Haochen Wang

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.

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.

In recent years, localized fast radio bursts (FRBs) have emerged as a powerful tool to study the structure of the baryonic matter in the Universe. Their dispersion measures (DMs) scale linearly with electron density independent of gas temperature, making them particularly well suited to studying the intragroup medium (IGrM), where traditional probes such as X-ray emission and the Sunyaev–Zeldovich effect are weak. Evidence suggests that the gas in group mass halos ( M ₅₀₀ ∼ 10 ¹³ –10 ¹⁴ M _⊙ ) is strongly affected by galactic feedback, causing deviations from cluster scaling relations. Three FRBs from the first CHIME/FRB Outrigger sample come from host galaxies found within or behind galaxy clusters and groups. We estimate the DM contribution of each ICM/IGrM by integrating different halo density profiles, accounting for uncertainties in halo mass and the host galaxy line-of-sight distance. For the more massive halos, predicted cluster DMs agree with the extragalactic DM budget. One burst, FRB 20230703A, intersects three groups yet has a low extragalactic DM. By comparing model predictions with the measured DM, we constrain the gas mass fraction f _g ( R ) in these halos. Comparing with published M – f _g relations, we find consistency with recent eROSITA results at R ₅₀₀ and mild tension at R ₂₀₀ and with earlier X-ray–based relations. As CHIME/FRB Outriggers build a large catalog of localized FRBs, many additional sightlines through groups and clusters will be obtained. These will enable systematic tests of intragroup and intracluster gas properties and sharpen constraints on the distribution of baryons in massive halos.

We present a catalog of 4539 fast radio bursts (FRBs) observed with the Canadian Hydrogen Intensity Mapping Experiment telescope between 2018 July 25 and 2023 September 15. These bursts originate from 3641 unique sources, including 981 bursts from 83 known repeating sources. For each FRB, the catalog provides an O ( 1 0 ′ ) estimate of sky location along with corresponding measurements of cumulative exposure time and survey sensitivity over the observing period. It includes a total-intensity dynamic spectrum between 400 and 800 MHz at 0.983 ms resolution. From this spectrum, we constrain a model of the burst morphology and measure key parameters such as arrival time, intrinsic temporal width, dispersion measure, scattering time, and flux density. This second catalog includes all FRBs from the first catalog, with every event reprocessed using a uniform and improved analysis framework. We show that previously published inferences remain valid under the updated measurements. We assess consistency of the detection rate across observational parameters, present initial distributions of burst properties, and outline ongoing and future studies that will use this catalog to investigate the nature of FRBs and their utility as astrophysical and cosmological probes.

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.

The dispersion measures (DMs) of fast radio bursts (FRBs) can be used as powerful probes of the distribution of extragalactic plasma. With a large enough sample, the free-electron–galaxy power spectrum P _eg can be measured by cross-correlating FRB DMs with galaxy positions. However, a precise measurement of P _eg requires a careful investigation of selection effects: the probability of both observing the FRB DM and obtaining a host galaxy redshift depends on their properties. We ray trace through the magnetohydrodynamic simulation IllustrisTNG to investigate the impact of expected observational selection effects on FRB dispersion–galaxy angular cross-correlations with a sample of 3000 FRBs at 0.3 ≤ z ≤ 0.4. Our results show that cross-correlations with such an FRB sample are robust to properties of the FRB host galaxy: this includes DM contributions from the FRB host and optical follow-up selection effects. We also find that such cross-correlations are robust to DM-dependent and scattering selection effects specific to the CHIME/FRB survey. However, a DM-dependent selection effect that cuts off the 10% most dispersed FRBs at a fixed redshift shell can bias the amplitude of the cross-correlation signal by over 50% at angular scales of ∼0 . ° 1, corresponding to ∼Mpc physical scales. Our findings highlight the importance of both measuring and accounting for selection effects present in existing FRB surveys, as well as mitigating DM-dependent selection effects in the design of upcoming FRB surveys aiming to probe large-scale structure with FRBs.

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) has emerged as the world’s premier facility for studying fast radio bursts (FRBs) through its fast transient search backend CHIME/FRB. The CHIME/FRB Outriggers project will augment this high detection rate of 2–3 FRBs per day with the ability to precisely localize them using very long baseline interferometry (VLBI). Using three strategically located stations in North America and deploying recently developed synoptic VLBI observing techniques, the Outriggers will provide ∼50 mas localization precision for the majority of detected FRBs. This paper presents an overview of the design and implementation of the Outriggers, covering their geographic distribution, structural design, and observational capabilities. We detail the scientific objectives driving the project, including the characterization of FRB populations, host galaxy demographics, and the use of FRBs as cosmological probes. We also discuss the calibration strategies available to mitigate ionospheric and instrumental effects, ensuring high-precision localization. With two stations currently in science operations, and the third in commissioning, the CHIME/FRB Outriggers project is poised to become a cornerstone of the FRB field, offering unprecedented insights into this enigmatic cosmic phenomenon.

Low-redshift fast radio bursts (FRBs) enable robust measurements of the host galaxy contribution to the dispersion measure (DM), offering valuable constraints on the circumgalactic medium (CGM) of FRB hosts. We curate a sample of 20 nearby FRBs with low scattering timescales and face-on host galaxies with stellar masses in the range 10⁹ < M ^*/M _⊙ < 10¹¹. We fit the distribution of the host galaxy DM to a quadratic model as a function of stellar mass with a mass-independent scatter and find that the more massive the host, the lower its host DM. We report that this relation has a negative slope of m = −97 ± 44 pc cm⁻³ dex⁻¹ in stellar mass. We compare this measurement against similar fits to three subgrid models implemented in the CAMELS suite of simulations from Astrid, IllustrisTNG, and SIMBA, which predict the CGM contribution to this relation, finding disagreement with the fiducial CAMELS-Astrid model, particularly for the most massive hosts (M ^* > 10^10.5 M _⊙). More generally, models that attribute a positive correlation between stellar mass and host DM (m > 0) to the CGM are in tension with our measurement unless compensated by fine-tuning of the host interstellar medium contribution as a function of stellar mass, e.g., at the low-mass end. We show that this conclusion is robust to a wide range of assumptions, such as the offset distribution of FRBs from their hosts and the statistics of the cosmic contribution to the DM budget along each sight line. Our results indirectly imply a lower limit on the strength of baryonic feedback in the local Universe (z < 0.2) in isolated ∼L ^* halos, complementing results from weak-lensing surveys and kSZ observations that target higher halo mass and redshift ranges.