Simon Foreman

Observations with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) have been used to measure the 21 cm intensity mapping autopower spectrum, at z ∼ 1, over a frequency range from 608.2 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 HI 2 ≡ 1 0 6 Ω HI 2 ( b HI 2 + 〈 f μ 2 〉 ) 2 , where Ω _HI is the cosmological density parameter for atomic hydrogen (H I ), b _HI is the linear bias for H I , and 〈 fμ 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 H I from the IllustrisTNG simulations, finding that the measurement disagrees with the TNG100 run at 3.1 σ and the TNG300 run at 4.0 σ . The disagreement is likely attributable to the effects of nonlinear H I clustering and redshift-space distortions in the simulations, rather than the total abundance of H I , and invites further investigation of the physical processes in the simulations that determine the behavior of H I 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 first detection of the cosmological 21 cm intensity mapping signal in autocorrelation 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 of 608.2–707.8 MHz ( z = 1.34–1.01) at 0.4 h Mpc ⁻¹ ≲ k ≲ 1.5 h Mpc ⁻¹ , with a detection significance of 12.4 σ . Our analysis employs significant improvements to the CHIME data processing pipeline compared to previous work, including novel radio frequency interference 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 8.6 σ and 9.1 σ , 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.

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 ^-1 Mpc, 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 Ly α 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.

We present the spatial part of the point-source detection strategy for the upcoming Canadian Hydrogen Observatory and Radio-transient Detector (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 total 6 meter dishes and observes in the 300–1500 MHz frequency range. One of its science goals is detecting galaxies by their neutral hydrogen (H I ) 21 cm spin-flip emission line. CHORD’s galaxy search will use a matched filter on visibility data. We present the search strategy, along with an analytic prediction tool that can efficiently estimate the output of the matched filter, without producing simulated visibility data. We use this to make quantitative predictions of the effects of spatial aliasing on the galaxy search and how these effects can be reduced by dithering the 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.

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.