We present an analysis of the hard X-ray emission from the central region of Abell 3667 using deep NuSTAR observations. While previous studies on the nature of the hard X-ray excess have been controversial, our analysis of the central region suggests that the excess is primarily thermal, best described by a two-temperature (2T) model, with the high-temperature component likely arising from merger-induced heating. This interpretation contrasts with some earlier suggestions of non-thermal emission due to inverse Compton scattering of relativistic electrons. Additionally, we set a lower limit on the magnetic field strength of $\sim 0.2 \, \unicode{x03BC}$G in the central region, consistent with values found in other dynamically active clusters and compatible with those inferred from equipartition and Faraday rotation measurements. Since our study is focused on the central region of the cluster, further high-resolution observations of the outer regions will be critical to fully disentangle the thermal and non-thermal contributions to the X-ray.

We analyse a volume-limited sample from the Sloan Digital Sky Survey to compare the spatial clustering and physical properties of active galactic nuclei (AGN) and star-forming galaxies (SFG) at fixed stellar mass. We find no statistically significant difference in clustering strength or local density between AGN and SFG. However, after matching their stellar mass distributions, we detect statistically significant differences (at a confidence level $\gt99.99\%$) in colour, star formation rate (SFR), $4000$Å break measurements (D4000), and morphology. These differences persist across both low- and high-density environments, suggesting that AGN are not driven by environmental factors. The development of favourable conditions for AGN activity within a galaxy may depend on the diverse evolutionary histories of galaxies. Our results imply that AGN activity may arise stochastically, modulated by the complex assembly history of galaxies.

The First Large Absorption Survey in H i (FLASH) is a large-area radio survey for neutral hydrogen in and around galaxies in the intermediate redshift range $0.4\lt z\lt1.0$, using the 21-cm H i absorption line as a probe of cold neutral gas. The survey uses the ASKAP radio telescope and will cover 24,000 deg$^2$ of sky over the next five years. FLASH breaks new ground in two ways – it is the first large H i absorption survey to be carried out without any optical preselection of targets, and we use an automated Bayesian line-finding tool to search through large datasets and assign a statistical significance to potential line detections. Two Pilot Surveys, covering around 3000 deg$^2$ of sky, were carried out in 2019-22 to test and verify the strategy for the full FLASH survey. The processed data products from these Pilot Surveys (spectral-line cubes, continuum images, and catalogues) are public and available online. In this paper, we describe the FLASH spectral-line and continuum data products and discuss the quality of the H i spectra and the completeness of our automated line search. Finally, we present a set of 30 new H i absorption lines that were robustly detected in the Pilot Surveys, almost doubling the number of known H i absorption systems at $0.4\lt z\lt1$. The detected lines span a wide range in H i optical depth, including three lines with a peak optical depth $\tau\gt1$, and appear to be a mixture of intervening and associated systems. Interestingly, around two-thirds of the lines found in this untargeted sample are detected against sources with a peaked-spectrum radio continuum, which are only a minor (5–20%) fraction of the overall radio-source population. The detection rate for H i absorption lines in the Pilot Surveys (0.3 to 0.5 lines per 40 deg$^2$ ASKAP field) is a factor of two below the expected value. One possible reason for this is the presence of a range of spectral-line artefacts in the Pilot Survey data that have now been mitigated and are not expected to recur in the full FLASH survey. A future paper in this series will discuss the host galaxies of the H i absorption systems identified here.

We present the first results of a pilot ‘TASmanian Search for Inclined Exoplanets’ (TASSIE) program. This includes observations and analysis of five short-period exoplanet candidates using data from TESS and the Harlingten 50 cm telescope at the Greenhill Observatory. We describe the instrumentation, data reduction process and target selection strategy for the program. We utilise archival multi-band photometry and new mid-resolution spectra to determine stellar parameters for five TESS Objects of Interest (TOIs). We then perform a statistical validation to rule out false positives, before moving on to a joint transit analysis of the remaining systems. We find that TOI3070, TOI3124 and TOI4266 are likely non-planetary signals, which we attribute to either short-period binary stars on grazing orbits or stellar spots. For TOI3097, we find a hot sub-Jovian to Jovian size planet ($R_{3097Ab}$ = 0.89 $\pm$ 0.04 $R_{J}$, $P_{3097Ab}$ = 1.368386 $\pm$ 0.000006 days) orbiting the primary K dwarf star in a wide binary system. This system shows indications of low metallicity ([Fe/H] $\approx$ -1), making it an unlikely host for a giant planet. For TOI3163, we find a Jovian-size companion on a circular orbit around a late F dwarf star, with $R_{3163b} = 1.42 \,\pm 0.05 \, R_{J}$ and $P_{3163b} = 3.074966 \pm 0.000022$ days. In future, we aim to validate further southern giant planet candidates with a particular focus on those residing in the sub-Jovian desert/savanna.

Searches for impulsive, astrophysical transients are often highly computationally demanding. A notable example is the dedispersion process required for performing blind searches for fast radio bursts (FRBs) in radio telescope data. We introduce a novel approach – efficient summation of arbitrary masks (ESAM) – which efficiently computes 1D convolution of many arbitrary 2D masks and can be used to carry out dedispersion over thousands of dispersion trials efficiently. Our method matches the accuracy of the traditional brute force technique in recovering the desired signal-to-noise ratio while reducing computational cost by around a factor of 10. We compare its performance with existing dedispersion algorithms, such as the fast dispersion measure transform algorithm, and demonstrate how ESAM provides freedom to choose arbitrary masks and further optimise computational cost versus accuracy. We explore the potential applications of ESAM beyond FRB searches.

The Australian SKA Pathfinder (ASKAP) offers powerful new capabilities for studying the polarised and magnetised Universe at radio wavelengths. In this paper, we introduce the Polarisation Sky Survey of the Universe’s Magnetism (POSSUM), a groundbreaking survey with three primary objectives: (1) to create a comprehensive Faraday rotation measure (RM) grid of up to one million compact extragalactic sources across the southern $\sim50$% of the sky (20,630 deg$^2$); (2) to map the intrinsic polarisation and RM properties of a wide range of discrete extragalactic and Galactic objects over the same area; and (3) to contribute interferometric data with excellent surface brightness sensitivity, which can be combined with single-dish data to study the diffuse Galactic interstellar medium. Observations for the full POSSUM survey commenced in May 2023 and are expected to conclude by mid-2028. POSSUM will achieve an RM grid density of around 30–50 RMs per square degree with a median measurement uncertainty of $\sim$1 rad m$^{-2}$. The survey operates primarily over a frequency range of 800–1088 MHz, with an angular resolution of 20” and a typical RMS sensitivity in Stokes Q or U of 18 $\mu$Jy beam$^{-1}$. Additionally, the survey will be supplemented by similar observations covering 1296–1440 MHz over 38% of the sky. POSSUM will enable the discovery and detailed investigation of magnetised phenomena in a wide range of cosmic environments, including the intergalactic medium and cosmic web, galaxy clusters and groups, active galactic nuclei and radio galaxies, the Magellanic System and other nearby galaxies, galaxy halos and the circumgalactic medium, and the magnetic structure of the Milky Way across a very wide range of scales, as well as the interplay between these components. This paper reviews the current science case developed by the POSSUM Collaboration and provides an overview of POSSUM’s observations, data processing, outputs, and its complementarity with other radio and multi-wavelength surveys, including future work with the SKA.

Radio-emitting active galactic nuclei (AGNs) are common in elliptical galaxies, and AGN feedback is one of the possible mechanisms for regulating star formation in massive galaxies. It is unclear if all passive galaxy populations host radio AGNs and if AGN feedback is a plausible mechanism for truncating or regulating star formation in these galaxies. To determine if radio AGNs are common in passive spiral galaxies, we have measured the radio emission of 38 low-redshift passive spiral galaxies using RACS-low at 887.5 MHz and VLASS at 3 GHz. We selected a subset of 2MRS galaxies with negligible WISE 12 $\mu$m emission from warm dust, and spiral morphologies from HyperLeda, RC3, 2MRS and manual inspection. In contrast to comparable early-type galaxies, our sample has no significant radio detections, with radio flux densities below 1 mJy, implying that radio AGNs are rare or non-existent in passive spirals. Using the combined radio images and assuming radio luminosity is proportional to K-band luminosity, we find $\textrm{log}\;L_\nu \lesssim 9.01-0.4\;M_K$. This falls below the radio luminosities of passive elliptical galaxies, implying radio luminosity in passive galaxies is correlated with host galaxy morphology and kinematics.

The theory of galaxy formation posits a clear correlation between the spin of galaxies and the orientation of the elements of the large-scale structure of the Universe, particularly cosmic filaments. A substantial number of observational and modelling studies have been undertaken with the aim of identifying the dependence of spin orientation on the components of the large-scale structure. However, the findings of these studies remain contradictory. In this paper, we present an analysis of the orientation of the spins of 2 861 galaxies with respect to the filaments of the large-scale structure of the Universe. All galaxies in our sample have an inclination to the line of sight greater than 85 degrees, enabling an unambiguous determination of the spin axis direction in space. We investigate the alignment of galaxy spin axes relative to cosmic web filaments as a function of various properties for galaxies. Our results reveal a statistically significant tendency for the galaxy spin axes to align along the filament axes of the large-scale structure.

We present a new radio continuum study of the Large Magellanic Cloud supernova remnant (SNR) MC SNR J0519–6902. With a diameter of $\sim$8 pc, this SNR shows a radio ring-like morphology with three bright regions towards the north, east, and south. Its linear polarisation is prominent with average values of $5\pm1$% and $6\pm1$% at 5 500 and 9 000 MHz, and we find a spectral index of ${-0.62\pm0.02}$, typical of a young SNR. The average rotation measure is estimated at ${-124\pm83}$ rad m$^{-2}$ and the magnetic field strength at ${\sim11}\,\mu$G. We also estimate an equipartition magnetic field of ${72\pm 5}\,\mu$G and minimum explosion energy of E$_\textrm{ min}$ = 2.6$\times10^{48}$ erg. Finally, we identified an H i cloud that may be associated with MC SNR J0519–6902, located in the southeastern part of the remnant, along with a potential wind-bubble cavity.

We undertake a comprehensive investigation into the distribution of in situ stars within Milky Way-like galaxies, leveraging TNG50 simulations and comparing their predictions with data from the H3 survey. Our analysis reveals that 28% of galaxies demonstrate reasonable agreement with H3, while only 12% exhibit excellent alignment in their profiles, regardless of the specific spatial cut employed to define in situ stars. To uncover the underlying factors contributing to deviations between TNG50 and H3 distributions, we scrutinise correlation coefficients among internal drivers (e.g. virial radius, star formation rate [SFR]) and merger-related parameters (such as the effective mass-ratio, mean distance, average redshift, total number of mergers, average spin-ratio, and maximum spin alignment between merging galaxies). Notably, we identify significant correlations between deviations from observational data and key parameters such as the median slope of virial radius, mean SFR values, and the rate of SFR change across different redshift scans. Furthermore, positive correlations emerge between deviations from observational data and parameters related to galaxy mergers. We validate these correlations using the Random Forest Regression method. Our findings underscore the invaluable insights provided by the H3 survey in unravelling the cosmic history of galaxies akin to the Milky Way, thereby advancing our understanding of galactic evolution and shedding light on the formation and evolution of Milky Way-like galaxies in cosmological simulations.

The abundance of dust within galaxies directly influences their evolution. Contemporary models attempt to match this abundance by simulating the processes of dust creation, growth, and destruction. While these models are accurate, they require refinement, especially at earlier epochs. This study aims to compare simulated and observed datasets and identify discrepancies between the two, providing a basis for future improvements. We utilise simulation data from the Simba cosmological simulation suite and observed data from the Galaxy and Mass Assembly (GAMA), a subset of the Cosmic Evolution Survey (G10-COSMOS), and the Hubble Space Telescope (3D-HST). We selected galaxies in the observed and simulated data in a stellar mass range of ($10^{8.59} \lt \text{M}_\odot \lt 10^{11.5}$) and at redshift bins centering around $z = 0.0$, $z = 0.1$, $z = 0.5$, $z = 1.0$, and $z = 1.5$ in a homogeneous dust mass range ($10^{6} \lt \text{M}_D [\text{ M}_\odot] \lt 10^{9}$). Our results show notable deviations between Simba and observed data for dust-poor and rich galaxies, with strong indications that differences in galaxy populations and Simba limitations are the underlying cause rather than the dust physics implemented in Simba itself.

The GLEAM 4-Jy (G4Jy) Sample is a thorough compilation of the ‘brightest’ radio sources in the southern sky (Declination $ \lt 30^{\circ}$), as measured at 151 MHz ($S_{\mathrm{151\,MHz}} \gt 4.0$ Jy) with the Murchison Widefield Array (MWA), through the GaLactic and Extragalactic All-sky MWA (GLEAM) Survey. In addition to flux-density measurements, the G4Jy catalogue (https://github.com/svw26/G4Jy.) provides host-galaxy identifications (through careful visual-inspection) and four sets of spectral indices. Despite their brightness in the radio, many of these sources are poorly studied, with the vast majority lacking a spectroscopic redshift in published work. This is crucial for studying the intrinsic properties of the sources, and so we conduct a multi-semester observing campaign on the Southern African Large Telescope (SALT), with optical spectroscopy enabling us to provide new redshifts to the astronomical community. Initial results show that not all of the host galaxies exhibit emission-line spectra in the optical ($\sim$4 500–7 500Å), which illustrates the importance of radio-frequency selection (rather than optical selection) for creating an unbiased sample of active galactic nuclei. By combining SALT redshifts with those from the 6-degree Field Galaxy Survey (6dFGS) and the Sloan Digital Sky Survey (SDSS), we calculate radio luminosities and linear sizes for 299 G4Jy sources (which includes one newly-discovered giant radio-galaxy, G4Jy 604). Furthermore, with the highest redshift acquired (so far) being $z \sim 2.2$ from SDSS, we look forward to evolution studies of this complete sample, as well as breaking degeneracies in radio properties with respect to, for example, the galaxy environment.

This study characterises the radio luminosity functions (RLFs) for star forming galaxies (SFGs) and active galactic nuclei (AGN) using statistical redshift estimation in the absence of comprehensive spectroscopic data. Sensitive radio surveys over large areas detect many sources with faint optical and infrared counterparts, for which redshifts and spectra are unavailable. This challenges our attempt to understand the population of radio sources. Statistical tools are often used to model parameters (such as redshift) as an alternative to observational data. Using the data from GAMA G23 and EMU early science observations, we explore simple statistical techniques to estimate the redshifts in order to measure the RLFs of the G23 radio sources as a whole and for SFGs and AGN separately. Redshifts and AGN/SFG classifications are assigned statistically for those radio sources without spectroscopic data. The calculated RLFs are compared with existing studies, and the results suggest that the RLFs match remarkably well for low redshift galaxies with an optical counterpart. We use a more realistic high redshift distribution to model the redshifts of (most likely) high redshift radio sources and find that the LFs from our approach match well with measured LFs. We also look at strategies to compare the RLFs of radio sources without an optical counterpart to existing studies.

The high redshift ‘little red dots’ (LRDs) detected with the James Webb Space Telescope are considered to be the cores of emerging galaxies that host active galactic nuclei (AGN). For the first time, we compare LRDs with local compact stellar systems and an array of galaxy-morphology-dependent stellar mass-black hole mass scaling relations in the $M_\mathrm{ bh}$–$M_{\star}$ diagrams. When considering the 2023–2024 masses for LRDs, they are not equivalent to nuclear star clusters (NSCs), with the latter having higher $M_\mathrm{ bh}/M_{\star}$ ratios. However, the least massive LRDs exhibit similar $M_\mathrm{ bh}$ and $M_\mathrm{ \star,gal}$ values as ultracompact dwarf (UCD) galaxies, believed to be the cores of stripped/threshed galaxies. We show that the LRDs span the $M_\mathrm{ bh}$–$M_\mathrm{ \star,gal}$ diagram from UCD galaxies to primaeval lenticular galaxies. In contrast, local spiral galaxies and the subset of major-merger-built early-type galaxies define $M_\mathrm{ bh}$–$M_{\star,gal}$ relations that are offset to higher stellar masses. Based on the emerging 2025 masses for LRDs, they may yet have similarities with NSCs, UCD galaxies, and green peas. Irrespective of this developing situation, we additionally observe that low-redshift galaxies with AGN align with the quasi-quadratic or steeper black hole scaling relations defined by local disc galaxies with directly measured black hole masses. This highlights the benefits of considering a galaxy’s morphology – which reflects its accretion and merger history – to understand the coevolution of galaxies and their black holes. Future studies of spatially resolved galaxies with secure masses at intermediate-to-high redshift hold the promise of detecting the emergence and evolution of the galaxy-morphology-dependent $M_\mathrm{ bh}$–$M_{\star}$ relations.

We examine the optical counterparts of the 1 829 neutral hydrogen (H i) detections in three pilot fields in the Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY) using data from the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys DR10. We find that 17% (315) of the detections are optically low surface brightness galaxies (LSBGs; mean g-band surface brightness within 1 $ R_e$ of $\gt 23$ mag arcsec$^{-2}$) and 3% (55) are optically ‘dark’. We find that the gas-rich WALLABY LSBGs have low star formation efficiencies, and have stellar masses spanning five orders of magnitude, which highlights the diversity of properties across our sample. 75% of the LSBGs and all of the dark H i sources had not been catalogued prior to WALLABY. We examine the optically dark sample of the WALLABY pilot survey to verify the fidelity of the catalogue and investigate the implications for the full survey for identifying dark H i sources. We assess the H i detections without optical counterparts and identify 38 which pass further reliability tests. Of these, we find that 13 show signatures of tidal interactions. The remaining 25 detections have no obvious tidal origin, so are candidates for isolated galaxies with high H i masses, but low stellar masses and star-formation rates. Deeper H i and optical follow-up observations are required to verify the true nature of these dark sources.

We present the Evolutionary Map of the Universe (EMU) survey conducted with the Australian Square Kilometre Array Pathfinder (ASKAP). EMU aims to deliver the touchstone radio atlas of the southern hemisphere. We introduce EMU and review its science drivers and key science goals, updated and tailored to the current ASKAP five-year survey plan. The development of the survey strategy and planned sky coverage is presented, along with the operational aspects of the survey and associated data analysis, together with a selection of diagnostics demonstrating the imaging quality and data characteristics. We give a general description of the value-added data pipeline and data products before concluding with a discussion of links to other surveys and projects and an outline of EMU’s legacy value.

The stars of the Milky Way carry the chemical history of our Galaxy in their atmospheres as they journey through its vast expanse. Like barcodes, we can extract the chemical fingerprints of stars from high-resolution spectroscopy. The fourth data release (DR4) of the Galactic Archaeology with HERMES (GALAH) Survey, based on a decade of observations, provides the chemical abundances of up to 32 elements for 917 588 stars that also have exquisite astrometric data from the Gaia satellite. For the first time, these elements include life-essential nitrogen to complement carbon, and oxygen as well as more measurements of rare-earth elements critical to modern-life electronics, offering unparalleled insights into the chemical composition of the Milky Way. For this release, we use neural networks to simultaneously fit stellar parameters and abundances across the whole wavelength range, leveraging synthetic grids computed with Spectroscopy Made Easy. These grids account for atomic line formation in non-local thermodynamic equilibrium for 14 elements. In a two-iteration process, we first fit stellar labels to all 1 085 520 spectra, then co-add repeated observations and refine these labels using astrometric data from Gaia and 2MASS photometry, improving the accuracy and precision of stellar parameters and abundances. Our validation thoroughly assesses the reliability of spectroscopic measurements and highlights key caveats. GALAH DR4 represents yet another milestone in Galactic archaeology, combining detailed chemical compositions from multiple nucleosynthetic channels with kinematic information and age estimates. The resulting dataset, covering nearly a million stars, opens new avenues for understanding not only the chemical and dynamical history of the Milky Way but also the broader questions of the origin of elements and the evolution of planets, stars, and galaxies.

Bow shocks generated by pulsars moving through weakly ionized interstellar medium (ISM) produce emission dominated by non-equilibrium atomic transitions. These bow shocks are primarily observed as H$\alpha$ nebulae. We developed a package, named Shu, that calculates non-LTE intensity maps in more than 150 spectral lines, taking into account geometrical properties of the pulsars’ motion and lines of sight. We argue here that atomic (C i, N i, O i) and ionic (S ii, N ii, O iii, Ne iv) transitions can be used as complementary and sensitive probes of ISM. We perform self-consistent 2D relativistic hydrodynamic calculations of the bow shock structure and generate non-LTE emissivity maps, combining global dynamics of relativistic flows, and detailed calculations of the non-equilibrium ionization states. We find that though typically $\text{H}_\alpha$ emission is dominant, spectral fluxes in [O iii], [S ii] and [N ii] may become comparable for relatively slowly moving pulsars. Overall, morphology of non-LTE emission, especially of the ionic species, is a sensitive probe of the density structures of the ISM.

Petabytes of archival high time resolution observations have been captured with the Murchison Widefield Array. The search for Fast Radio Bursts within these using established software has been limited by its inability to scale on supercomputing infrastructure, necessary to meet the associated computational and memory requirements. Hence, past searches used a coarse integration time, in the scale of seconds, or analysed an insufficient number of hours of observations. This paper introduces BLINK, a novel radio interferometry imaging software for low-frequency FRB searches to be run on modern supercomputers. It is implemented as a suite of software libraries executing all computations on GPU, supporting both AMD and NVIDIA hardware vendors. These libraries are designed to interface with each other and to define the BLINK imaging pipeline as a single executable program. Expensive I/O operations between imaging stages are not necessary because the stages now share the same memory space and data representation. BLINK is the first imaging pipeline implementation able to fully run on GPUs as a single process, further supporting AMD hardware and enabling Australian researchers to take advantage of Pawsey’s Setonix supercomputer. In the millisecond-scale time resolution imaging test case illustrated in this paper, representative of what is required for FRB searches, the BLINK imaging pipeline achieves a 3 687x speedup compared to a traditional MWA imaging pipeline employing WSClean.

The outer solar system is theoretically predicted to harbour an undiscovered planet, often referred to as Planet Nine. Simulations suggest that its gravitational influence could explain the unusual clustering of minor bodies in the Kuiper Belt. However, no observational evidence for Planet Nine has been found so far, as its predicted orbit lies far beyond Neptune, where it reflects only a faint amount of Sunlight. This work aims to find Planet Nine candidates by taking advantage of two far-infrared all-sky surveys, which are IRAS and AKARI. The epochs of these two surveys were separated by 23 years, which is large enough to detect Planet Nine’s $\sim3'$/year orbital motion. We use a dedicated AKARI Far-Infrared point source list for the purpose of our Planet Nine search — AKARI-FIS Monthly Unconfirmed Source List (AKARI-MUSL), which includes sources detected repeatedly only in hours timescale, but not after months. AKARI-MUSL is more advantageous than the AKARI Bright Source Catalogue (AKARI-BSC) for detecting moving and faint objects like Planet Nine with a twice-deeper flux detection limit. We search for objects that moved slowly between IRAS and AKARI detections given in the catalogues. First, we estimated the expected flux and orbital motion of Planet Nine by assuming its mass, distance, and effective temperature to ensure it can be detected by IRAS and AKARI, then applied the positional and flux selection criteria to narrow down the number of sources from the catalogues. Next, we produced all possible candidate pairs including one IRAS source and one AKARI source whose angular separations were limited between 42′ and $69.6'$, corresponding to the heliocentric distance range of 500 – 700 AU and the mass range of 7 – 17M$_{\oplus}$. There are 13 candidate pairs obtained after the selection criteria. After image inspection, we found one good candidate, of which the IRAS source is absent from the same coordinate in the AKARI image after 23 years and vice versa. However, AKARI and IRAS detections are not enough to determine the full orbit of this candidate. This issue leads to the need for follow-up observations, which will determine the Keplerian motion of our Planet Nine candidate.