arXiv subjects

Juergen Ott

Publications and source records attributed to Juergen Ott.

At least 19 recordsLinked to original sources

arXiv2026

The Karl G. Jansky Very Large Array Sky Survey (VLASS). Data Products

The VLA Sky Survey (VLASS) is a radio continuum survey of the entire sky visible to the VLA (dec > -40deg) at ~2.5 arcsecond resolution over 2-4 GHz with full polarization. Data processing and quality assurance have proven challenging due to limited computing and staff resources, and require innovative approaches. Although "Quick Look" images in Stokes I continuum were produced within a few weeks of observation, high accuracy images that utilize the full potential of the dataset are computationally intensive and take much longer to produce. Half of the sky will need correction for w-terms to account for sky curvature, increasing the computational cost per image by ~100x. This problem will be tackled using GPUs and High-Throughput Computing (HTC) centers. Quality assurance of the tens of thousands of individual images has used a mix of algorithmic and machine learning techniques to automatically identify common artifacts and anomalies in the data. Archiving and dissemination of VLASS data products also pose a significant challenge.

astro-ph.IM↗

arXiv2026

Reconciling 3D Models for the Central 10 parsecs of the Milky Way

The construction of an accurate 3D model of the Milky Way center is necessary to understand inflow processes that drive its overall evolution, and to compare our Galactic nucleus to other galaxies' nuclei. A main point of contention is the line-of-sight location of sources observed toward the central 10 pc of the Galaxy, including recent star formation (the Sgr A East supernova remnant and Sgr A HII regions) and copious gas (the 50 and 20 km/s molecular clouds, the Circumnuclear Disk, and the Sgr A West ionized "minispiral" that encircles the central supermassive black hole, Sgr A*). Some models place all of these structures within a radius of 5 pc from Sgr A*, while others place the 20 and 50 km/s clouds at a distance of at least 30 - 50 pc away from Sgr A* along the line of sight. We present new radio and millimeter observations of the molecular gas toward the central ~10 pc, from which we have constructed an alternative 3D model that is consistent with both prior radio observations and orbital gas kinematics. Our model places the 20 km/s cloud, 50 km/s cloud, and Sgr A East more than 10 pc in front of Sgr A*. While this model does not conclusively rule out a connection between the 50 and 20 km/s clouds and the circumnuclear disk, we argue that prior evidence for these connections is tenuous, especially given the complex spatial and kinematic overlap of structures along the line of sight.

astro-ph.GA↗

arXiv2026

ALMA Central Molecular Zone Exploration Survey (ACES) I: Overview

The mass flows and energy cycles within the inner regions of galaxies exert a powerful influence on the evolution of the galaxy population. The centre of the Milky Way is the only galactic nucleus for which it is possible to resolve the physical mechanisms that drive these cycles, namely star formation and feedback, while also tracing global (>100 pc) processes which determine where and when star formation and feedback occur. We present an overview of ACES, the 'Atacama Large Millimeter/submillimeter Array (ALMA) CMZ Exploration Survey', a ~1.5" angular resolution, 0.2-3 km/s spectral resolution ALMA Band 3 (85-102 GHz), survey of the 'Central Molecular Zone' (CMZ) -- the inner-100 pc of the Galaxy (l = 359.4 deg to 0.8 deg). ACES spectral setup is tuned to observe optimal tracers of the physical, chemical, and kinematic conditions in over 70 spectral features (e.g. HCO+, HNCO, SiO, H40alpha, complex molecules) of the gas in the CMZ, to derive the properties of all potentially star-forming Galactic Centre gas, from global scales (100 pc) to dense ~0.05 pc structures that are expected to host individual star-forming cores, down to sub-sonic (<0.4 km/s) velocity resolution. In this overview paper, we provide the scientific justification for the ACES survey, explain the choice of observational setup, and describe the data legacy products. Finally, we show some of the initial ACES data which highlight the power of ACES' combination of high angular resolution, unprecedented spatial dynamic range, sensitivity, spectral resolution and spectral bandwidth as an illustration of how ACES aims to understand how global processes set the location, intensity, and timescales for star formation and feedback in the CMZ.

astro-ph.GA↗

arXiv2026

JWST Observations of Starbursts: PAHs Closely Trace the Cool Phase of M82's Galactic Wind

Stellar feedback drives multiphase gas outflows from starburst galaxies, but the interpretation of dust emission in these winds remains uncertain. To investigate this, we analyze new JWST mid-infrared images tracing polycyclic aromatic hydrocarbon (PAH) emission at 7.7 and 11.3~$μ$m from the outflow of the prototypical starburst M82 out to $3.2$ kpc. We find that PAH emission shows significant correlations with CO, H$α$, and X-ray emission within the outflow, though the strengths and behaviors of these correlations vary with gas phase and distance from the starburst. PAH emission correlates strongly with cold molecular gas, with PAH--CO scaling relations in the wind nearly identical to those in galaxy disks despite the very different conditions. The H$α$--PAH correlation indicates that H$α$ traces the surfaces of PAH-bearing clouds, consistent with arising from ionized layers produced by shocks. Meanwhile the PAH--X-ray correlation disappears once distance effects are controlled for past 2~kpc, suggesting that PAHs are decoupled from the hot gas and the global correlation merely reflects the large-scale structure of the outflow. The PAH-to-neutral gas ratio remains nearly flat to 2~kpc, with variations following changes in the radiation field. This implies that the product of PAH abundance and dust-to-gas ratio does not change significantly over the inner portion of the outflow. Together, these results demonstrate that PAHs robustly trace the cold phase of M82's wind, surviving well beyond the starburst and providing a powerful, high-resolution proxy for mapping the life cycle of entrained cold material in galactic outflows.

astro-ph.GA↗

arXiv2025

Where do stars explode in the ISM? -- The distribution of dense gas around evolved massive stars in M33

The effect of supernovae (SNe) on star-formation in the interstellar medium (ISM) depends sensitively on where SNe explode with respect to ISM clouds. Observationally, SN ISM environments characterized by spatially-resolved gas maps can empirically guide the placement of SNe in subgrid models, but unfortunately such measurements remain scarce, as SNe are rare and often distant. Here we demonstrate a new approach -- mapping the ISM around evolved massive stars that are soon to explode. These provide a substantially larger sample of `explosion sites' (than just historical SNe) in nearby galaxies that have high-resolution atomic and molecular ISM maps from Jansky VLA and ALMA. We demonstrate this technique in the well-resolved Local Group spiral M33 by analyzing the 50 pc-scale projected ISM densities around red supergiants (RSGs, 8-30 M$_{\odot}$ stars) Wolf-Rayet stars (WRs, $>$30 M$_{\odot}$ stars), and supernova remnants (SNRs). We find a \emph{mass-dependent} correlation between stars and gas clouds, with atleast 45\% of WRs and upto 77\% of RSGs having no detectable H$_2$ at their pixel locations. In the sample with H$_2$ detections, we find that more massive younger progenitors are coincident with denser gas. We show that the density distributions for stars $>$15 M$_{\odot}$ are statistically distinct from random alignment of stars and gas in M33. Our work provides the first observationally-derived estimate of the fraction of the SN-producing stellar population correlated with ISM density peaks. We demonstrate how this can be compared with galaxy simulations, and advocate similar comparisons to the community for constraining sub-grid models.

astro-ph.GA↗

arXiv2025

Nonthermal Pressures: Key to Energy Balance and Structure Formation Near Sgr A* in the Milky Way

The circumnuclear region of the Galactic Center offers a unique laboratory to study energy balance and structure formation around Sgr A$\star$. This work investigates thermal and nonthermal processes within 7 pc distance from Sgr A$\star$. Using MeerKAT 1.3 GHz radio continuum data and ALMA H40 radio recombination line emission from the ACES survey, we separate free-free and synchrotron components at $\sim$0.2 pc resolution. With a thermal fraction of $\simeq$13%, the 1.3 GHz emission shows tight correlations with the Herschel PACS infrared data. The correlation between the equipartition magnetic field and molecular gas traced by JCMT $^{12}$CO (J=3$\rightarrow$2) observations reveals a balance between the magnetic field, cosmic rays, and molecular gas pressures south of the circumnuclear disk on $\sim$0.7 pc scales. Unlike the magnetic field and ionized gas, the molecular gas density declines in the cavity (R$\leq$2 pc) toward the center, likely due to feedback from Sgr A$\star$. We find that nonthermal pressure from turbulent gas nearly balances magnetic and cosmic ray pressures and exceeds thermal pressure by two orders of magnitude. The medium surrounding Sgr A$\star$ is filled by a low-$β$ (thermal-to-magnetic energy), supersonic plasma, with an Alfvén Mach number $\simeq$ 4 (assuming equipartition). Analysis of the mass-to-magnetic flux ratio suggests that the circumnuclear region is mostly subcritical and, therefore, the magnetic field can help stabilize gas clouds against gravitational collapse.

astro-ph.GA↗

arXiv2025

Serendipitous discovery of a young cluster of galaxies at $z \sim 0.5$ projected next to the nearby tadpole galaxy KUG 1138 + 327

Using a 90 ks Chandra ACIS-S observation in the 0.3--7 keV band, along with complementary Low-Frequency Array and Karl G.~Jansky Very Large Array data in the 120--168 MHz and 1--2 GHz ranges, we study the diffuse emission around the nearby dwarf galaxy KUG~1138+327. Our analysis reveals a diffuse X-ray feature on the southern side, disconnected from the galactic disk. This feature exhibits a hard X-ray spectrum, which is highly unusual for an outflow from a dwarf galaxy. We interpret the irregularly shaped feature as hot plasma in a young galaxy cluster at redshift 0.5, supported by X-ray spectral fitting and consistent with the optical redshift of the central elliptical galaxy of a known cluster identified by a red sequence. Additionally, we detect a radio lobe east of the X-ray feature, likely produced by an AGN offset from the cluster center and confined primarily by ram pressure from the ambient medium. The lobe shows a steep nonthermal radio spectrum, suggesting a cosmic-ray age of $\gtrsim 5 \times 10^{7}$~yr. Assuming energy equipartition between cosmic rays and magnetic fields, we estimate the lobe's total energy to be $\sim 9 \times 10^{56}~\mathrm{erg}$, comparable to the thermal energy in the same volume. This study thus identifies a background young cluster projected next to KUG~1138+327 and highlights the potentially significant role of off-center AGN feedback in shaping the intracluster medium.

astro-ph.HE↗

arXiv2025

Discovery of a Giant Molecular Cloud at the Midpoint of the Galactic Bar Dust Lanes: M4.7-0.8

We present the detection of a previously unknown giant molecular cloud (GMC) located at the midpoint of the Galactic Bar Dust Lanes (M4.7--0.8), using spectral line observations taken with the Green Bank Telescope (GBT). This $\sim$60 pc long GMC is associated with accreting material that is transitioning from the quieter Galactic disk environment to the more extreme central molecular zone (CMZ) environment. Our 24 GHz single-dish radio observations targeted the NH$_3$ (1,1)$-$(4,4) and HC$_5$N (9$-$8), known dense gas tracers. The observations reveal the main features of the GMC, which we have dubbed the `Nexus' and `Filament', covering a 0$.\!\!^\circ$5$\times$0$.\!\!^\circ$25 area at 31$''$ angular resolution. In this publication we investigate the gas kinematics within the observed region and compare the distribution of molecular emission to previous infrared surveys to better understand the dust component. The observed gas tracers show centrally condensed cores corresponding to the positions of high dust column densities and low dust temperatures. We report the detection of a previously unknown NH$_3$ (3,3) maser, along with a 70$μ$m source association, which supports the identification of this region as being actively star-forming. Gas emission in this region shows broad linewidths, comparable to values seen in CMZ clouds. The overall description of this cloud that we present is that of a highly dynamic region comprising dense gas and dust. This encapsulates a wide range of features associated with star formation, in addition to material transport related to the CMZ.

astro-ph.GA↗

arXiv2025

Turbulent Pressure Heats Gas and Suppresses Star Formation in Galactic Bar Molecular Clouds

The Central Molecular Zone (CMZ) of the Milky Way is fed by gas inflows from the Galactic disk along almost radial trajectories aligned with the major axis of the Galactic bar. However, despite being fundamental to all processes in the nucleus of the galaxy, these inflows have been studied significantly less than the CMZ itself. We present observations of various molecular lines between 215 and 230 GHz for 20 clouds with $|\ell| < 10^\circ$, which are candidates for clouds in the Galactic bar due to their warm temperatures and broad lines relative to typical Galactic disk clouds, using the Atacama Large Millimeter/submillimeter Array (ALMA) Atacama Compact Array (ACA). We measure gas temperatures, shocks, star formation rates, turbulent Mach numbers, and masses for these clouds. Although some clouds may be in the Galactic disk despite their atypical properties, nine clouds are likely associated with regions in the Galactic bar, and in these clouds, turbulent pressure is suppressing star formation. In clouds with no detected star formation, turbulence is the dominant heating mechanism, whereas photo-electric processes heat the star-forming clouds. We find that the ammonia (NH$_3$) and formaldehyde (H$_2$CO) temperatures probe different gas components, and in general each transition appears to trace different molecular gas phases within the clouds. We also measure the CO-to-H$_2$ X-factor in the bar to be an order of magnitude lower than the typical Galactic value. These observations provide evidence that molecular clouds achieve CMZ-like properties before reaching the CMZ

astro-ph.GA↗

arXiv2025

Candidate intermediate-mass black hole discovered in an extremely young low-metallicity cluster in the tadpole galaxy KUG 1138+327

We explore what unusual products a starburst of about 6% solar metallicity and a mean estimated age of ~0.5 Myr can produce in KUG 1138 + 327 at a distance of 24.5 Mpc. Chandra X-ray observations show a dominant point-like source with an average 0.3-10 keV luminosity of 10^{40.3} erg/s and variability by a factor of ~2 over months. This extreme ultraluminous X-ray source (ULX) is apparently associated with the young central cluster. A multicolor disk modeling of the X-ray spectrum of the source suggests a standard accretion around a black hole. It also has a morphologically elongated nonthermal radio continuum counterpart on the scale of ~200 pc, probably the longest detected from such a source. The radio, optical, and X-ray findings suggest that it could well be an intermediate-mass black hole undergoing sub-Eddington accretion from a massive star companion. Accounting for the presence of the ULX and the prominent emission lines HeII\lambda4658 and [ArIV]\lambda4711 while lacking Wolf-Rayet spectral features, we estimate the true age of the starburst to be about 2-4 Myr. Only with such a moderate age can the starburst host this extraordinary ULX, probably triggered by a recent influx of extremely low-metallicity gas. This study demonstrates the potential of multiwavelength studies of low-metallicity starbursts to provide insights into what may commonly occur in high-redshift galaxies.

astro-ph.HE↗

arXiv2025

The Young Ages of 70 μm-dark Clumps Inferred from Carbon Chain Chemistry

The physical conditions of the earliest environment of high-mass star formation are currently poorly understood. To that end, we present observations of the carbon chain molecules HC$_5$N , CCS, and HC$_7$N in the 22-25 GHz band towards 12 high-mass 70 micron-dark clumps (SMDC) with the Jansky Very Large Array (VLA). We detect HC$_5$N and CCS towards 11 of these SMDC sources. We calculate column densities and abundances relative to H$_2$ for HC$_5$N and CCS. We do not find any clear HC$_7$N detections in the 11 sources individually, but by stacking the HC$_7$N spectra, we do detect HC$_7$N on average in these sources. We also calculate the ratio of the column densities of HC$_5$N to HC$_7$N using the stacked spectra of both species. We compare our measured abundances of HC$_5$N and our measured ratio of HC$_5$N to HC$_7$N to the UMIST dark cloud chemistry models to constrain an age for the gas assuming a fixed volume density and temperature. The chemical models favor a chemical evolutionary age less than 1 Myr at densities of n(H2) = 2 x 10$^4$ cm$^{-3}$. The consistent carbon-chain detections and young model-derived ages support the conclusion that these 11 70 micron-dark clumps lack high mass protostars because they are young and not because they are inefficient and incapable of high mass star formation.

astro-ph.GA↗

arXiv2024

JWST Observations of Starbursts: Massive Star Clusters in the Central Starburst of M82

We present a near infrared (NIR) candidate star cluster catalog for the central kiloparsec of M82 based on new JWST NIRCam images. We identify star cluster candidates using the F250M filter, finding 1357 star cluster candidates with stellar masses $>10^4$ M$_\odot$. Compared to previous optical catalogs, nearly all (87%) of the candidates we identify are new. The star cluster candidates have a median intrinsic cluster radius of $\approx$1 pc and have stellar masses up to $10^6$ M$_\odot$. By comparing the color-color diagram to dust-free yggdrasil stellar population models, we estimate that the star cluster candidates have A$_{\rm V}\sim3-24$ mag, corresponding to A$_{\rm 2.5μm}\sim0.3-2.1$ mag. There is still appreciable dust extinction towards these clusters into the NIR. We measure the stellar masses of the star cluster candidates, assuming ages of 0 and 8 Myr. The slope of the resulting cluster mass function is $β=1.9\pm0.2$, in excellent agreement with studies of star clusters in other galaxies.

astro-ph.GA↗

arXiv2024

Evidence of a Cloud-Cloud Collision from Overshooting Gas in the Galactic Center

The Milky Way is a barred spiral galaxy with "bar lanes" that bring gas towards the Galactic Center. Gas flowing along these bar lanes often overshoots, and instead of accreting onto the Central Molecular Zone, it collides with the bar lane on the opposite side of the Galaxy. We observed G5, a cloud which we believe is the site of one such collision, near the Galactic Center at (l,b) = (+5.4, -0.4) with the ALMA/ACA. We took measurements of the spectral lines $^{12}$CO J=2-1, $^{13}$CO J=2-1, C$^{18}$O J=2-1, H$_2$CO J=3$_{03}$-2$_{02}$, H$_{2}$CO J=3$_{22}$-2$_{21}$, CH$_{3}$OH J=4$_{22}$-3$_{12}$, OCS J=18-17 and SiO J=5-4. We observed a velocity bridge between two clouds at $\sim$50 km/s and $\sim$150 km/sin our position-velocity diagram, which is direct evidence of a cloud-cloud collision. We measured an average gas temperature of $\sim$60 K in G5 using H$_2$CO integrated intensity line ratios. We observed that the $^{12}$C/$^{13}$C ratio in G5 is consistent with optically thin, or at most marginally optically thick $^{12}$CO. We measured 1.5 x 10$^{19}$ cm$^{-2}$(K km/s)$^{-1}$ for the local X$_{CO}$, 10-20x less than the average Galactic value. G5 is strong direct observational evidence of gas overshooting the Central Molecular Zone (CMZ) and colliding with a bar lane on the opposite side of the Galactic center.

astro-ph.GA↗

arXiv2024

JWST Observations of Starbursts: Polycyclic Aromatic Hydrocarbon Emission at the Base of the M 82 Galactic Wind

We present new observations of the central 1 kpc of the M 82 starburst obtained with the James Webb Space Telescope (JWST) near-infrared camera (NIRCam) instrument at a resolution ~0.05"-0.1" (~1-2 pc). The data comprises images in three mostly continuum filters (F140M, F250M, and F360M), and filters that contain [FeII] (F164N), H2 v=1-0 (F212N), and the 3.3 um PAH feature (F335M). We find prominent plumes of PAH emission extending outward from the central starburst region, together with a network of complex filamentary substructure and edge-brightened bubble-like features. The structure of the PAH emission closely resembles that of the ionized gas, as revealed in Paschen alpha and free-free radio emission. We discuss the origin of the structure, and suggest the PAHs are embedded in a combination of neutral, molecular, and photoionized gas.

astro-ph.GA↗

arXiv2023

Widespread Hot Ammonia in the Central Kiloparsec of the Milky Way

The inner 300-500 pc of the Milky Way has some of the most extreme gas conditions in our Galaxy. Physical properties of the Central Molecular Zone (CMZ), including temperature, density, thermal pressure, and turbulent pressure, are key factors for characterizing gas energetics, kinematics, and evolution. The molecular gas in this region is more than an order of magnitude hotter than gas in the Galactic disk, but the mechanism responsible for heating the gas remains uncertain. We characterize the temperature for 16 regions, extending out to a projected radius of $\sim$450 pc. We observe \am\, J,K=(1,1)-(6,6) inversion transitions from SWAG (Survey of Water and Ammonia in the Galactic Center) using the Australia Telescope Compact Array (ATCA), and ammonia lines (J,K) = (8,8)-(14,14) using the 100\,m Green Bank Telescope. Using these two samples we create full Boltzmann plots for every source and fit two rotational temperature components to the data. For the cool component we detect rotational temperatures ranging from 20-80\,K, and for the hot component we detect temperature ranging from 210-580\,K. With this sample of 16 regions, we identify some of the most extreme molecular gas temperatures detected in the Galactic center thus far. We do not find a correlation between gas temperature and Galactocentric radius, and we confirm that these high temperatures are not exclusively associated with actively star-forming clouds. We also investigate temperature and line widths and find (1) no correlation between temperature and line width and (2) the lines are non-thermally broadened indicating that non-thermal motions are dominant over thermal.

astro-ph.GA↗

arXiv2023

CMZoom III: Spectral Line Data Release

We present an overview and data release of the spectral line component of the SMA Large Program, \textit{CMZoom}. \textit{CMZoom} observed $^{12}$CO(2-1), $^{13}$CO(2-1) and C$^{18}$O(2-1), three transitions of H$_{2}$CO, several transitions of CH$_{3}$OH, two transitions of OCS and single transitions of SiO and SO, within gas above a column density of N(H$_2$)$\ge 10^{23}$\,cm$^{-2}$ in the Central Molecular Zone (CMZ; inner few hundred pc of the Galaxy). We extract spectra from all compact 1.3\,mm \emph{CMZoom} continuum sources and fit line profiles to the spectra. We use the fit results from the H$_{2}$CO 3(0,3)-2(0,2) transition to determine the source kinematic properties. We find $\sim 90$\% of the total mass of \emph{CMZoom} sources have reliable kinematics. Only four compact continuum sources are formally self-gravitating. The remainder are consistent with being in hydrostatic equilibrium assuming that they are confined by the high external pressure in the CMZ. Based on the mass and density of virially bound sources, and assuming star formation occurs within one free-fall time with a star formation efficiency of $10\% - 75\%$, we place a lower limit on the future embedded star-formation rate of $0.008 - 0.06$\,M$_{\odot}$\,yr$^{-1}$. We find only two convincing proto-stellar outflows, ruling out a previously undetected population of very massive, actively accreting YSOs with strong outflows. Finally, despite having sufficient sensitivity and resolution to detect high-velocity compact clouds (HVCCs), which have been claimed as evidence for intermediate mass black holes interacting with molecular gas clouds, we find no such objects across the large survey area.

astro-ph.GA↗

arXiv2022

CASA, the Common Astronomy Software Applications for Radio Astronomy

CASA, the Common Astronomy Software Applications, is the primary data processing software for the Atacama Large Millimeter/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA), and is frequently used also for other radio telescopes. The CASA software can handle data from single-dish, aperture-synthesis, and Very Long Baseline Interferometery (VLBI) telescopes. One of its core functionalities is to support the calibration and imaging pipelines for ALMA, VLA, VLA Sky Survey (VLASS), and the Nobeyama 45m telescope. This paper presents a high-level overview of the basic structure of the CASA software, as well as procedures for calibrating and imaging astronomical radio data in CASA. CASA is being developed by an international consortium of scientists and software engineers based at the National Radio Astronomical Observatory (NRAO), the European Southern Observatory (ESO), the National Astronomical Observatory of Japan (NAOJ), and the Joint Institute for VLBI European Research Infrastructure Consortium (JIV-ERIC), under the guidance of NRAO.

astro-ph.IM↗

arXiv2022

Polarized Emission From Four Supernova Remnants In The THOR Survey

We present polarization and Faraday rotation for the supernova remnants (SNRs) G46.8-0.3, G43.3-0.2, G41.1-0.3, and G39.2-0.3 in L-band (1-2 GHz) radio continuum in The HI/OH/Recombination line (THOR) survey. We detect polarization from G46.8-0.3, G43.3-0.2 and G39.2-0.3 but find upper limits at the 1% level of Stokes I for G41.1-0.3. For G46.8-0.3 and G39.2-0.3 the fractional polarization varies on small scales from 1% to ~6%. G43.3-0.2 is less polarized with fractional polarization <~3%. We find upper limits at the 1% level for the brighter regions in each SNR with no evidence for associated enhanced Faraday depolarization. We observe significant variation in Faraday depth and fractional polarization on angular scales down to the resolution limit of 16". Approximately 6% of our polarization detections from G46.8-0.3 and G39.2-0.3 exhibit two-component Faraday rotation and 14% of polarization detections in G43.3-0.2 are multi-component. For G39.2-0.3 we find a bimodal Faraday depth distribution with a narrow peak and a broad peak for all polarization detections as well as for the subset with two-component Faraday rotation. We identify the narrow peak with the front side of the SNR and the broad peak with the back side. Similarly, we interpret the observed Faraday depth distribution of G46.8-0.3 as a superposition of the distributions from the front side and the back side. We interpret our results as evidence for a partially filled shell with small-scale magnetic field structure and internal Faraday rotation.

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