arXiv subjects

J. R. Dawson

Publications and source records attributed to J. R. Dawson.

At least 19 recordsLinked to original sources

arXiv2026

A Detailed GASKAP-HI View of Shells and Loops in the Magellanic Bridge

We present 8 pc-scale GASKAP-HI observations of an HI shell (diameter \sim130 pc) associated with the Hαshell DEM171 in the Magellanic Bridge. The Bridge's diffuse, HI-dominated environment, with minimal galactic shearing and fewer overlapping star-forming regions, provides an ideal environment to study cold gas formation driven by stellar feedback. To investigate the HI multi-phase structure and kinematics of the shell, we perform Gaussian decomposition of HI line profiles. We identify cold HI components with velocity dispersions σ< 2.5 km/s located within the shell. Complementary Herschel far-infrared (FIR) 250\micron and Hαmaps show that while the central cavity is ionized, the shell walls contain cold HI, molecular gas, and dust, suggesting that stellar feedback has promoted cold gas formation or swept pre-existing cold gas into the shell walls. We also identify cold HI clumps outside the shell tracing a larger-scale structure, MB-Loop I. To investigate the large-scale context, we apply a Fourier transform method to HI emission-line profiles to map the lower limit of cold-gas column densities across a section of the Bridge, revealing structures from large loops to smaller shells. The association between MB-Loop I and the HI shell suggests that cold HI gas may pre-exist before shell expansion. Finally, the shell exhibits asymmetric expansion, with a preferred orientation roughly perpendicular to the arc along MB-Loop I. Our results show that cold HI gas exists across a wide range of spatial scales in the Magellanic Bridge, highlighting the dynamic interplay that shapes the surrounding interstellar medium.

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arXiv2026

Discovery of Two Consecutive Profile Change Events in the Millisecond Pulsar PSR J0437-4715

The closest and brightest millisecond pulsar (MSP), PSR J0437-4715, is a high-priority pulsar for precision timing experiments, including the Parkes Pulsar Timing Array (PPTA) project, where it is used for both pulsar timing and for polarization calibration. Using approximately seven years of observations obtained with the Ultra-Wideband Low-Frequency receiver on Murriyang, CSIRO's Parkes radio telescope, we investigate the long-term spectro-polarimetric variability of PSR J0437-4715. We report the discovery of two consecutive discrete profile change events exhibiting the same phase-localised morphological variations, the first such behaviour reported in any MSP. Both events affect three phase-localised regions of the pulse profile that exhibit coupled and anti-correlated temporal evolution. We also identified, and corrected for, systematic profile variability induced by polarization calibration errors arising from long-term drift of the polarimetric properties of the receiver. The profile change exhibits frequency-dependent evolution in total intensity, together with variations in linear polarization, position angle and fractional linear polarization. These results confirm a magnetospheric origin for the events rather than propagation effects caused by the interstellar medium, and provide potential new constraints on MSP emission models.

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arXiv2026

Exploring the Galactic plasma with pulsars in the SKA Era

The ionised media that permeate the Milky Way have been active topics of research since the discovery of pulsars in 1967. In fact, pulsars allow one to study several aspects of said plasma, such as their column density, turbulence, scattering measures, and discrete, intervening structures between the neutron star and the observer, and aspects of the magnetic field throughout. Such sources of information allow us to characterise the electron distribution in the terrestrial ionosphere, the Solar Wind, and our Galaxy and have an important impact on other experiments involving pulsars such as Pulsar Timing Arrays. In this article, we review the state-of-the-art of plasma research using pulsars, the aspects that should be taken into consideration for optimal plasma studies, and we provide future perspectives on improvements to those enabled by the SKA.

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arXiv2026

Spectroscopic surveys with the SKA probing the ionized and molecular Milky Way

Radio spectroscopic surveys provide us with a comprehensive picture of the Milky Way across many physical and chemical regimes. Spectral lines primarily probe the multi-phase gaseous interstellar medium (ISM) from its ionized to atomic and molecular phases, and constrain both local and galaxy-scale kinematics and structure through Doppler shifts. By investigating the physical and chemical properties and distribution of the ISM, the processes driving star formation and galaxy evolution can be studied in detail. This motivates line surveys of our Galaxy that allow us to use the range of physical conditions found in the Milky Way as a template for understanding star formation in extragalactic environments. In this chapter, we describe the science enabled by the spectroscopy of small molecules and radio recombination lines of atoms toward a range of Galactic environments with the SKA. We address questions concerning the processes that dictate the formation of molecular clouds (OH, CH), the properties of warm, ionized gas and the potential of HII regions in understanding the structure of the Galaxy (radio recombination lines), and the impact of CO-dark molecular gas across various density regimes on star formation and galaxy evolution (OH, H2CO). We propose a survey that includes the inner and outer Galaxy disk, characterized by a broad range of densities, temperatures, and metallicities. Deep, wide-field observations of small molecules will be uniquely accessible with SKA, providing key insights on the condition of interstellar medium in galaxies and its impact on star formation.

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arXiv2026

The Multi-phase HI of the Milky Way and Nearby Galaxies

Atomic hydrogen (HI) is the dominant baryonic component of the interstellar medium (ISM) in Milky Way-like galaxies and the reservoir from which molecular clouds and stars ultimately form. The condensation of diffuse HI into cold structures is governed by a complex interplay between radiative cooling, turbulence, magnetic fields, stellar feedback, and galactic dynamics, acting over scales ranging from astronomical units to kiloparsecs. Understanding how these processes regulate the thermal structure of the HI, the formation of cold clouds, and the transfer of matter and energy across scales is essential for connecting the small-scale physics of the ISM to the evolution of galaxies. Recent advances from SKA precursors have transformed our view of the atomic ISM, revealing a highly structured and filamentary cold medium, increasing the density of HI absorption measurements by orders of magnitude, and enabling new approaches to infer the thermodynamic and magnetic properties of the gas from spectral-line datasets. SKA-mid will provide the first comprehensive characterization of HI as a multi-phase, turbulent, and magnetized medium across the Milky Way and nearby galaxies. Its combination of sensitivity, angular resolution, spectral resolution, and survey speed will enable matched emission-absorption studies, dense optical-depth grids, and detailed mapping of the atomic-to-molecular transition over a broad range of environments. Combined with polarization, Zeeman, recombination-line, and multi-wavelength observations, SKA-mid will establish a unified observational framework to study the evolution of diffuse matter in galaxies, in connection with star formation, from the Solar neighborhood to galactic scales.

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arXiv2026

CO-dark molecular gas traced by HCO$^+$ in the diffuse interstellar medium

A classic problem in the study of the interstellar medium (ISM) is the near-invisibility of molecular hydrogen (H$_2$) in cold environments. Observations of CO emission are typically used to indirectly trace H$_2$, but a significant fraction of H$_2$ in the diffuse ISM is not associated with any detectable CO emission (``CO-dark'' molecular gas). Meanwhile, observations of H$_2$ absorption trace nearly all of the H$_2$ in diffuse directions. In particular, a kinematically broad HCO$^+$ absorption signature traces extremely diffuse, CO-dark H$_2$. We have used sensitive observations of HCO$^+$, CO, and atomic hydrogen (HI) in absorption to constrain the properties of such diffuse molecular gas in five directions. The diffuse molecular gas revealed by broad HCO$^+$ absorption has a lower fraction of cold HI ($f_{\mathrm{CNM}} = 0.38^{+0.28}_{-0.27}$) and a lower fraction of hydrogen in H$_2$ ($f_{\mathrm{mol}}=0.09^{+0.06}_{-0.03}$) than gas traced by CO in the same directions. We detect almost no CO absorption from the gas traced by broad HCO$^+$ absorption. We constrain the CO abundance relative to H$_2$ to be $\lesssim10^{-6}$-$10^{-5}$ for gas traced by both broad and narrow HCO$^+$ absorption, consistent with chemical model predictions for the diffuse ISM. We further show that neither CO emission nor absorption is likely to be detected where $N(\mathrm{H_2})\lesssim\mathrm{few}\times10^{19}$ $\mathrm{cm^{-2}}$ - a result of both the low CO abundance and the low H$_2$ column - while HCO$^+$ absorption is readily detected for $N(\mathrm{H_2})\gtrsim\text{few}\times10^{18}$ $\mathrm{cm^{-2}}$. These results demonstrate that even modest amounts of cold HI can bear H$_2$, providing critical constraints on the HI-to-H$_2$ transition in the ISM.

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arXiv2026

Characterising and mitigating Bluetooth and WiFi radio frequency interference at the Parkes Observatory

We present a detailed characterisation of radio frequency interference (RFI) in the 2.4 GHz band around Murriyang, CSIRO's Parkes radio telescope. The dominant sources of interference are Wi-Fi and Bluetooth transmissions. We quantify how the intensity and directionality of this RFI vary with time of day and document its evolution over several years. Although most observers currently discard data within this band, our analysis shows that the interference is confined in both timeand frequency and can be effectively mitigated. Using 10 seconds of 16-bit voltage data recorded during observations of the Vela Pulsar (PSR J0835-4510), we demonstrate that the majority of the channelised data remain unaffected by RFI. We compare three RFI detection and mitigation algorithms and evaluate their relative performance. All methods perform effectively, and any could be implemented in real time to enable productive use of this observing band. A real time implementation would allow the scientific use of this 128 MHz observing band to increase, from almost 70% of the band being completely unusable all of the time, to over 90% of becoming accessible for science. Given its simplicity and efficiency, a basic power-threshold approach offers a relatively straightforward solution.

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arXiv2026

Murriyang cryogenic phased array feed: spectral-line results and noise-reduction methods

Spectral-line results from a new cryogenic phased array feed (cryoPAF) on the Murriyang telescope at Parkes are presented. This array offers a significant improvement in field of view, aperture efficiency, bandwidth, chromaticity and survey speed compared with conventional horn-fed receivers. We demonstrate this with measurements of sky calibrators and observations of 21-cm neutral hydrogen (HI) in the LMC and the nearby galaxy NGC 6744. Within 0.3 deg of the optical axis, the ratio of system temperature to dish aperture efficiency is 25 K and the ratio with beam efficiency is 21 K (at 1.4 GHz). For the previously measured $T_{sys} = 17$ K, respective efficiency values 0.7 and 0.8 are derived. Our HI observational results are in good agreement with previous results, although detailed comparison with multibeam observations of the LMC suggests that the earlier observations may have missed an extended component of low-column-density gas ($8\times 10^{18}$ cm$^{-2}$). We use the cryoPAF zoom-band and wideband data to make a preliminary investigation of whether the large number of simultaneous beams (72) permits the use of novel data reduction methods to reduce the effects of foreground/background continuum contamination and RFI. We also investigate if these methods can better protect against signal loss for the detection of faint, extended cosmological signals such as HI intensity maps. Using robust higher-order singular value decomposition (SVD) techniques, we find encouraging results for the detection of both compact and extended sources, including challenging conditions with high RFI occupancy and significant sky continuum structure. Examples are shown that demonstrate that 3D SVD techniques offer a significant improvement in noise reduction and signal capture compared with more traditional layered 2D techniques.

astro-ph.IM↗

arXiv2025

Exploring Galactic plasma with pulsars in the SKA era

The ionised media that permeate the Milky Way have been active topics of research since the discovery of pulsars in 1967. In fact, pulsars allow one to study several aspects of said plasma, such as their column density, turbulence, scattering measures, and discrete, intervening structures between the neutron star and the observer, as well as aspects of the magnetic field throughout. Such sources of information allow us to characterise the electron distribution in the terrestrial ionosphere, the Solar Wind, and our Galaxy, as well as the impact on other experiments involving pulsars, such as Pulsar Timing Arrays. In this article, we review the state-of-the-art in plasma research using pulsars, the aspects that should be taken into consideration for optimal plasma studies, and we provide future perspectives on improvements enabled by the SKA.

astro-ph.HE↗

arXiv2025

The Local Group L-band Survey: Probing Cold Atomic Gas in IC10 with Neutral Hydrogen Absorption

We present the first localized detections of the cold neutral medium (CNM) in IC10, offering a rare view of dense atomic gas in a low-metallicity (0.27 solar metallicity) dwarf galaxy. As a low-metallicity starburst, IC10's interstellar medium conditions could reflect small-scale physical conditions that mirror those of early galaxies, providing a unique window into the heating and cooling processes that shaped the interstellar medium in early-Universe environments. Leveraging the high angular (<5'' ~ 15pc) and spectral (0.4 km/s) resolution of the Local Group L-band Survey, we searched for HI absorption against nine continuum radio sources and detected absorption along three sightlines corresponding to internal radio emission sources within IC10. Using Gaussian decomposition and radiative transfer, we characterize the CNM, deriving spin temperatures of ~30-55 K, column densities of (0.6-3.0)x 1$0^{21}$ cm$^{-2}$, cold HI fractions of ~ 21-37%, and line widths of ~ 5.6-13.6 km/s. For each individual detection of HI absorption, we find corresponding molecular emission from 12CO (J=1-0), HCO+ (J=1-0), and HCN (J=1-0) at similar velocities and with comparable linewidths, indicating a well-mixed cold atomic and molecular medium. In IC10, the CNM shows a clear kinematic connection to the high-density ISM, implying a stronger dynamical coupling with molecular gas than in the Milky Way, in line with expectations for low-metallicity environments. At the ~ 15 pc scales probed by slightly extended HII regions in IC10, unresolved CNM clouds likely contribute to line blending, so the observed broad HI linewidths may partly reflect spatial and kinematic averaging.

astro-ph.GA↗

arXiv2025

Tracing Gas Kinematics and Interactions between H II Regions and Molecular Clouds using VLA Observations of Recombination Lines and Hydroxyl

Observational studies of HII region-molecular cloud interactions constrain models of feedback and quantify its impact on the surrounding environment. A recent hypothesis proposes that a characteristic spectral signature in ground state hyperfine lines of hydroxyl (OH) -- the OH flip -- may trace gas that is dynamically interacting with an expanding HII region, offering a new means of probing such interactions. We explore this hypothesis using dedicated Jansky Very Large Array (VLA) observations of three Galactic HII regions, G049.205$-$0.343, G034.256+0.145 and G024.471+0.492, in 1--2 GHz continuum emission, all four 18-cm ground-state OH lines, and multiple hydrogen radio recombination lines. A Gaussian decomposition of the molecular gas data reveals complex OH emission and absorption across our targets. We detect the OH flip towards two of our sources, G049.205$-$0.343 and G034.256+0.145, finding agreement between key predictions of flip hypothesis and the observed multi-wavelength spectra, kinematics and morphology. Specifically, we demonstrate a strong spatial and kinematic association between the OH flip and the ionized gas of the HII regions -- the first time this has been demonstrated for resolved sources -- and evidence from $^{13}$CO(1--0) data that the expected OH component originates from the non-disturbed gas of the parent cloud. While we detect no flip in G024.471+0.492, we do find evidence of interacting molecular gas traced by OH, providing further support for OH's ability to trace HII region-molecular cloud interactions.

astro-ph.GA↗

arXiv2025

Multi-wavelength probes of the Milky Way's Cold Interstellar Medium: Radio HI and Optical KI Absorption with GASKAP and GALAH

We present a comparative analysis of interstellar hydrogen (HI) and potassium (KI) absorption from the radio and optical surveys, GASKAP and GALAH, to study the physical and kinematic properties of the cold interstellar medium (ISM) in the Milky Way foreground towards the Magellanic Clouds. By comparing GASKAP HI absorption with interstellar KI absorption detected in GALAH spectra of nearby stars (within 12 arcmin angular distance or a spatial separation of ~0.75 pc), we reveal a strong kinematic correlation between these two tracers of the cold neutral ISM. The velocity offsets between matched HI and KI absorption components are small, with a mean (median) offset of -1.3 (-1.2) km s-1 and a standard deviation of 2.3 km s-1. The high degree of kinematic consistency suggests a close spatial association between Ki and cold HI gas. Correlation analyses reveal a moderate positive relationship between HI and KI line-of-sight properties, such as KI column density with HI column density or HI brightness temperature. We observe a ~63% overlap in the detection of both species towards 290 (out of 462) GASKAP HI absorption lines of sight, and estimate a median KI/HI abundance ratio of ~2.3 x 10^(-10), in excellent agreement with previous findings. Our work opens up an exciting avenue of Galactic research that uses large-scale surveys in the radio and optical wavelengths to probe the neutral interstellar medium through its diverse tracers.

astro-ph.GA↗

arXiv2025

Ultra-Wideband Polarimetry of the April 2021 Profile Change Event in PSR J1713+0747

The millisecond pulsar PSR J1713+0747 is a high-priority target for pulsar timing array experiments due to its long-term timing stability, and bright, narrow pulse profile. In April 2021, PSR~J1713$+$0747 underwent a significant profile change event, observed by several telescopes worldwide. Using the broad-bandwidth and polarimetric fidelity of the Ultra-Wideband Low-frequency receiver on Murriyang, CSIRO's Parkes radio telescope, we investigated the long-term spectro-polarimetric behaviour of this profile change in detail. We highlight the broad-bandwidth nature of the event, which exhibits frequency dependence that is inconsistent with cold-plasma propagation effects. We also find that spectral and temporal variations are stronger in one of the orthogonal polarisation modes than the other, and observe mild variations ($\sim 3$ - $5\,σ$ significance) in circular polarisation above 1400 MHz following the event. However, the linear polarisation position angle remained remarkably stable in the profile leading edge throughout the event. With over three years of data post-event, we find that the profile has not yet recovered back to its original state, indicating a long-term asymptotic recovery, or a potential reconfiguration of the pulsar's magnetic field. These findings favour a magnetospheric origin of the profile change event over a line-of-sight propagation effect in the interstellar medium.

astro-ph.HE↗

arXiv2025

Faint absorption of the ground state hyperfine-splitting transitions of hydroxyl at 18 cm in the Galactic Disk

The interstellar hydride hydroxyl (OH) is a potential tracer of CO-dark molecular gas. We present new absorption line observations of OH at 18-cm wavelength towards four continuum sources. We compare these to the [CII] line at 1.9 THz obtained with SOFIA, observations of the neutral atomic hydrogen 21 cm line with the VLA, and CO lines obtained with APEX. We trace OH over a large range of molecular hydrogen column densities, and derive OH abundances with respect to molecular and total hydrogen column densities. Increased sensitivity and spectral resolution allowed us to detect weak and narrow features. We identify only one OH absorption component out of 23 without CO counterpart, yet several with intermediate molecular gas fractions. A potential association of [CII] 158 mu m emission with an OH absorption component is seen toward one sightline. Our results confirm that OH absorption traces molecular gas across diffuse and dense environments of the interstellar medium. At the sensitivity limits of the present observations our detection of only one CO-dark molecular gas feature appears in agreement with previous studies. We conclude that if OH absorption was to be used as a CO-dark molecular gas tracer, deeper observations or stronger background targets are necessary to unveil its full potential as a CO-dark molecular gas tracer, and yet it will never be an exclusive tracer of CO-dark molecular gas. For OH hyperfine-splitting transitions in the vicinity of photodissociation regions in W43-South, we detect a spectral and spatial offset between the peak of the inversion of the OH 1612 MHz line and the absorption of the OH 1720 MHz line on the one hand, and the absorption of the OH main lines on the other hand, which provides additional constraints on the interpretation of the OH 18 cm line signatures typical of HII regions.

astro-ph.GA↗

arXiv2025

Unlocking the hidden potential of pulsar astronomy

Pulsars have traditionally been used for research into fundamental physics and astronomy. In this paper, we investigate the expanding applications of radio pulsars in societal and industrial domains beyond their conventional scientific roles. We describe emerging applications in positioning, navigation, timing and synchronization, random number generation, space weather monitoring, public engagement, antenna calibration techniques, and leveraging extensive pulsar data sets generated by large-scale observatories. Such pulsar data sets have already been used to demonstrate quantum-computing algorithms. We evaluate the potential for compact radio receiver systems for pulsar detection by describing optimal observing bands. We show that relatively simple and compact receiver systems can detect the brightest pulsar, Vela. The equivalent of an ~4m-diameter dish with a small bandwidth operating around 700 MHz would be able to detect many more pulsars. Such a detector would be able to localise itself to around 10 km using pulsar navigation techniques. The space weather community requires direct measurements of the integrated electron density at a range of solar elongations. The only method to get model-independent values is through pulsar observations and we explore the possibility of measuring dispersion measures (DMs) and rotation measures with a range of telescopes (observing from low to mid-frequencies) as well as using a typical model to predict the variation of the DM as a function of solar radii. We review how pulsars can be used to produce random sequences and demonstrate that such sequences can be produced using the scintillation properties of pulsars as well as from pulse jitter.

astro-ph.IM↗

arXiv2025

Tiny-Scale Properties within the Interstellar Medium towards PSR J1644$-$4559: I. Observational Evidence of Turbulence-induced Tiny Scale Atomic Structures

We investigated HI absorption toward a single pulsar, PSR J1644$-$4559, and its variability over timescales from days to years, using Murriyang, CSIRO's Parkes Radio Telescope. Our 19 epochs of spectral observations, spanning 1.2 years with intervals as short as 1 day, provide the most comprehensive cadence coverage for monitoring HI absorption to date. We identified two significant detections of tiny-scale atomic structure (TSAS) with spatial scales ranging from a lower limit of $\sim$11 au to an upper limit of 165 au, both exhibiting integrated signal-to-noise ratios exceeding 5.0. We find a relationship between linear size and optical depth variation in the cold neutral medium (CNM) component hosting the TSAS, described by a power-law relationship, $Δτ_{\rm int} = Δτ_0 (ΔL)^{(α-2)/2}$, with $α= 4.1 \pm 0.4$. This is the first observational evidence explicitly connecting TSAS to turbulence in CNM. This power-law index is significantly steeper than previously reported values for the CNM, where $α$ ranges from 2.3 to 2.9, but similar to those observed in the warm ionized gas. Additionally, we observe no significant variation in $α$ across the entire range of spatial scales traced in our study, indicating that turbulence may be cascading down and dissipating at smaller scales. While there is no precise proper motion measurement for this pulsar, our estimates for the turbulence dissipation in the CNM place the lower and upper limits at less than 0.03 au and 0.4 au, respectively.

astro-ph.GA↗

arXiv2025

First Search for Pulsed CH Maser Emission Stimulated by a Pulsar

We present the first search for pulsed CH maser emission potentially stimulated by PSR J1644$-$4559, conducted using the ultra-wide-bandwidth low-frequency receiver on Murriyang, CSIRO's Parkes Radio Telescope. Observations targeted three CH $Λ$-doublet transitions at 3264, 3335, and 3349 MHz, with a variability timescale of 78 ms. We detected ten CH emission features at 3335 and 3349 MHz, and seven features at 3264 MHz, during both pulsar-ON and pulsar-OFF phases. The observed velocities align with the OH emission and absorption reported by a previous study, suggesting a close spatial association between CH and OH molecules. The derived column densities for CH clouds within the Parkes beam range from $0.05$ to $9.8 \times 10^{13}$ cm$^{-2}$, indicating that these clouds are likely in diffuse and translucent states. Upper limits for CH column densities within the pulsar beam ranged from $0.3$ to $9.8 \times 10^{13}$ cm$^{-2}$. Comparison of these column densities suggests that CH clouds may exhibit clumpiness and substructure. No significant stimulated emission feature was detected in the optical depth spectra. Additionally, as part of our search for pulsed stimulated emission, we investigated the potential CH absorption of the pulsar signal and found none, in agreement with astrophysical expectations. The upper limits for the potential maser amplification factors towards PSR J1644$-$4559 at 3264, 3335, and 3349 MHz are 1.014, 1.009, and 1.009, respectively. This study demonstrates the feasibility of detecting pulsed CH maser emission in the interstellar medium stimulated by pulsar photons.

astro-ph.GA↗

arXiv2024

Multi-phase HI clouds in the Small Magellanic Cloud halo

Context. The Galactic ASKAP collaboration (GASKAP) is undertaking an HI emission survey of the 21cm line to map the Magellanic system and the Galactic plane with the Australian Square Kilometre Array Pathfinder (ASKAP). One of the first areas observed in the Pilot Phase I of the survey was the Small Magellanic Cloud (SMC). Previous surveys of the SMC have uncovered new structures in the periphery of the SMC, along relatively low column density lines of sight. Aims. In this work we aimed to uncover the phase distribution of three distinct structures in the periphery of the SMC. This work will add to the constraints we have on the existence and survival of the cold neutral medium (CNM) in the SMC. Methods. We used ROHSA, a Gaussian decomposition algorithm, to model the emission across each cloud and classify the HI emission into their respective phases based on the linewidths of the fitted Gaussians. We created maps of velocity and column density of each phase of the HI across these three clouds. We measured the HI mass and CNM number density for each cloud. We also compared the HI results across the different phases with other gas tracers. Results. We find that in two clouds, the ends of each cloud are almost completely CNM dominated. Analysis of these two clouds indicates they are experiencing a compressive force from the direction of the SMC main body. In the third cloud we find a uniform CNM distribution along one wall of what is likely a supershell structure. Comparison with previous measurements of CO clumps in two of the clouds show the CO and HI are co-moving within a few km/s in regions of high HI column density, particularly when considering just the CNM.

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