Alex Schwaner Hill

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.

Faraday rotation of diffuse Galactic synchrotron emission is a powerful tracer of the complex, magnetised interstellar medium (ISM), whose structures span a wide range of spatial scales, requiring both interferometric and single-antenna broadband radio polarisation observations for full characterisation. We compared Faraday rotation in the interferometric LOw-Frequency ARray Two-Metre Sky Survey (LoTSS; 120–168 MHz) and the single-antenna Dominion Radio Astrophysical Observatory Global Magneto-Ionic Medium Survey of the Northern Sky (DRAGONS; 350–1030 MHz), which are complementary in their sensitivity to spatial and Faraday-depth scales. We calculated first moments (M1) of polarised intensity (PI) versus Faraday depth, thus producing PI–weighted mean Faraday depth maps of the regions common to both surveys. These maps show remarkable agreement between the surveys despite the lack of overlap in frequency or spatial-scale coverage. In the northern Galactic region the M1 maps are morphologically similar, with only small spatial shifts in the boundaries between positive and negative M1 and strong pixel-by-pixel correlation. In the southern Galactic region, both surveys trace the Faraday-depth gradient with Galactic longitude previously identified in LoTSS. Faraday depth spectra show consistent numbers and locations of peaks for more than half of the pixels. The strong structural similarity between the surveys, demonstrated by computing structure functions, suggests coupling across spatial scales in the magnetised ISM, thus enabling both interferometric and single-antenna observations to trace the same features. Instances of differences point to ISM configurations where observational effects such as depolarisation dominate or where this coupling breaks down due to local physical conditions.

The Wisconsin H α Mapper surveyed the entire Galactic sky in H α (∣ v _LSR ∣ ≲ 100 km s ⁻¹ ) to approximately 0.1 Rayleigh ( R ), albeit with a 1° beam. Reynolds et al. (2005) reported “point sources” which stood out against the Galactic background in space and velocity. Half of the sources are associated with plausible planetary nebulae and OB stars. Reynolds et al. suggested sub dwarfs for one quarter of the sources. Here, we investigate one such source, WPS 46, for which Reynolds et al. suggested the sub-dwarf PG 0931+691 to provide the source of ionization. With the Keck Cosmic Web Imager we found numerous nebular emission lines within the vicinity of WPS 46, but we failed to find H α emission in the arc-minute vicinity of PG 0931+691. The line ratios (BPT diagram and [S ii ]/H α ) combined with the morphology are more consistent with AGN or LI(N)ER-like ionization than with pure warm ionized medium or H ii region-like photoionization. Separately, we offer compelling reasons to argue that PG 0931+691 cannot be the source of ionizing power for WPS 46. We suggest that WPS 46 is associated with an intermediate velocity complex (IVC) and that H α and nebula emission may arise as a result of a shock. We conclude by outlining a plan of action of using SDSS’s Local Volume Mapper along with deep narrow band imagery obtained by amateur astronomers to explore and study the ionized sky on sub-degree scales, in general, and specifically studies of IVC and high-velocity complexes.

We developed a computer vision-based methodology to achieve precise 3D segmentation and tracking of superbubbles within magnetohydrodynamic simulations of the supernova-driven interstellar medium. Leveraging advanced 3D transformer models, our approach effectively captures the complex morphology and dynamic evolution of these astrophysical structures. To demonstrate the technique, we specifically focused on a superbubble exhibiting interesting interactions with its surrounding medium, driven by a series of successive supernova explosions. Our model successfully generated detailed 3D segmentation masks, enabling us to visualize and analyze the bubble’s structural evolution over time. The results reveal insights into the superbubble’s growth patterns, energy retention, and interactions with the surrounding interstellar matter. This interdisciplinary approach not only enhances our understanding of superbubble dynamics but also offers a robust framework for investigating other complex phenomena in the cosmos.

We present a distance-resolved reconstruction of the local line-of-sight Galactic magnetic field, B_||, by combining a 3D electron density (n_e) map derived from dust map-informed simulations and a full-sky map of Faraday rotation measure (RM). The forward model evaluates RM on the same 3D grid as the n_e map and compares to the Galactic Faraday rotation sky. We infer B_|| with a Gaussian-process prior whose power spectrum is inferred from the data using geometric variational inference. The result is a local (within 1.25 kpc where b > 5^○) map of B_|| with uncertainties. The reconstructed RM sky reproduces prominent features of Faraday rotation sky, with a root mean square average strength of B_|| of |$1.63\pm 0.16\rm \mu G$|⁠. In face-on views, the magnetic field exhibits coherent patches with alternating sign and hints of kpc-scale modulations, but with significant structure seen on scales of order 100 pc. The B_|| field is seen to exhibit a 3D power spectrum with an average slope of −2.73 ± 0.19. We validate our B_|| reconstruction with Galactic pulsars. Predicted RMs (computed by integrating n_eB_|| to each pulsar's distance) correlates with observed RMs, and predicted dispersion measures (DMs) from the n_e map also correlate with measured DMs, albeit with significant scatter.

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.

We probe the three-dimensional geometry of the large-scale Galactic magnetic field within 1 kpc of the Sun using the Dominion Radio Astrophysical Observatory Global Magneto-Ionic Medium Survey (GMIMS) of the Northern Sky (DRAGONS). DRAGONS is a new full polarization survey of the northern sky from 350 to 1030 MHz covering decl. of –20° < δ < 90° and a component of GMIMS. The first moment of the Faraday depth (FD) spectra produced from DRAGONS above 500 MHz reveals large-angular-scale FD structures with signs that alternate only once in the southern Galactic hemisphere and twice in the northern hemisphere, patterns shared by other Faraday rotation datasets. DRAGONS is the first survey to achieve high FD resolution while maintaining sensitivity to broad FD structures, enabling the first use of Galactic longitude–FD plots. These plots reveal Faraday-complex structures across the sky, indicating a slablike scenario in which emission and Faraday rotation are mixed. This complexity is overlaid on the same large-scale FD patterns that appear in the first moment map. We model these patterns as a magnetic reversal slicing through the disk on a diagonal and passing above the Sun in Galactic coordinates. We describe this reversal as a plane with a normal vector parallel to the line directed along ( ℓ , b ) = (168 . ° 5, −60°) and estimate its distance to be between 0.25 and 0.55 kpc. Our results show that much of the observed Faraday sky may be dominated by the local magnetic field configuration.

Polarized synchrotron emission at meter to centimeter wavelengths provides an effective tracer of the Galactic magnetic field. Calculating Faraday depth, the most useful parameter for mapping the line-of-sight magnetic field, requires observations covering wide frequency bands with many channels. As part of the Global Magneto-Ionic Medium Survey (GMIMS), we have observed polarized emission spanning 350–1030 MHz over the northern sky, in the declination range −20° ≤ δ ≤ 90°. We used the 15 m telescope at the Dominion Radio Astrophysical Observatory (DRAO), equipped to receive orthogonal circular polarizations, with the Onsala Space Observatory band 1 feed developed for the SKA Project. Angular resolution varies across the band from 1 . ° 3 to 3 . ° 6. A digital spectrometer provided 42 kHz frequency resolution. Data were taken with the telescope moving rapidly in azimuth and are absolutely calibrated in intensity. Approximately 25% of the data were lost due to radio-frequency interference. The resolution in Faraday depth is ∼6 rad m ⁻² , and features as wide as ∼38 rad m ⁻² are represented. The median sensitivity of the Faraday depth cube is 11 mK. Approximately 55% of sight lines in this survey show Faraday complexity. This dataset, called “DRAO GMIMS of the Northern Sky,” is the first to probe Faraday depth of the northern sky in its frequency range and will support many scientific investigations. The data will be used to calibrate surveys with higher angular resolution, particularly Galactic foreground maps from the Canadian Hydrogen Intensity Mapping Experiment, and to provide information on large structures for aperture-synthesis telescopes, particularly the DRAO Synthesis Telescope. The data are available through the Canadian Astronomy Data Centre.

The Smith Cloud is a high-velocity cloud (HVC) on its final approach to the Milky Way that shows evidence of interaction with the Galaxy’s disk. We investigate the metallicity and gas-phase chemical depletion patterns in this HVC using UV absorption-line observations toward two background QSOs taken with the Hubble Space Telescope/Cosmic Origin Spectrograph and H i 21 cm emission-line observations taken with Green Bank Telescope. We find evidence of silicon gas-phase depletion with [Si/S] = − 0.7 2 − 0.26 + 0.24 and [Si/O]_3σ ≲ −0.05, implying the presence of dust within the Smith Cloud. Because dust is galactic in origin, this HVC could trace the return leg of a Galactic fountain or a dwarf galaxy that passed through the Galactic plane.

Structures in the magnetoionic medium exist across a wide range of angular sizes owing to large-scale magnetic fields coherent over the Galactic spiral arms combined with small-scale fluctuations in the magnetic field and electron density resulting from energy injection processes such as supernovae. For the first time, we produce diffuse Galactic synchrotron emission Faraday rotation maps covering all spatial scales down to 3 ′ resolution for magnetic field studies. These maps complement total and polarized intensity maps combining single-antenna and interferometric data that have been produced, such as the Canadian Galactic Plane Survey (CGPS). Combined maps have sensitivity to large scales from the single-antenna component and angular resolution from the interferometric component. We combine Global Magneto-Ionic Medium Survey High-Band North single-antenna and CGPS aperture-synthesis polarization data after spatial filtering, producing Stokes Q and U maps for the four CGPS frequency channels. We calculate rotation measures (RMs) for all pixels using a linear fit to polarization angle versus wavelength squared. Smooth polarized emission regions require the large-scale sensitivity of the single antenna to illuminate the Faraday rotation, while aperture synthesis reveals small-scale RM variability. While these maps show magnetic field structures on the full range of spatial scales they probe, the RM values should be interpreted with caution, as the narrow λ ² coverage limits sensitivity to Faraday complexity. Despite this limitation of the CGPS 35 MHz bandwidth, we demonstrate that useful Faraday rotation information can be obtained from the combined data set, highlighting the important synergy between future broadband interferometric and single-antenna polarization surveys.