we present comparisons with independent probes of the CHIME beam, which indicate the presence of polarized beam leakage. Holographic measurements of the beam have already been applied in science with CHIME, e.g., in estimating the detection significance of far-sidelobe fast radio bursts, and in validating the beam models used for CHIME's first detections of 21 cm emission (in cross-correlation with measurements of large-scale structure from galaxy surveys and the Ly alpha forest). Measurements presented in this paper, and future holographic results, will provide a unique data set to characterize the CHIME beam and improve the experiment's prospects for a detection of the baryon acoustic oscillation signal.

We report the detection of 21 cm emission at an average redshift z =2.3 in the cross-correlation of data from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) with measurements of the Ly alpha forest from eBOSS. Data collected by CHIME over 88 days in the 400-500 MHz frequency band (1.8 < z < 2.5) are formed into maps of the sky and high-pass delay filtered to suppress the foreground power, corresponding to removing cosmological scales with k(||) less than or similar to 0.13 Mpc(-1) at the average redshift. Line-of-sight spectra to the eBOSS background quasar locations are extracted from the CHIME maps and combined with the Ly alpha forest flux transmission spectra to estimate the 21 cm-Ly alpha cross-correlation function. Fitting a simulation-derived template function to this measurement results in a 9 sigma detection significance. The coherent accumulation of the signal through cross-correlation is sufficient to enable a detection despite excess variance from foreground residuals similar to 6-10 times brighter than the expected thermal noise level in the correlation function. These results are the highest-redshift measurement of 21 cm emission to date, and they set the stage for future 21 cm intensity mapping analyses at z > 1.8.

A direct consequence of Faraday rotation is that the polarized radio sky does not resemble the total intensity sky at long wavelengths. We analyze G137+7, which is undetectable in total intensity but appears as a depolarization feature. We use the first polarization maps from the Canadian Hydrogen Intensity Mapping Experiment. Our $400-729$ MHz bandwidth and angular resolution, $17'$ to $30'$, allow us to use Faraday synthesis to analyze the polarization structure. In polarized intensity and polarization angle maps, we find a "tail" extending $10^\circ$ from the "head" and designate the combined object the "tadpole". Similar polarization angles, distinct from the background, indicate that the head and tail are physically associated. The head appears as a depolarized ring in single channels, but wideband observations show that it is a Faraday rotation feature. Our investigations of H I and H$\alpha$ find no connections to the tadpole. The tail suggests motion of either the gas or an ionizing star through the ISM; the B2(e) star HD 20336 is a candidate. While the head features a coherent, $\sim -8$ rad m$^2$ Faraday depth, Faraday synthesis also identifies multiple components in both the head and tail. We verify the locations of the components in the spectra using QU fitting. Our results show that $\sim$octave-bandwidth Faraday rotation observations at $\sim 600$ MHz are sensitive to low-density ionized or partially-ionized gas which is undetectable in other tracers.