We present the fourth data release (DR4) of the SkyMapper Southern Survey (SMSS), the last major step in our hemispheric survey with six optical filters: u, v, g, r, i, z. SMSS DR4 covers 26 000 deg$^{2}$ from over 400 000 images acquired by the 1.3 m SkyMapper telescope between 2014-03 and 2021-09. The 6-band sky coverage extends from the South Celestial Pole to $\delta=+16^{\circ}$, with some images reaching $\delta\sim +28^{\circ}$. In contrast to previous DRs, we include all good-quality images from the facility taken during that time span, not only those explicitly taken for the public Survey. From the image dataset, we produce a catalogue of over 15 billion detections made from $\sim$700 million unique astrophysical objects. The typical 10$\sigma$ depths for each field range between 18.5 and 20.5 mag, depending on the filter, but certain sky regions include longer exposures that reach as deep as 22 mag in some filters. As with previous SMSS catalogues, we have cross-matched with a host of other imaging and spectroscopic datasets to facilitate additional science outcomes. SMSS DR4 is now available to the worldwide astronomical community.

The discovery that blazars dominate the extra-galactic $\gamma$-ray sky is a triumph in the Fermi era. However, the exact location of $\gamma$-ray emission region still remains in debate. Low-synchrotron-peaked blazars (LSPs) are estimated to produce high-energy radiation through the external Compton process, thus their emission regions are closely related to the external photon fields. We employed the seed factor approach proposed by Georganopoulos et al. It directly matches the observed seed factor of each LSP with the characteristic seed factors of external photon fields to locate the $\gamma$-ray emission region. A sample of 1 138 LSPs with peak frequencies and peak luminosities was adopted to plot a histogram distribution of observed seed factors. We also collected some spectral energy distributions (SEDs) of historical flare states to investigate the variation of $\gamma$-ray emission region. Those SEDs were fitted by both quadratic and cubic functions using the Markov-chain Monte Carlo method. Furthermore, we derived some physical parameters of blazars and compared them with the constraint of internal $\gamma\gamma$-absorption. We find that dusty torus dominates the soft photon fields of LSPs and most $\gamma$-ray emission regions of LSPs are located at 1–10 pc. The soft photon fields could also transition from dusty torus to broad line region and cosmic microwave background in different flare states. Our results suggest that the cubic function is better than the quadratic function to fit the SEDs.

The proto-Milky Way epoch forms the earliest stars in our galaxy and sets the initial conditions for the subsequent disk formation. Recent observations showed that the [$\alpha$/Fe] ratio among in situ metal-poor stars declined between [Fe/H] $=-3$ and $-1.3$ until it reached the lowest value ($\sim$0.25) and rose to the traditional value associated with the high-[$\alpha$/Fe] thick disk ($\sim$0.3) at [Fe/H] $\approx$ -1.0. It was suggested that the rise in [$\alpha$/Fe] could be caused by an increase in the star formation efficiency (SFE), known as the ‘simmering’ phase scenario. However, gas inflow also plays a vital role in shaping the star formation history and chemical evolution of galaxies, especially during the earliest epoch of the universe. We investigate this unexpected [$\alpha$/Fe]-rise with an experiment involving a galactic chemical evolution model. Our model has five free parameters: the mass of the initial reservoir of the cold interstellar medium (ISM) at birth, the frequency of Type Ia supernovae (SNe Ia), the cooling timescale of the warm ISM, the SFE, and the inflow rate of fresh gas. The last two free parameters were allowed to change after [$\alpha$/Fe] reached its lowest value, dividing the proto-Galaxy epoch into two phases. The models that reproduced the observed [Fe/H]-[$\alpha$/Fe]-track provided estimates for these fundamental parameters of the proto-Milky Way. We find that the rise in [$\alpha$/Fe] could also be caused by a large inflow of high-[$\alpha$/Fe] gas and conclude that the [$\alpha$/Fe]-rise could be a signature of the gas accretion that fuelled the formation of the Milky Way disk.

We investigate the relationship between a galaxy cluster’s hydrostatic equilibrium state, the entropy profile, K, of the intracluster gas, and the system’s non-thermal pressure (NTP), within an analytic model of cluster structures. When NTP is neglected from the cluster’s hydrostatic state, we find that the gas’ logarithmic entropy slope, $k\equiv \mathrm{d}\ln K/\mathrm{d}\ln r$, converges at large halocentric radius, r, to a value that is systematically higher than the value $k\simeq1.1$ that is found in observations and simulations. By applying a constraint on these ‘pristine equilibrium’ slopes, $k_\mathrm{eq}$, we are able to predict the required NTP that must be introduced into the hydrostatic state of the cluster. We solve for the fraction, $\mathcal{F}\equiv p_\mathrm{nt}/p$, of NTP, $p_\mathrm{nt}$, to total pressure, p, of the cluster, and we find $\mathcal{F}(r)$ to be an increasing function of halocentric radius, r, that can be parameterised by its value in the cluster’s core, $\mathcal{F}_0$, with this prediction able to be fit to the functional form proposed in numerical simulations. The minimum NTP fraction, as the solution with zero NTP in the core, $\mathcal{F}_0=0$, we find to be in excellent agreement with the mean NTP predicted in non-radiative simulations, beyond halocentric radii of $r\gtrsim0.7r_{500}$, and in tension with observational constraints derived at similar radii. For this minimum NTP profile, we predict $\mathcal{F}\simeq0.20$ at $r_{500}$, and $\mathcal{F}\simeq0.34$ at $2r_{500}$; this amount of NTP leads to a hydrostatic bias of $b\simeq0.12$ in the cluster mass $M_{500}$ when measured within $r_{500}$. Our results suggest that the NTP of galaxy clusters contributes a significant amount to their hydrostatic state near the virial radius and must be accounted for when estimating the cluster’s halo mass using hydrostatic equilibrium approaches.

The study of very short-period contact binaries provides an important laboratory in which the most important and problematic astrophysical processes of stellar evolution take place. Short-period contact systems, such as CC Com, are particularly important for binary evolution. Close binary systems, especially those with multiple system members, have significant period variations, angular momentum loss mechanisms predominance, and pre-merger stellar evolution, making them valuable astrophysical laboratories. In this study, observations of CC Com, previously reported as a binary system, and new observations from the TÜBİTAK National Observatory (TUG) and the space-based telescope TESS have revealed that there is a third object with a period of about eight years and a fourth object with a period of about a century orbiting the binary system. From simultaneous analysis of all available light curves and radial velocities, the sensitive orbital and physical parameters of the system components are derived. The orbital parameters of the components are P$_\mathrm{A}=0.2206868 \pm 0.0000002$ days, P$_\mathrm{B}=7.9\pm0.1$ yr, P$_\mathrm{C}=98\pm5$ yr, $e_3$ = 0.06, $e_4$ = 0.44 and the physical parameters as M$_\mathrm{A1}=0.712\pm0.009$ M$_{\odot}$, M$_\mathrm{ A2}=0.372\pm0.005$ M$_{\odot}$, $m_{B;i^{\prime}=90^\circ}=0.074$ M$_{\odot}$, $m_{C;i^{\prime}=90^\circ}=0.18$ M$_{\odot}$, R$_\mathrm{A1}=0.693\pm0.006$ R$_{\odot}$, R$_\mathrm{A2}=0.514\pm0.005$ R$_{\odot}$, L$_\mathrm{A1}$ = 0.103 L$_\odot$, L$_\mathrm{A2}$ = 0.081 L$_\odot$. Finally, the evolutionary status of the multiple system CC Com and its component stars is discussed.

PSR J0837$-$2454 is a young 629 ms radio pulsar whose uncertain distance has important implications. A large distance would place the pulsar far out of the Galactic plane and suggest it is the result of a runaway star, while a short distance would mean the pulsar is extraordinarily cold. Here we present further radio observations and the first deep X-ray observation of PSR J0837$-$2454. Data from the Parkes Murriyang telescope show flux variations over short and long timescales and also yield an updated timing model, while the position and proper motion (and, less strongly, parallax) of the pulsar are constrained by a number of low-significance detections with the Very Long Baseline Array. XMM-Newton data enable detection of X-ray pulsations for the first time from this pulsar and yield a spectrum that is thermal and blackbody-like, with a cool blackbody temperature $\approx$$70\ \mbox{eV}$ or atmosphere temperature $\approx$$50\ \mbox{eV}$, as well as a small hotspot. The spectrum also indicates the pulsar is at a small distance of $\lesssim$$1\ \mbox{kpc}$, which is compatible with the marginal VLBA parallax constraint that favours a distance of $\gtrsim$330 pc. The low implied luminosity ($\sim7.6\times10^{31}\mbox{erg\, s}^{-1}$ at 0.9 kpc) suggests PSR J0837$-$2454 has a mass high enough that fast neutrino emission from direct Urca reactions operates in this young star and points to a nuclear equation of state that allows for direct Urca reactions at the highest densities present in neutron star cores.

We present the first of two papers dedicated to verifying the Australian Epoch of Reionisation pipeline (AusEoRPipe) through simulation. The AusEoRPipe aims to disentangle 21-cm radiation emitted by gas surrounding the very first stars from contaminating foreground astrophysical sources and has been in the development for close to a decade. In this paper, we build an accurate 21-cm sky model that can be used by the WODEN simulation software to create visibilities containing a predictable 21-cm signal. We verify that the power spectrum (PS) estimator CHIPS can recover this signal in the absence of foregrounds. We also investigate how measurements in Fourier-space are correlated and how their gridded density affects the PS. We measure and fit for this effect using Gaussian-noise simulations of the Murchison Widefield Array (MWA) phase I layout. We find a gridding density correction factor of 2.651 appropriate for integrations equal to or greater than 30 minutes of data, which contain observations with multiple primary beam pointings and LSTs. Paper II of this series will use the results of this paper to test the AusEoRPipe in the presence of foregrounds and instrumental effects.

We present high-resolution observations of nearby ($z\lesssim0.1$) galaxies that have hosted Type Ia supernovae to measure systemic spectroscopic redshifts using the wide field spectrograph (WiFeS) instrument on the Australian National University 2.3 m telescope at Siding Spring Observatory. While most of the galaxies targeted have previous spectroscopic redshifts, we provide demonstrably more accurate and precise redshifts with competitive uncertainties, motivated by potential systematic errors that could bias estimates of the Hubble constant ($H_0$). The WiFeS instrument is remarkably stable; after calibration, the wavelength solution varies by $\lesssim$0.5 Å in red and blue with no evidence of a trend over the course of several years. By virtue of the $25\times 38$ arcsec field of view, we are always able to measure the redshift of the galactic core, or the entire galaxy in the cases where its angular extent is smaller than the field of view, reducing any errors due to galaxy rotation. We observed 185 southern SN Ia host galaxies and measured the redshift of each via at least one spatial region of (a) the core and (b) the average over the full-field/entire galaxy. Overall, we find stochastic differences between historical redshifts and our measured redshifts on the order of $\lesssim10^{-3}$ with a mean offset of 4.3${\times 10^{-5}}$ and normalised median absolute deviation of 1.2${\times 10^{-4}}$. We show that a systematic redshift offset at this level is not enough to bias cosmology, as $H_0$ shifts by $+0.1$ km s$^{-1}$ Mpc$^{-1}$ when we replace Pantheon+ redshifts with our own, but the occasional large differences are interesting to note.

The radio interferometric closure phases can be a valuable tool for studying cosmological HI from the early Universe. Closure phases have the advantage of being immune to element-based gains and associated calibration errors. Thus, calibration and errors therein, which are often sources of systematics limiting standard visibility-based approaches, can be avoided altogether in closure phase analysis. In this work, we present the first results of the closure phase power spectrum of HI 21-cm fluctuations using the Murchison Widefield Array (MWA), with $\sim12$ h of MWA phase II observations centred around redshift, $z\approx 6.79$, during the Epoch of Reionisation. On analysing three redundant classes of baselines – 14, 24, and 28 m equilateral triads, our estimates of the $2\sigma$ (95% confidence interval) 21-cm power spectra are $\lesssim(184)^2 pseudo\,\mathrm{mK}^2$ at ${k}_{||} = 0.36 pseudo\ h \mathrm{Mpc}^{-1}$ in the EoR1 field for the 14 m baseline triads, and $\lesssim(188)^2 pseudo\,\mathrm{mK}^2$ at $k_{||} = 0.18 \,pseudo\ h \mathrm{Mpc}^{-1}$ in the EoR0 field for the 24 m baseline triads. The ‘pseudo’ units denote that the length scale and brightness temperature should be interpreted as close approximations. Our best estimates are still 3-4 orders high compared to the fiducial 21-cm power spectrum; however, our approach provides promising estimates of the power spectra even with a small amount of data. These data-limited estimates can be further improved if more datasets are included into the analysis. The evidence for excess noise has a possible origin in baseline-dependent systematics in the MWA data that will require careful baseline-based strategies to mitigate, even in standard visibility-based approaches.

The angular correlation is a method for measuring the distribution of structure in the Universe, through the statistical properties of the angular distribution of galaxies on the sky. We measure the angular correlation of galaxies from the second data release of the GaLactic and Extragalactic All-sky Murchison Widefield Array eXtended survey (GLEAM-X) survey, a low-frequency radio survey covering declinations below $+30^\circ$. We find an angular distribution consistent with the $\Lambda$CDM cosmological model assuming the best fitting cosmological parameters from Planck Collaboration et al. (2020, A&A, 641, A6). We fit a bias function to the discrete tracers of the underlying matter distribution, finding a bias that evolves with redshift in either a linear or exponential fashion to be a better fit to the data than a constant bias. We perform a covariance analysis to obtain an estimation of the properties of the errors, by analytic, jackknife, and sample variance means. Our results are consistent with previous studies on the topic, and also the predictions of the $\Lambda$CDM cosmological model.

We present deep near-infrared $K_\textrm{s}$-band imaging for 35 of the 53 sources from the high-redshift ($z \gt 2$) radio galaxy candidate sample defined in Broderick et al. (2022, PASA, 39, e061). These images were obtained using the High-Acuity Widefield K-band Imager (HAWK-I) on the Very Large Telescope. Host galaxies are detected for 27 of the sources, with $K_\textrm{s} \approx 21.6$–23.0 mag (2$^{\prime\prime}$ diameter apertures; AB). The remaining eight targets are not detected to a median $3\unicode{x03C3}$ depth of $K_\textrm{s} \approx 23.3$ mag (2$^{\prime\prime}$ diameter apertures). We examine the radio and near-infrared flux densities of the 35 sources, comparing them to the known $z \gt 3$ powerful radio galaxies with 500-MHz radio luminosities $L_{500\,\textrm{MHz}} \gt 10^{27}$ W Hz$^{-1}$. By plotting 150-MHz flux density versus $K_\textrm{s}$-band flux density, we find that, similar to the sources from the literature, these new targets have large radio to near-infrared flux density ratios, but extending the distribution to fainter flux densities. Five of the eight HAWK-I deep non-detections have a median $3\unicode{x03C3}$ lower limit of $K_\textrm{s} \gtrsim 23.8$ mag (1$.\!^{\prime\prime}$5 diameter apertures); these five targets, along with a further source from Broderick et al. (2022, PASA, 39, e061) with a deep non-detection ($K_\textrm{s} \gtrsim 23.7$ mag; $3\unicode{x03C3}$; 2$^{\prime\prime}$ diameter aperture) in the Southern H-ATLAS Regions $K_\textrm{s}$-band Survey, are considered candidates to be ultra-high-redshift ($z \gt 5$) radio galaxies. The extreme radio to near-infrared flux density ratios ($\gt 10^5$) for these six sources are comparable to TN J0924$-$2201, GLEAM J0856$+$0223 and TGSS J1530$+$1049, the three known powerful radio galaxies at $z \gt 5$. For a selection of galaxy templates with different stellar masses, we show that $z \gtrsim 4.2$ is a plausible scenario for our ultra-high-redshift candidates if the stellar mass $M_\textrm{*} \gtrsim 10^{10.5}$ M$_\odot$. In general, the 35 targets studied have properties consistent with the previously known class of infrared-faint radio sources. We also discuss the prospects for finding more UHzRG candidates from wide and deep near-infrared surveys.

We conducted a study on the X-ray polarisation properties of MCG-5-23-16 by analysing long-term monitoring data from NuSTAR jointly with IXPE observations made in May and November 2022. The re-analysis of IXPE data gives model-dependent polarisation degree, PD (%) = $1.08\pm0.66$ in the energy band 2–8 keV, which agrees with previous studies within error bars. The model-independent analysis of PD poses an upper limit of $\leq3.8$ ($1\sigma$ level) for the same energy band. The observed upper limit of PD, along with broadband spectral analysis (2–79 keV) using an accretion-ejection based model, allowed us to derive the corona geometry (i.e. radius and height) and the accretion disc inclination ($\sim33^\circ$). Additional NuSTAR observations were also analysed to gain insights into the accretion flow properties of the source and to estimate the expected polarisation during those epochs with PD $\sim 4.3\%$. The radius and height of the corona varies between $28.2\pm3.1 - 39.8\pm4.6$ r$_s$ and $14.3\pm1.7-21.4\pm1.9$ r$_s$ respectively, with a mass outflow rate from the corona measuring $0.14\pm0.03-0.2\pm0.03$ Eddington rate ($\dot m_{\mathrm{Edd}}$). The estimated PD values were nearly constant up to a certain radial distance and height of the corona and then decreased for increasing corona geometry. The spectral analysis further provided an estimate for the mass of the central black hole $\sim2\times 10^7$ M$_\odot$ and the velocity of the outflowing gas $\sim0.16-0.19c$. A comparative broadband spectral study using reflection-based models estimates the disc inclination between $\sim 31^\circ\pm8^\circ-45^\circ\pm7^\circ$, and yields an expected PD of 3.4–6.0%. We also found a weak reflection fraction and a less ionised distant reflecting medium. The expected PD measured using accretion-ejection and reflection models is less compared to the expected PD measured for a given disc inclination of $45^\circ$. Our modelling of the disc-corona-outflows and polarisation connection can be extended and validated with data from the recently launched XPoSat, India’s first X-ray Polarimeter Satellite, offering potential applications to other sources.

We explore the globular cluster population of NGC 1052-DF4, a dark matter deficient galaxy, using Bayesian inference to search for the presence of rotation. The existence of such a rotating component is relevant to the estimation of the mass of the galaxy, and therefore the question of whether NGC 1052-DF4 is truly deficient of dark matter, similar to NGC 1052-DF2, another galaxy in the same group. The rotational characteristics of seven globular clusters in NGC 1052-DF4 were investigated, finding that a non-rotating kinematic model has a higher Bayesian evidence than a rotating model, by a factor of approximately 2.5. In addition, we find that under the assumption of rotation, its amplitude must be small. This distinct lack of rotation strengthens the case that, based on its intrinsic velocity dispersion, NGC 1052-DF4 is a truly dark matter deficient galaxy.

The interaction between stellar winds and the partially ionized local interstellar medium (LISM) is quite common in astrophysics. However, the main difficulty in describing the neutral components lies in the fact that the mean free path of an interstellar atom, l, can be comparable to the characteristic size of an astrosphere, L (i.e. the Knudsen number, which is equal to l/L, is approximately equal to 1, as in the case of the heliosphere). In such cases, a single-fluid approximation becomes invalid, and a kinetic description must be used for the neutral component. In this study, we consider a general astrosphere and use a kinetic-gas dynamics model to investigate how the global structure of the astrosphere depends on the Knudsen number. We present numerical results covering an extremely wide range of Knudsen numbers (from 0.0001 to 100). Additionally, we explore the applicability of single-fluid approaches for modelling astrospheres of various sizes. We have excluded the influence of interstellar and stellar magnetic fields in our model to make parametric study of the kinetic effects feasible. The main conclusion of this work is that, for large astrospheres (with a distance to the bow shock greater than 600 AU) a heated rarefied plasma layer forms in the outer shock layer near the astropause. The formation of this layer is linked to localized heating of the plasma by atoms (specifically, ENAs) that undergo charge exchange again behind the astropause. This process significantly alters the flow structure in the outer shock layer and the location of the bow shock, and it cannot be described by a single-fluid model. Additionally, this paper discusses how atoms weaken the bow shocks at near-heliospheric conditions.

As TeV gamma-ray astronomy progresses into the era of the Cherenkov Telescope Array (CTA), instantaneously following up on gamma-ray transients is becoming more important than ever. To this end, a worldwide network of Imaging Atmospheric Cherenkov Telescopes has been proposed. Australia is ideally suited to provide coverage of part of the Southern Hemisphere sky inaccessible to H.E.S.S. in Namibia and the upcoming CTA-South in Chile. This study assesses the sources detectable by a small, transient-focused array in Australia based on CTA telescope designs. The TeV emission of extragalactic sources (including the majority of gamma-ray transients) can suffer significant absorption by the extragalactic background light. As such, we explored the improvements possible by implementing stereoscopic and topological triggers, as well as lowered image cleaning thresholds, to access lower energies. We modelled flaring gamma-ray sources based on past measurements from the satellite-based gamma-ray telescope Fermi-LAT. We estimate that an array of four Medium-Sized Telescopes (MSTs) would detect $\sim$24 active galactic nucleus flares >5$\sigma$ per year, up to a redshift of $z\approx1.5$. Two MSTs achieved $\sim$80–90% of the detections of four MSTs. The modelled Galactic transients were detectable within the observation time of one night, 11 of the 21 modelled gamma-ray bursts were detectable, as were $\sim$10% of unidentified transients. An array of MST-class telescopes would thus be a valuable complementary telescope array for transient TeV gamma-ray astronomy.

Drift scan observations provide the broad sky coverage and instrumental stability needed to measure the Epoch of Reionization (EoR) 21-cm signal. In such observations, the telescope’s pointing centre (PC) moves continuously on the sky. The Tracking Tapered Gridded Estimator (TTGE) combines observations from different PC to estimate $P(k_{\perp}, k_{\parallel})$ the 21-cm power spectrum, centred on a tracking centre (TC) which remains fixed on the sky. The tapering further restricts the sky response to a small angular region around TC, thereby mitigating wide-field foregrounds. Here we consider $154.2\,\mathrm{MHz}$ ($z = 8.2$) Murchison Widefield Array (MWA) drift scan observations. The periodic pattern of flagged channels, present in MWA data, is known to introduce artefacts which pose a challenge for estimating $P(k_{\perp}, k_{\parallel})$. Here we have validated the TTGE using simulated MWA drift scan observations which incorporate the flagged channels same as the data. We demonstrate that the TTGE is able to recover $P(k_{\perp}, k_{\parallel})$ without any artefacts and estimate $P(k)$ within $5 \%$ accuracy over a large $k$-range. We also present preliminary results for a single PC, combining 9 nights of observation $(17 \, \mathrm{min}$ total). We find that $P(k_{\perp}, k_{\parallel})$ exhibits streaks at a fixed interval of $k_{\parallel}=0.29 \, \mathrm{Mpc}^{-1}$, which matches $\Delta \nu_\mathrm{per}=1.28 \, \mathrm{MHz}$ that is the period of the flagged channels. Since the simulations demonstrate that the TTGE is impervious to the flagged channels, the streaks seen for the actual data are possibly caused by some systematic that has the same period as the flagged channels. These streaks are more than 3–4 orders of magnitude smaller than the peak foreground power $\mid P(k_{\perp}, k_{\parallel}) \mid \approx 10^{16} \, \mathrm{mK^2}\, \mathrm{Mpc^3}$ at $k_{\parallel}=0$. The streaks are not as pronounced at larger $k_{\parallel}$, and in some cases they do not appear to extend across the entire $k_{\perp}$ range. The rectangular region $0.05 \leq k_{\perp} \leq 0.16 \, \mathrm{Mpc^{-1}}$ and $0.9 \leq k_{\parallel}\leq 4.6 \, \mathrm{Mpc^{-1}}$ is found to be relatively free of foreground contamination and artefacts, and we have used this to place the $2\unicode{x03C3}$ upper limit $\Delta^2(k) < (1.85\times10^4)^2\, \mathrm{mK^2}$ on the EoR 21-cm mean squared brightness temperature fluctuations at $k=1 \,\mathrm{Mpc}^{-1}$.

We investigated the kinematics and dynamics of gas structures on galaxy-cloud scales in two spiral galaxies NGC5236 (M83) and NGC4321 (M100) using CO (2$-$1) line. We utilised the FILFINDER algorithm on integrated intensity maps for the identification of filaments in two galaxies. Clear fluctuations in velocity and density were observed along these filaments, enabling the fitting of velocity gradients around intensity peaks. The variations in velocity gradient across different scales suggest a gradual and consistent increase in velocity gradient from large to small scales, indicative of gravitational collapse, something also revealed by the correlation between velocity dispersion and column density of gas structures. Gas structures at different scales in the galaxy may be organised into hierarchical systems through gravitational coupling. All the features of gas kinematics on galaxy-cloud scale are very similar to that on cloud-clump and clump-core scales studied in previous works. Thus, the interstellar medium from galaxy to dense core scales presents multi-scale/hierarchical hub-filament structures. Like dense core as the hub in clump, clump as the hub in molecular cloud, now we verify that cloud or cloud complex can be the hub in spiral galaxies. Although the scaling relations and the measured velocity gradients support the gravitational collapse of gas structures on galaxy-cloud scales, the collapse is much slower than a pure free-fall gravitational collapse.

We applied a Density-Based Clustering algorithm on samples of galaxies and galaxy systems belonging to 53 rich superclusters from the Main SuperCluster Catalogue to identify the presence of “central regions’’, or cores, in these large-scale structures. Cores are defined here as large gravitationally bound galaxy structures, comprised of two or more clusters and groups, with sufficient matter density to survive cosmic expansion and virialize in the future. We identified a total of 105 galaxy structures classified as cores, which exhibit a high density contrast of mass and galaxies. The Density-based Core Catalogue, presented here, includes cores that were previously reported in well-known superclusters of the Local Universe, and also several newly identified ones. We found that 83% of the rich superclusters in our sample have at least one core. While more than three cores with different dynamical state are possible, the presence of a single core in the superclusters is more common. Our work confirms the existence of nucleation regions in the internal structure of most rich superclusters and points to the fact that these cores are the densest and most massive features that can be identified in the cosmic web with high probability for future virialization.

In order to study the structure and temperature distribution within high-mass star-forming clumps, we employed the Australia Telescope Compact Array to image the $\mathrm{NH}_3$ (J,K) = (1,1) through (6,6) and the (2,1) inversion transitions, the $\mathrm{H}_2\mathrm{O}$ $6_{16}$-$5_{23}$ maser line at 22.23508 GHz, several $\mathrm{CH}_3\mathrm{OH}$ lines and hydrogen and helium recombination lines. In addition, 22- and 24-GHz radio continuum emission was also imaged.

The $\mathrm{NH}_3$ lines probe the optical depth and gas temperature of compact structures within the clumps. The $\mathrm{H}_2\mathrm{O}$ maser pinpoints the location of shocked gas associated with star formation. The recombination lines and the continuum emission trace the ionised gas associated with hot OB stars. The paper describes the data and presents sample images and spectra towards select clumps. The technique for estimating gas temperature from $\mathrm{NH}_3$ line ratios is described. The data show widespread hyperfine intensity anomalies in the $\mathrm{NH}_3$ (1,1) images, an indicator of non-LTE $\mathrm{NH}_3$ excitation. We also identify several new $\mathrm{NH}_3$ (3,3) masers associated with shocked gas. Towards AGAL328.809+00.632, the $\mathrm{H}_2\mathrm{O}$ $6_{16}$-$5_{23}$ line, normally seen as a maser, is instead seen as a thermally excited absorption feature against a strong background continuum. The data products are described in detail.

The Vela pulsar (J0835$-$4510) is known to exhibit variations in Faraday rotation and dispersion on multi-decade timescales due to the changing sightline through the surrounding Vela supernova remnant and the Gum Nebula. Until now, variations in Faraday rotation towards Vela have not been studied on timescales less than around a decade. We present the results of a high-cadence observing campaign carried out with the Aperture Array Verification System 2 (AAVS2), a prototype SKA-Low station, which received a significant bandwidth upgrade in 2022. We collected observations of the Vela pulsar and PSR J0630$-$2834 (a nearby pulsar located outside the Gum Nebula), spanning $\sim$1 and $\sim$0.3 yr, respectively, and searched for linear trends in the rotation measure (RM) as a function of time. We do not detect any significant trends on this timescale ($\sim$months) for either pulsar, but the constraints could be greatly improved with more accurate ionospheric models. For the Vela pulsar, the combination of our data and historical data from the published literature have enabled us to model long-term correlated trends in RM and dispersion measure (DM) over the past two decades. We detect a change in DM of $\sim$0.3 $\mathrm{cm}^{-3}\,\mathrm{pc}$ which corresponds to a change in electron density of $\sim$$10^5\,\mathrm{cm}^{-3}$ on a transverse length scale of $\sim$1–2 au. The apparent magnetic field strength in the time-varying region changes from $240^{+30}_{-20}\,\mu\mathrm{G}$ to $-6.2^{+0.7}_{-0.9}\,\mu\mathrm{G}$ over the time span of the dataset. As well as providing an important validation of polarimetry, this work highlights the pulsar monitoring capabilities of SKA-Low stations, and the niche science opportunities they offer for high-precision polarimetry and probing the microstructure of the magneto-ionic interstellar medium.