arXiv subjects

Jennifer West

Publications and source records attributed to Jennifer West.

At least 19 recordsLinked to original sources

arXiv2026

3D Magnetic Field Vectors in Space: Bubbles, Clouds, and Filaments

Magnetic fields play important roles in the star-formation process across different spatial scales. The interplay between magnetic field strength (a key component of the interstellar medium's energy budget) and field orientation relative to density structures impacts how interstellar material evolves toward star formation. To understand galactic evolution toward stars, planets, and ultimately life, we need to map three-dimensional (3D) magnetic field vectors in 3D space. However, determining full vector information remains challenging due to projection effects and the complex relationship between observable tracers and field geometry. We outline the observational techniques that can be used to probe the 3D magnetic field structures of objects such as supernova remnants (SNR), superbubbles, HII regions, and HI filaments in the diffuse interstellar medium (ISM), and objects in the dense ISM such as molecular clouds, filaments, and cores. The main SKA-specific observational techniques include synchrotron emission and Faraday rotation of both compact sources and the diffuse emission. We discuss how SKA AA4 will allow implementation of the techniques we describe, leveraging the vastly improved sensitivity, resolution and uv-coverage compared to existing datasets. This will enhance our ability to reconstruct 3D magnetic field vectors, advancing our understanding of magnetic fields in Galactic evolution and star formation.

astro-ph.GA↗

arXiv2026

The magnetic field in the Milky Way Galaxy: from large to small scales

The Milky Way is the galaxy in which we can study its magnetic field to the finest details, providing an ideal laboratory to understand the fundamental questions: how magnetic field is generated and evolves, and how it influences other components in the Galaxy. An SKA-Mid polarization survey will produce an all-sky rotation measure (RM) grid with a density of about 100 per square degree, which is approximately two orders of magnitude larger than what is currently available, and produce total intensity, polarized intensity, and RM all-sky images of diffuse emission covering scales from about 10 arcseconds upward after combination with single-dish observations. The dense RM grid and images of diffuse emission will allow us to determine the most complete picture of the magnetic field in the southern Galactic hemisphere from large to small scales.

astro-ph.GA↗

arXiv2026

Imprints of the Neutral Interstellar Medium on Polarized Synchrotron Emission and Faraday Rotation

The interstellar medium (ISM) is a complex, multiphase medium, where disentangling the distribution of gas and magnetic field structure across different phases remains a considerable challenge. Recently, Faraday tomography enabled by broadband polarized radio observations has emerged as a promising probe of 3D ISM gas and magnetic field structures. However, the interpretation of these observations is obscured by our limited understanding of the different ISM components probed by the distinct Faraday depth features. In this work, we present a comprehensive multi-frequency ($\sim$300 MHz - 23 GHz) analysis comparing features in the Faraday-rotated, polarized synchrotron emission and HI structures over the full high-latitude (|b|>30 degrees) diffuse sky. Using measures of HI structure complexity along the line of sight (LOS), we observe enhanced depolarization across synchrotron radio frequencies in regions with high HI complexity characterized by multiple HI velocity components. We also find that the first and second moments of the Faraday depth spectra are linked to the underlying neutral gas structure. These results indicate that regions of the ISM that are dominated by neutral gas could directly contribute a significant portion of the diffuse synchrotron emission and Faraday rotation. These findings establish new observational constraints for Galactic magnetic field models that synthesize multiphase tracers into a single coherent picture.

astro-ph.GA↗

arXiv2026

The radial component of the local Galactic magnetic field in 3D

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 $|b|>5^{\circ}$) 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 $1.63\pm 0.16$ $\mu$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 $-2.73 \pm 0.19$. We validate our $B_{||}$ reconstruction with Galactic pulsars. Predicted RMs (computed by integrating $n_{e}B_{||}$ 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.

astro-ph.GA↗

arXiv2026

A Robust Analysis of QU-fitting Behavior for 800-1088 MHz and 1296-1440 MHz

QU-fitting is a powerful tool for interpreting spectro-polarimetric radio continuum observations by linking them to physical models, enabling estimates of the magnetic fields in, for example, the Milky Way, galaxy clusters, and radio jets. We present a comprehensive investigation into the effectiveness and limitations of QU-fitting within the ASKAP POSSUM survey frequency ranges (800-1088 MHz and 1296-1440 MHz) with projections to other spectro-polarimetric radio observations. We simulate different physical polarization sources: Faraday simple, Burn slab, internal turbulence, external turbulence, and two-component models in the POSSUM frequencies, and assess their observational degeneracies and fit accuracies. Our results highlight the model-dependent nature of reliable fitting and identify specific regions of parameter space where model selection, and therefore characterization of the physical medium, becomes ambiguous. For QU-fitting we find the Bayes factor, computed using the marginal likelihood, outperforms more traditionally used goodness-of-fit metrics such as Bayesian Information Criterion (BIC), Akaike Information Criterion (AIC), and chi-squared for model selection. We provide empirical relationships to delineate the boundaries where model distinguishability is impossible. Finally, we evaluate how accurately QU-fitting recovers model parameters and their associated uncertainties, thereby assessing its ability to correctly characterize the Faraday-rotating medium in both point and extended sources in Faraday depth space.

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arXiv2025

Revealing the magnetization of the intracluster medium of Abell 3581 using background Faraday rotation measures from the POSSUM survey

The line-of-sight magnetic field of galaxy clusters can be probed using Faraday rotation measure (RM) data. However, our understanding of cluster magnetism is limited due to the scarcity of polarized background radio sources, with most previous studies being constrained to $\sim 10$ sources per cluster. Leveraging the increased source density of the POlarisation Sky Survey of the Universe's Magnetism (POSSUM), we probe the magnetic field properties of the galaxy cluster Abell 3581 {(A3581)} with 111 RMs. We find that the standard deviation in the RM declines monotonically with increasing radius up to 0.75 Mpc{, agreeing with a radially declining magnetic field and electron density profile modeled as Gaussian and lognormal random fields, respectively. {We compare our observations of the inner 0.75 Mpc of A3581 to various semi-analytic models of the magnetic field and electron density, and obtain several best-fit models.} For the first time, we compare the observed RMs in a cluster to full magnetohydrodynamic simulated clusters from TNG-Cluster and find that the non-monotonic trend in RM standard deviation past 0.75 Mpc in A3581 is likely caused by past or present merger activity. We identify a possible candidate for a merger to be the galaxy group [DZ2015b] 276, which would be the first group detected in RMs that is not strongly emitting in X-rays. We find a possible merger axis of A3581 with this group at a position angle of $\theta = 52\pm 4$ deg.

astro-ph.GA↗

arXiv2024

Interpolation techniques for reconstructing Galactic Faraday rotation

The line-of-sight structure of the Galactic magnetic field (GMF) can be studied using Faraday rotation measure (RM) grids. We analyze how the choice of interpolation kernel can affect the accuracy and reliability of reconstructed RM maps. We test the following kernels: inverse distance weighting (IDW), natural neighbour interpolation (NNI), inverse multiquadric interpolation (IM), thin-plate spline interpolation (TPS), and a Bayesian rotation measure sky (BRMS); all techniques were tested on two simulated Galactic foreground RMs (one assuming the GMF has patchy structures and the other assuming it has filamentary structures) using magnetohydrodynamic simulations. Both foregrounds were sampled to form RM grids with densities of $\sim$40 sources deg$^{-2}$ and area $\sim$144 deg$^2$. The techniques were tested on data sets with different noise levels and Gaussian random extragalactic RM contributions. The data set that most closely emulates expected data from current surveys, such as the POlarization Sky Survey of the Universe's Magnetism (POSSUM), had extragalactic contributions and a noise standard deviation of $\sim 1.5$ rad m$^{-2}$. For this data set, the accuracy of the techniques for the patchy structures from best to worst was: BRMS, NNI, TPS, IDW and IM; while in the filamentary simulate foreground it was: BRMS, NNI, TPS, and IDW. IDW is the most computationally expensive technique, while TPS and IM are the least expensive. BRMS and NNI have the same, intermediate computational cost. This analysis lays the groundwork for Galactic RM studies with large radio polarization sky surveys, such as POSSUM.

astro-ph.IM↗

arXiv2023

A Catalogue of Radio Supernova Remnants and Candidate Supernova Remnants in the EMU/POSSUM Galactic Pilot Field

We use data from the pilot observations of the EMU/POSSUM surveys to study the "missing supernova remnant (SNR) problem", the discrepancy between the number of Galactic SNRs that have been observed and the number that are estimated to exist. The Evolutionary Map of the Universe (EMU) and the Polarization Sky Survey of the Universe's Magnetism (POSSUM) are radio sky surveys that are conducted using the Australian Square Kilometre Array Pathfinder (ASKAP). We report on the properties of 7 known SNRs in the joint Galactic pilot field, with an approximate longitude and latitude of 323$^\circ\leq$ l $\leq$ 330$^\circ$ and -4$^\circ\leq$ b $\leq$ 2$^\circ$ respectively, and identify 21 SNR candidates. Of these, 4 have been previously identified as SNR candidates, 3 were previously listed as a single SNR, 13 have not been previously studied, and 1 has been studied in the infrared. These are the first discoveries of Galactic SNR candidates with EMU/POSSUM and, if confirmed, they will increase the SNR density in this field by a factor of 4. By comparing our SNR candidates to the known Galactic SNR population, we demonstrate that many of these sources were likely missed in previous surveys due to their small angular size and/or low surface brightness. We suspect that there are SNRs in this field that remain undetected due to limitations set by the local background and confusion with other radio sources. The results of this paper demonstrate the potential of the full EMU/POSSUM surveys to uncover more of the missing Galactic SNR population.

astro-ph.GA↗

arXiv2023

Polarized accretion shocks from the cosmic web

On the largest scales, galaxies are pulled together by gravity to form clusters, which are connected by filaments making a web-like pattern. Radio emission is predicted from this cosmic web, which should originate from the strong accretion shocks around the cosmic structures. We present the first observational evidence that Fermi-type acceleration from strong shocks surrounding the filaments of the cosmic web, as well as in peripherals of low-mass clusters, is at work in the Universe. Using all-sky radio maps and stacking on clusters and filaments, we have detected the polarization signature of the synchrotron emission with polarization fractions >= 20%, which is best explained by the organization of local magnetic fields by strong shock waves both at the cluster peripheries and between clusters. Our interpretation is well supported by a detailed comparison with state-of-the-art cosmological simulations.

astro-ph.CO↗

arXiv2022

Structure in the Magnetic Field of the Milky Way Disk and Halo traced by Faraday Rotation

Magnetic fields in the ionized medium of the disk and halo of the Milky Way impose Faraday rotation on linearly polarized radio emission. We compare two surveys mapping the Galactic Faraday rotation, one showing the rotation measures of extragalactic sources seen through the Galaxy (from Hutschenreuter et al 2022), and one showing the Faraday depth of the diffuse Galactic synchrotron emission from the Global Magneto-Ionic Medium Survey. Comparing the two data sets in 5deg x 10deg bins shows good agreement at intermediate latitudes, 10 < |b| < 50 deg, and little correlation between them at lower and higher latitudes. Where they agree, both tracers show clear patterns as a function of Galactic longitude: in the Northern Hemisphere a strong sin(2 x longitude) pattern, and in the Southern hemisphere a sin(longitude + pi) pattern. Pulsars with height above or below the plane |z| > 300 pc show similar longitude dependence in their rotation measures. Nearby non-thermal structures show rotation measure shadows as does the Orion-Eridanus superbubble. We describe families of dynamo models that could explain the observed patterns in the two hemispheres. We suggest that a field reversal, known to cross the plane a few hundred pc inside the solar circle, could shift to positive z with increasing Galactic radius to explain the sin(2xlongitude) pattern in the Northern Hemisphere. Correlation shows that rotation measures from extragalactic sources are one to two times the corresponding rotation measure of the diffuse emission, implying Faraday complexity along some lines of sight, especially in the Southern hemisphere.

astro-ph.GA↗

arXiv2022

A method for reconstructing the Galactic magnetic field using dispersion of fast radio bursts and Faraday rotation of radio galaxies

With the rapid increase of fast radio burst (FRB) detections within the past few years, there is now a catalogue being developed for all-sky extragalactic dispersion measure (DM) observations in addition to the existing collection of all-sky extragalactic Faraday rotation measurements (RMs) of radio galaxies. We present a method of reconstructing all-sky information of the Galactic magnetic field component parallel to the line of sight, $B_{\parallel}$, using simulated observations of the RM and DM along lines of sight to radio galaxies and FRB populations, respectively. This technique is capable of distinguishing between different input Galactic magnetic field and thermal electron density models. Significant extragalactic contributions to the DM are the predominant impediment in accurately reconstructing the Galactic DM and $\left $ skies. We look at ways to improve the reconstruction by applying a filtering algorithm on the simulated DM lines of sight and we derive generalized corrections for DM observations at $|b|$ > 10 deg that help to disentangle Galactic and extragalactic DM contributions. Overall, we are able to reconstruct both large-scale Galactic structure and local features in the Milky Way's magnetic field from the assumed models. We discuss the application of this technique to future FRB observations and address possible differences between our simulated model and observed data, namely: adjusting the priors of the inference model, an unevenly distributed population of FRBs on the sky, and localized extragalactic DM structures.

astro-ph.GA↗

arXiv2021

Discovery of Magnetic Fields Along Stacked Cosmic Filaments as Revealed by Radio and X-Ray Emission

Diffuse filaments connect galaxy clusters to form the cosmic web. Detecting these filaments could yield information on the magnetic field strength, cosmic ray population and temperature of intercluster gas, yet, the faint and large-scale nature of these bridges makes direct detections very challenging. Using multiple independent all-sky radio and X-ray maps we stack pairs of luminous red galaxies as tracers for cluster pairs. For the first time, we detect an average surface brightness between the clusters from synchrotron (radio) and thermal (X-ray) emission with $\gtrsim 5σ$ significance, on physical scales larger than observed to date ($\geq 3\,$Mpc). We obtain a synchrotron spectral index of $α\simeq -1.0$ and estimates of the average magnetic field strength of $ 30 \leq B \leq 60 \,$nG, derived from both equipartition and Inverse Compton arguments, implying a 5 to 15$\,$per cent degree of field regularity when compared with Faraday rotation measure estimates. While the X-ray detection is inline with predictions, the average radio signal comes out higher than predicted by cosmological simulations and dark matter annihilation and decay models. This discovery demonstrates that there are connective structures between mass concentrations that are significantly magnetised, and the presence of sufficient cosmic rays to produce detectable synchrotron radiation.

astro-ph.CO↗

arXiv2020

CHANG-ES XXII: Coherent Magnetic Fields in the Halos of Spiral Galaxies

Context. The magnetic field in spiral galaxies is known to have a large-scale spiral structure along the galactic disk and is observed as X-shaped in the halo of some galaxies. While the disk field can be well explained by dynamo action, the 3-dimensional structure of the halo field and its physical nature is still unclear. Aims. As first steps towards understanding the halo fields, we want to clarify whether the observed X-shaped field is a wide-spread pattern in the halos of spiral galaxies and whether these halo fields are just turbulent fields ordered by compression or shear (anisotropic turbulent fields), or have a large-scale regular structure. Methods. The analysis of the Faraday rotation in the halo is the tool to discern anisotropic turbulent fields from large-scale magnetic fields. This, however, has been challenging until recently because of the faint halo emission in linear polarization. Our sensitive VLA broadband observations C-band and L-band of 35 spiral galaxies seen edge-on (called CHANG-ES) allowed us to perform RM-synthesis in their halos and to analyze the results. We further accomplished a stacking of the observed polarization maps of 28 CHANG-ES galaxies at C-band. Results. Though the stacked edge-on galaxies were of different Hubble types, star formation and interaction activities, the stacked image clearly reveals an X-shaped structure of the apparent magnetic field. We detected a large-scale (coherent) halo field in all 16 galaxies that have extended polarized intensity in their halos. We detected large-scale field reversals in all of their halos. In six galaxies they are along lines about vertical to the galactic midplane (vertical RMTL) with about 2 kpc separation. Only in NGC 3044 and possibly in NGC 3448 we observed vertical giant magnetic ropes (GMRs) similar to those detected recently in NGC 4631.

astro-ph.GA↗

arXiv2019

Canada and the SKA from 2020-2030

This white paper submitted for the 2020 Canadian Long-Range Planning process (LRP2020) presents the prospects for Canada and the Square Kilometre Array (SKA) from 2020-2030, focussing on the first phase of the project (SKA1) scheduled to begin construction early in the next decade. SKA1 will make transformational advances in our understanding of the Universe across a wide range of fields, and Canadians are poised to play leadership roles in several. Canadian key SKA technologies will ensure a good return on capital investment in addition to strong scientific returns, positioning Canadian astronomy for future opportunities well beyond 2030. We therefore advocate for Canada's continued scientific and technological engagement in the SKA from 2020-2030 through participation in the construction and operations phases of SKA1.

astro-ph.IM↗

arXiv2019

Cosmic Magnetism

Magnetic fields are involved in every astrophysical process on every scale: from planetary and stellar interiors to neutron stars, stellar wind bubbles and supernova remnants; from the interstellar medium in galactic disks, nuclei, spiral arms and halos to the intracluster and intergalactic media. They are involved in essentially every particle acceleration process and are thus fundamental to non-thermal physics in the Universe. Key questions include the origin of magnetic fields, their evolution over cosmic time, the amplification and decay processes that modify their strength, and their impact on other processes such as star formation and galaxy evolution. Astrophysical plasmas provide a unique laboratory for testing magnetic dynamo theory. The study of magnetic fields requires observations that span the wavelength range from radio through infrared, optical, UV, X-ray, and gamma-ray. Canada has an extremely strong record of research in cosmic magnetism, and has a significant leadership role in several ongoing and upcoming global programs. This white paper will review the science questions to be addressed in the study of cosmic magnetic fields and will describe the observational and theoretical opportunities and challenges afforded by the telescopes and modelling capabilities of today and tomorrow.

astro-ph.GA↗

arXiv2019

Canadian Investigations of the Interstellar Medium

The interstellar medium mediates galactic evolution as the reservoir of material for future star formation and the repository of energy and matter output by stellar processes. Canadians have played leading roles in ISM science for decades. The Canadian Galactic Plane Survey identified a wealth of small-scale structure in H I emission as well as self-absorption and in the structure of polarized emission. These observations demonstrated that no phase of the ISM, including the transition from atomic gas to star formation, can be understood in isolation. Canadians have also played leading roles in the characterization of dust with Planck and balloon-borne telescopes. Canadians have also used pulsar scintillometry to measure structure in the ISM at the smallest scales, below 1 AU. The 2020s offer many opportunities for ISM science in Canada. A major but cost-effective upgrade to the Synthesis Telescope with broadband (400-1800 MHz) single-pixel feeds would enable broadband polarimetry as well as wide-area, arcminute surveys of radio recombination lines. The next generation of balloon-borne telescopes will investigate magnetic fields and dust properties. Large single dishes, particularly the Green Bank Telescope, remain essential for our understanding of the diffuse structure which characterizes the ISM. Very long baseline interferometry capability enables parallax measurements of pulsars and masers and for further progress in scintillometry. ISM astronomers will continue to participate in cosmological experiments including CHIME and CHORD. Protecting quiet radio frequency interference environments will be ever more important as broadband observations are ever more central to ISM science. Computational capability is essential both for numerical work and for handling the observational data.

astro-ph.GA↗

arXiv2019

A Broadband Spectro-polarimetric View of the NVSS Rotation Measure Catalogue I: Breaking the nπ-ambiguity

The NRAO VLA Sky Survey (NVSS) Rotation Measure (RM) catalogue is invaluable for the study of cosmic magnetism. However, the RM values reported in it can be affected by n$π$-ambiguity, resulting in deviations of the reported RM from the true values by multiples of +-652.9 rad m-2. We therefore set off to observationally constrain the fraction of sources in the RM catalogue affected by this ambiguity. New broadband spectro-polarimetric observations were performed with the Karl G. Jansky Very Large Array (VLA) at 1--2 GHz, with 23 n$π$-ambiguity candidates selected by their peculiarly high |RM| values. We identified nine sources with erroneous RM values due to n$π$-ambiguity and 11 with reliable RM values. In addition, we found two sources to be unpolarised and one source to be inconsistent with neither n$π$-ambiguity nor reliable RM cases. By comparing the statistical distributions of the above two main classes, we devised a measure of how much a source's RM deviates from that of its neighbours: $Δ/σ$, which we found to be a good diagnostic of n$π$-ambiguity. With this, we estimate that there are at least 50 sources affected by n$π$-ambiguity among the 37,543 sources in the catalogue. Finally, we explored the Faraday complexities of our sources revealed by our broadband observations.

astro-ph.GA↗

arXiv2019

A Broadband Spectro-polarimetric View of the NVSS Rotation Measure Catalogue II: Effects of Off-axis Instrumental Polarisation

The NRAO VLA Sky Survey (NVSS) Rotation Measure (RM) catalogue has enabled numerous studies in cosmic magnetism, and will continue being a unique dataset complementing future polarisation surveys. Robust comparisons with these new surveys will however require further understandings in the systematic effects present in the NVSS RM catalogue. In this paper, we make careful comparisons between our new on-axis broadband observations with the Karl G. Jansky Very Large Array and the NVSS RM results for 23 sources. We found that two unpolarised sources were reported as polarised at about 0.5% level in the RM catalogue, and noted significant differences between our newly derived RM values and the catalogue values for the remaining 21 sources. These discrepancies are attributed to off-axis instrumental polarisation in the NVSS RM catalogue. By adopting the 0.5% above as the typical off-axis instrumental polarisation amplitude, we quantified its effect on the reported RMs with a simulation, and found that on average the RM uncertainties in the catalogue have to be increased by $\approx$ 10% to account for the off-axis instrumental polarisation effect. This effect is more substantial for sources with lower fractional polarisation, and is a function of the source's true RM. Moreover, the distribution of the resulting RM uncertainty is highly non-Gaussian. With the extra RM uncertainty incorporated, we found that the RM values from the two observations for most (18 out of 21) of our polarised targets can be reconciled. The remaining three are interpreted as showing hints of time variabilities in RM.

astro-ph.GA↗