arXiv subjects

Ian Kemp

Publications and source records attributed to Ian Kemp.

10 recordsLinked to original sources

arXiv2026

The GASKAP-HI Survey towards the Magellanic Clouds: Cold Atomic Gas Survival and Evolution in the Large Magellanic Cloud

We use atomic hydrogen (HI) absorption detections from the GASKAP-HI survey to investigate the properties of cold atomic gas in the Large Magellanic Cloud (LMC). Using the radiative transfer method, we decompose 155 sightlines into 330 cold neutral medium (CNM), 2 thermally unstable neutral medium (UNM), and 310 warm neutral medium (WNM) components. We find that the CNM in the LMC exhibits higher optical depths (median 0.46), lower spin temperatures (median $\sim$37 K), broader linewidths (median $\sim$4.9 km s$^{-1}$), and slightly lower CNM fractions (median $\sim$23%) than in the Milky Way. We examine the connection between the CNM, molecular gas, and star formation, finding that CNM correlates more closely with molecular gas than WNM, while molecular gas shows a tighter relation with star formation. Molecular hydrogen (H$_2$) formation begins near $N{_\mathrm{HI,CNM}}\sim10^{20}~\mathrm{cm^{-2}}$, and nearly all sightlines with $N_{\mathrm{HI,CNM}}>10^{21}~\mathrm{cm^{-2}}$ contain molecular gas. The CNM fraction increases with visual extinction ($A_V$), and the LMC maintains CNM fractions comparable to those in the Milky Way at substantially lower $A_V$, likely due to higher local densities and a longer line-of-sight depth. Sightlines near expanding shells tend to show higher CNM fractions, although this is partly driven by higher total HI column densities. Finally, the CNM kinematics generally follow the HI disk rotation, with about 7% of components showing velocity offsets greater than $25~\mathrm{km~s^{-1}}$, likely tracing inflows or outflows driven by stellar feedback or large-scale interactions within the Magellanic System.

astro-ph.GA↗

arXiv2026

Revealing the cold skeleton of the Magellanic Clouds and the Magellanic Bridge with ASKAP

We present the GASKAP-HI pilot absorption survey of neutral hydrogen (HI) in the Magellanic system. This survey provides 3219 sightlines across the Large (LMC) and Small Magellanic Clouds (SMC) and the Magellanic Bridge (MB) towards 1.4-GHz continuum sources, representing a 15-fold increase on pre--GASKAP-HI sampling of the Magellanic System. We find 344 candidate detections of cold gas at Magellanic velocities (vLSRK >= 90 km s-1), with signal-to-noise ratio > 3 detection rates of 44% (LMC; 192 of 438), 73% (SMC; 85 of 117) and 4% (MB; 35 of 793). We examine the candidate detections within the MB, Gaussian decompose these and examine the cold gas across the MB. Here we find that the majority of cold gas detections are found closer to the SMC. We also find potential evidence of the recent formation of cold gas on the outskirts of a shell within the MB. We find a mean cold gas fraction of fCNM = 0.12 +- 0.08 for the MB, which is very similar to the SMC and lower than the LMC value of 0.14. Overall, we reveal cold gas distributed extensively across the Magellanic system, including within the MB, and surmise that the cold gas in the MB is either pulled from the SMC as part of the formation of the MB, or formed in the turbulence of those same interactions.

astro-ph.GA↗

arXiv2025

A High-resolution Study of the Cold Neutral Medium in and around 30 Doradus

With the aim of evaluating the roles of the cold neutral medium (CNM) in the cloud-scale baryon cycle, we perform a high-resolution study of the CNM in and around the extreme star-forming region 30 Doradus (30 Dor). For our study, we use Galactic Australian Square Kilometre Array Pathfinder H I Survey data and produce H I emission and absorption cubes on 7 pc scales. To examine the CNM structures toward 30 Dor, we decompose the H I absorption cube into 862 Gaussian components and find that these components are distributed at four velocity ranges (B1, B2, B3, and B4, respectively): 200$-$230 km s$^{-1}$, 230$-$260 km s$^{-1}$, 260$-$277 km s$^{-1}$, and 277$-$300 km s$^{-1}$. We derive line-of-sight average spin temperatures and opacity-corrected total H I column densities and show that the B1$-$B4 structures have systematically different properties, indicating that they are physically distinct. As for the nature of the observed CNM structures, we find that B2 is associated with the main dense structure where ionized, atomic, and molecular gases are concentrated. B3 and B4 trace inflows whose combined mass flux rate of 0.14 $M_{\odot}$ yr$^{-1}$ is comparable to the current star formation rate, while B1 probes outflows with a much lower mass flux rate of 0.007 $M_{\odot}$ yr$^{-1}$. Interestingly, the H I column densities in B1$-$B4 are nearly uniform with a factor of two spatial variations, implying the presence of H I shielding layers for H$_{2}$ formation.

astro-ph.GA↗

arXiv2025

Study of HI Turbulence in the SMC Using Multi-point Structure Functions

Turbulence in the interstellar medium (ISM) plays an important role in many physical processes, including forming stars and shaping complex ISM structures. In this work, we investigate the HI turbulent properties of the Small Magellanic Cloud (SMC) to reveal what physical mechanisms drive the turbulence and at what scales. Using the high-resolution HI data of the Galactic ASKAP (GASKAP) survey and multi-point structure functions (SF), we perform a statistical analysis of HI turbulence in 34 subregions of the SMC. Two-point SFs tend to show a linear trend, and their slope values are relatively uniform across the SMC, suggesting that large-scale structures exist and are dominant in the two-point SFs. On the other hand, seven-point SF enables us to probe small-scale turbulence by removing large-scale fluctuations, which is difficult to achieve with the two-point SFs. In the seven-point SFs, we find break features at scales of 34-84 pc, with a median scale of $\sim$50 pc. This result indicates the presence of small-scale turbulent fluctuations in the SMC and quantifies its scale. In addition, we find strong correlations between slope values of the seven-point SFs and the stellar feedback-related quantities (e.g., H$\alpha$ intensities, the number of young stellar objects, and the number of HI shells), suggesting that stellar feedback may affect the small-scale turbulent properties of the HI gas in the SMC. Lastly, estimated sonic Mach numbers across the SMC are subsonic, which is consistent with the fact that the HI gas of the SMC primarily consists of the warm neutral medium.

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

Experiences of Commercial Supercomputing in Radio Astronomy Data Processing

The ongoing exponential growth of computational power, and the growth of the commercial High Performance Computing (HPC) industry, has led to a point where ten commercial systems currently exceed the performance of the highest-used HPC system in radio astronomy in Australia, and one of these exceeds the expected requirements of the Square Kilometre Array (SKA) Science Data Processors. In order to explore implications of this emerging change in the HPC landscape for radio astronomy, we report results from a survey conducted via semi-structured interviews with 14 Australian scientists and providers with experience of commercial HPC in astronomy and similar data intensive fields. We supplement these data with learnings from two earlier studies in which we investigated the application of commercial HPC to radio astronomy data processing, using cases with very different data and processing considerations. We use the established qualitative research approach of thematic analysis to extract key messages from our interviews. We find that commercial HPC can provide major advantages in accessibility and availability, and may contribute to increasing researchers' career productivity. Significant barriers exist, however, including the need for access to increased expertise in systems programming and parallelisation, and a need for recognition in research funding. We comment on potential solutions to these issues.

astro-ph.IM↗

arXiv2025

A neutral hydrogen absorption study of cold gas in the outskirts of the Magellanic Clouds using the GASKAP-HI survey

Cold neutral hydrogen (HI) is a crucial precursor for molecular gas formation and can be studied via HI absorption. This study investigates HI absorption in low column density regions of the Small and Large Magellanic Clouds (SMC and LMC) using the Galactic-ASKAP HI (GASKAP-HI) survey, conducted by the Australian Square Kilometer Array Pathfinder (ASKAP). We select 10 SMC directions in the outer regions and 18 LMC directions, with 4 in the outskirts and 14 within the main disk. Using the radiative transfer method, we decompose the emission and absorption spectra into individual cold neutral medium (CNM) and warm neutral medium (WNM) components. In the SMC, we find HI peak optical depths of 0.09-1.16, spin temperatures of ~20-50 K, and CNM fractions of 1-11%. In the LMC, optical depths range from 0.03 to 3.55, spin temperatures from ~10 to 100 K, and CNM fractions from 1% to 100%. The SMC's low CNM fractions likely result from its low metallicity and large line-of-sight depth. Additionally, the SMC's outskirts show lower CNM fractions than the main body, potentially due to increased CNM evaporation influenced by the hot Magellanic Corona. Shell motions dominate the kinematics of the majority of CNM clouds in this study and likely supply cold HI to the Magellanic Stream. In the LMC, high CNM fraction clouds are found near supergiant shells, where thermal instability induced by stellar feedback promotes WNM-to-CNM transition. Although no carbon monoxide (CO) has been detected, enhanced dust shielding in these areas helps maintain the cold HI.

astro-ph.GA↗

arXiv2025

Considerations with stacking absorption spectra: cold HI gas in cirrus region of the Milky Way

We use the Milky Way neutral hydrogen (HI) absorption and emission spectra from the Galactic Australian Square Kilometre Array Pathfinder (GASKAP) Phase II Pilot survey along with toy models to investigate the effects of stacking multicomponent spectra on measurements of peak optical depth and spin temperature. Shifting spectra by the peak in emission, 'primary' components shifted to 0 km s$^{-1}$ are correctly averaged. Additional components on individual sightlines are averaged with non-centred velocities, producing a broader and shallower 'secondary' component in the resulting stack. Peak optical depths and brightness temperatures of the secondary components from stacks are lower limits of their true average values due to the velocity offset of each component. The spin temperature however is well correlated with the truth since the velocity offset of components affects the emission and absorption spectra equally. Stacking 462 GASKAP absorption-emission spectral pairs, we detect a component with a spin temperature of 1320 $\pm$ 263 K, consistent with gas from the unstable neutral medium and higher than any previous GASKAP detection in this region. We also stack 2240 pilot survey spectra containing no Milky Way absorption, revealing a primary narrow and secondary broad component, with spin temperatures belonging to the cold neutral medium (CNM). Spatially binning and stacking the non-detections across the plane-of-sky by their distance from CNM absorption detections, the primary component's optical depth decreases with distance from known locations of cold gas. The spin temperature however remains stable in both components, over an approximate physical plane-of-sky distance of $\sim$ 100 pc.

astro-ph.GA↗

arXiv2024

Local HI Absorption towards the Magellanic Cloud foreground using ASKAP

We present the largest Galactic neutral hydrogen HI absorption survey to date, utilizing the Australian SKA Pathfinder Telescope at an unprecedented spatial resolution of 30''. This survey, GASKAP-HI, unbiasedly targets 2,714 continuum background sources over 250 square degrees in the direction of the Magellanic Clouds, a significant increase compared to a total of 373 sources observed by previous Galactic absorption surveys across the entire Milky Way. We aim to investigate the physical properties of cold (CNM) and warm (WNM) neutral atomic gas in the Milky Way foreground, characterized by two prominent filaments at high Galactic latitudes (between $-45^{\circ}$ and $-25^{\circ}$). We detected strong HI absorption along 462 lines of sight above the 3$\sigma$ threshold, achieving an absorption detection rate of 17%. GASKAP-HI's unprecedented angular resolution allows for simultaneous absorption and emission measurements to sample almost the same gas clouds along a line of sight. A joint Gaussian decomposition is then applied to absorption-emission spectra to provide direct estimates of HI optical depths, temperatures, and column densities for the CNM and WNM components. The thermal properties of CNM components are consistent with those previously observed along a wide range of Solar neighborhood environments, indicating that cold HI properties are widely prevalent throughout the local interstellar medium. Across our region of interest, CNM accounts for ~30% of the total HI gas, with the CNM fraction increasing with column density toward the two filaments. Our analysis reveals an anti-correlation between CNM temperature and its optical depth, which implies that CNM with lower optical depth leads to a higher temperature.

astro-ph.GA↗

arXiv2024

An image-based blind search for Fast Radio Bursts in 88 hours of data from the EoR0 Field, with the Murchison Widefield Array

This work is part of ongoing efforts to detect Fast Radio Bursts (FRBs) using the Murchison Widefield Array (MWA) in a spectral window below 300 MHz. We used an image-based method based on the pilot study of Tingay et al. 2015, scaled up via massively parallel processing using a commercial supercomputer. We searched 87.6 hours of 2-second snapshot images, each covering 1165 square degrees of the EoR0 field, over a dispersion measure range of 170 to 1035 pc cm$^{-3}$. The large amount of data necessitated the construction of a series of filters to classify and reject the large number of false positives. Our search was more sensitive than any previous blind search using the MWA telescope, but we report no FRB detections, a result which is consistent with the extrapolation into the low-frequency domain of the results of Sokolowski et al. (2024). We obtain upper bounds on the event rate ranging from <1783 sky$^{-1}$day$^{-1}$ at a fluence of 392 Jy ms, to <31 sky$^{-1}$day$^{-1}$ at 8400 Jy ms, for our spectral window of 167-198 MHz. Our method was shown to be computationally efficient and scalable by the two or three orders of magnitude required to seriously test the model of Sokolowski et al. Our process is especially sensitive to detections of satellites and meteor trails and may find applications in the identification of these transients. We comment on future surveys using this method, with both the MWA and the SKA.

astro-ph.IM↗