We report the results of a four-epoch very long baseline interferometry experiment targeting seventeen water maser sources in the Magellanic Clouds with the Long Baseline Array. We have measured accurate proper motions for water masers associated with two star formation regions N180 and N157A, both in the Large Magellanic Cloud. The water masers have proper motions of (1.79 ± 0.13, 0.89 ± 0.03) milliarcseconds per year and (1.74 ± 0.29, 0.55 ± 0.08) milliarcseconds per year in right ascension and declination respectively. These results are consistent with the proper motions of nearby stars and the bulk motion of the Large Magellanic Cloud as measured by Gaia, providing independent non-optical evidence of the Large Magellanic Cloud’s motion.

Recent studies by B. Villarroel and colleagues have assembled and analysed datasets of unidentified features measured from digital scans of photographic plates captured by the first-epoch Palomar Observatory Sky Survey (POSS1) in the pre-Sputnik era. These studies have called attention to (i) a purported deficit of features within Earth’s shadow; (ii) the sporadic presence of linear clusters; and (iii) a positive correlation between the timing of feature observations and nuclear tests as well as Unidentified Aerial Phenomena (UAP) sighting reports. These observations were cited as evidence that some fraction of the unidentified features represent glinting artificial objects near Earth. We have examined these claims using two previously published datasets that are closely related to those used in the Villarroel et al. studies. For these datasets, the Villarroel et al. assumption of a spatially uniform-random background distribution of features, essential to the Earth shadow analysis, is shown to be false. After finding the null distribution of feature count deviations from the background, we find no statistically significant deficit in the shadow. We also determine that a third of the features in the reported linear clusters were not confidently distinguished from catalogue stars. We find that the reported correlation between the timing of feature observations and nuclear tests becomes insignificant after properly normalizing by the relevant number of observation days, and is almost completely determined by the observation schedule of the Palomar telescope. We uncover important inconsistencies in the definitions of the datasets used in these studies, as well as the use of unvalidated datasets containing catalogue stars, scan artefacts, and plate defects. It has not been shown that any of the features in these datasets represent optical transients. We examine the spatial distribution of the plate-derived features, finding an overall gradual increase in number density toward the corners and edges of plates, as well as examples of (i) empty north-south strips that span multiple plates; (ii) clusters and voids having geometric shapes; and (iii) amorphous clusters. We also highlight a circular argument used in these studies, that leverages the results of an inferential analysis to justify conclusions about the origin of the features as well as the validity of the measurements. Finally, we also review the literature concerning historical searches for optical transients in photographic plates corresponding to gamma ray bursts (GRBs); following decades of work, researchers were unable to make a confident identification of a GRB-associated optical transient.

Obtaining robust galaxy number counts is crucial for understanding galaxy evolution, and submillimetre counts in particular have proven valuable for revising subgrid physics models in cosmological simulations. In confusion-limited surveys, which are common at these wavelengths, statistical methods such as P(D) fluctuation analysis are required to recover counts of faint, unresolved galaxies. However, the standard P(D) framework assumes that galaxies are Poisson-distributed, whereas in reality galaxies are clustered. Using simulations, we demonstrate that this clustering systematically biases P(D)-derived number counts, and present an empirical method that simultaneously measures and corrects for this bias by combining the 1- and 2-point statistics in the map, thereby maximising the information extracted from the data. Applying this method to deep Herschel-SPIRE observations of the GOODS-N field, we provide revised galaxy number counts at 250, 350 and 500 μm. Our results indicate that at 500 μm clustering inflates the apparent counts by a factor of 1.6 around 10 mJy and slightly suppresses the faintest sub-mJy counts, with milder effects at 350 μm and 250 μm owing to the smaller beam sizes. This methodology is broadly applicable to other confusion-limited data sets with well-characterised beam and noise properties, including Spitzer, SCUBA-2, and the upcoming CCAT, enabling unbiased exploitation of the full statistical information in many surveys in the mid-IR to radio wavebands.

Wide-angle outflows, or winds, are associated with a broad range of astrophysical systems, including protostars, massive stars, X-ray binaries, tidal disruption events (TDEs), luminous fast blue optical transients (LFBOTs), and starburst galaxies. When these winds first “turn on,” they inflate a “bubble” into their surroundings, bounded by two shocks and a contact discontinuity, and evolve through distinct adiabatic phases prior to the onset of significant radiative cooling. For sufficiently overdense ejecta, the flow quickly relaxes into an interactiondominated similarity state at early times and later enters an energy-conserving regime. We present a systematic study of these phases for adiabatic winds expanding into power-law density profiles ρ ∝ r−n with 0 ≤ n ≤ 2. Using analytic scalings together with one-dimensional shock-capturing hydrodynamic simulations, we quantify both the relaxation timescales and the accuracy with which the corresponding similarity solutions reproduce the fluid velocity, density, and pressure throughout the shocked bubble.We show that the interaction solutions are attained within only a few dynamical times and remain valid until the reverse-shocked shell is no longer thin relative to the forwardshocked shell, corresponding in practice to an instantaneous overdensity of order unity. For n <2, the flow subsequently converges to the generalized energy-conserving scaling Rs ∝ t3/(5−n), while the special case n = 2 exhibits a single persistent similarity state. We discuss the durations and implications of these phases for stellar and galactic outflows, TDEs, and LFBOTs.

Array (LPDA) and associated receivers, developed and deployed at the Gauribidanur observatory (13.604 N, 77.427 E). It studies bright pulsars and solar transients in the frequency range of 130−350MHz. The LPDAs are arranged in a checkerboard pattern. The orthogonal polarisation feeds are formed using pairs of opposite LPDAs. A diamond-shaped (tilted square) array configuration was chosen for high sidelobe suppression in the East-West and North-South directions. The tile measures 5.9 by 5.9 meters, with diagonals along North-South and East-West directions, each about 8.3 meters. The tile operates as a phased array in transit-observing mode and has been successfully detecting strong pulsars and solar flares for eleven months. The current instantaneous observation bandwidth is 16MHz, limited by the present digital backend. Array operations are streamlined for remote use. In addition to its scientific role, the system serves as a training platform for radio astronomy using a simple, low-cost telescope. This paper presents details of the array and antenna studies, brief descriptions of the front-end and backend instrumentation, and illustrative results from pulsar and solar flare observations. It also briefly discusses future upgrade plans, focusing on tile sensitivity and backend bandwidth, to extend observation capabilities.

We describe the latest iteration of upgrades (designated Phase III) to the Murchison Widefield Array (MWA), in the fourth paper in a series that covers the evolution of the telescope from design concept to initial operational facility, and through two major upgrades. As part of the Phase III upgrade of the MWA, we report the completion of work to design, build, and deploy a new fleet of digital receivers that further optimise the MWA for Epoch of Reionisation observations. These receivers complement existing receivers, such that the MWA now supports the full correlation of all 256 antenna tiles currently in the array. This step releases the MWA from the prior constraint of having to correlate only 128 of the 256 tiles at any given time, which means that the maximum instantaneous sensitivity of the MWA is doubled and the maximum number of interferometric baselines is approximately quadrupled. The upgrade is fundamentally enabled by the new MWAX correlator and various other improvements to the MWA sub-systems. In this paper we describe the new digital receivers and the other improvements that result in the Phase III system. A range of operational benefits arise from the upgrade and scientific flexibility is increased. We also comment on the transition from the MWA to the SKA-Low facility near the end of the decade, including a description of some unique science opportunities utilising joint MWA/SKA-Low data during the Science Verification phase of the SKA-Low Array Assembly 2 (AA2) period.

Understanding the evolution of the cosmic star formation rate density (SFRD) is key to uncovering how the Universe arrived at its present state. This paper presents a novel and efficient method to estimate the local SFRD, which uses supervised machine learning to first identify a population of star-forming galaxies (SFGs). Next, star-formation rates (SFRs) are determined using the 1.4-GHz radio-continuum emission detected by the Australian Square Kilometre Array Pathfinder (ASKAP). Specifically, a gradient-boosted decision tree model was imple-mented to classify extragalactic sources from the Beck et al. (2022) catalogue as either galaxies or quasars using RACS-mid and WISE photometry. The full sample, consisting of 389,392 sources, was partitioned into a 70%-15%-15% split for training, validating, and testing. The optimised model achieved a weighted F1 score of 0.93 and an accuracy of 0.94 on the test dataset, ultimately classifying 336,674 sources as galaxies and 52,718 sources as quasars. Using the resulting z <0.1 depth-matched galaxy sample and the photometric redshift predictions from Beck et al. (2022), a modified 1.4-GHz SFR calibration was determined, yielding a local, completeness-corrected, z < 0.1 SFRD of (1.4 ± 0.5) × 10−2 M⊙ yr−1 Mpc−3 using 11,293 sources. This value is consistent with previous results. Thus, this study demonstrates the feasibility of using supervised learning to identify large populations of SFGs in order to investigate the SFRD evolution. This presents an exciting prospect for future, deeper surveys such as EMU, which will enable the cosmic SFRD to be probed out to higher redshifts.

The Southern-sky MWA Rapid Two-metre (SMART) survey, which capitalises on the MWA’s large field of view and voltage recording capability, is an ambitious effort to conduct sensitive searches for pulsars and fast transients in the 140–170MHz band. The novelty of voltage recording, long dwell times (4800 s) and the high-time and -frequency resolutions (100 μs/10-kHz) exchange a large survey speed (∼ 450 deg2 h–1) for high computational cost. The survey covers the entire sky south of +30° in declination through a series of dedicated observing campaigns, accumulating nearly four petabytes of data. The large volumes of data and the processing challenges at low frequencies necessitate data processing to be approached in multiple phases, and the initial searches focused on a first-pass (shallow) survey of parts of the skies, as reported in earlier papers in this series. These data are also processed for re-detections of hundreds of known pulsars in the southern sky, many of which are also the first detections at frequencies below 400 MHz. This paper is motivated by the need to address the inherent difficulties (for the wider community) in handling large amounts of voltage data and software/processing challenges for routine pulsar detections, and also by the fast-evolving landscape of the SKA Observatory (SKAO).With the construction and commissioning ramping up towards the full-scale SKA-Low, a low-frequency catalogue of detectable pulsars in the southern sky will prove to be a valuable reference for the science verification exercise. A growing sample of low-frequency pulsar detections and measurements will also prove invaluable in a variety of science applications including population studies, survey simulations and emission beam models, refining interstellar medium models for electron densities and the spatial distribution of turbulence, and also for forecasting the detection prospects and survey yield from pulsar surveys planned with SKA-Low. We also present an electronic catalogue of various data products, including pulse profiles, time series and multi-channel folded archives, along with the measurements of dispersion and rotation measures, and mean flux densities for the detected pulsars, and this will be periodically updated as more detections flow on from the ongoing data processing.

Measurements of peculiar velocities in the local Universe are a powerful tool to study the nature of dark energy at low (z < 0.1) redshifts. Here we present the largest single set of z < 0.1 peculiar velocity measurements to date, obtained using the Fundamental Plane (FP) of galaxies in the first data release (DR1) of the Dark Energy Spectroscopic Instrument (DESI).We describe the photometric and spectroscopic selection criteria used to define the sample, as well as extensive quality control checks on the photometry and velocity dispersion measurements. Additionally, we perform detailed systematics checks for the many analysis parameters in our pipeline. Our DESI DR1 catalogue contains FP-based distances and peculiar velocities for 98, 292 unique early-type galaxies, increasing the total number of z < 0.1 FP distances ever measured by a factor of ∼ 2. We achieve a precision of 26% random error in our distance measurements which is comparable to previous surveys. A series of companion DESI papers use the distances and peculiar velocities presented in this paper to measure cosmological parameters.

We present a new data release from the Rapid ASKAP Continuum Survey (RACS), a widefield snapshot radio survey conducted with the Australian SKA Pathfinder (ASKAP). This data release contains the second RACS epoch to make use of ASKAP’s low-frequency band, centred on 887.5 MHz with a bandwidth of 288 MHz, referred to as RACS-low2. This RACS-low2 data release includes both Stokes I and V imaging and catalogue data products covering the whole sky up to a declination of $\approx+48^{\circ}$. RACS-low2 largely follows the observation footprint of the first low-band epoch, though includes additional coverage in the Northern Hemisphere. The observation scheduling and data processing follow previous RACS epochs, making use of autonomous scheduling and holography-derived primary beam models. The Stokes I and V catalogues are derived from images with a median point-spread function (PSF) of $14.2^{\prime\prime} \times 11.8^{\prime\prime}$ and have median root-mean-square noise properties of $\approx 195$ and $\approx 163\,\unicode{x03BC}$Jy PSF$^{-1}$, for the Stokes I and V images, respectively. The consolidated Stokes I catalogue contains 3 922 151 sources. We also constructed a catalogue of Stokes V sources by measuring Stokes V images at the Stokes I source positions. For the 221 Stokes V measurements above the estimated leakage and detection thresholds, we provide likely identifications, including detections of 61 radio stars, 85 pulsars, 43 AGN (many likely to be residual leakage), and one source that is not associated with a known astronomical object. These data products, including calibrated visibilities, images, source lists, and consolidated catalogues, are made publicly available through the CSIRO ASKAP Science Data Archive.

Fast radio bursts (FRBs) emit broadband radio emission that may, in rare cases, encode atomic hydrogen (Hi) absorption signals as they traverse the interstellar medium of their host galaxies. Though considered in the early FRB literature, the demanding observational prerequisites and the rarity of suitable events have meant that no thorough search for Hi absorption in FRB spectra has yet been undertaken. Here, we present an updated systematic analysis assessing the likelihood of modern facilities to detect such absorption features. As a proof of concept, we search for absorption in the spectrum of the bright ASKAP-localised FRB 20211127I, finding a $3\sigma$ opacity upper limit of 0.51. While this test case offers little constraining power, we find that narrow FRBs with fluences exceeding 20/70/150 Jy ms observed with MeerKAT/ASKAP/DSA can probe opacities below 0.1 – a regime in which absorption detections become physically meaningful. We further highlight that stacking thousands of bursts from hyperactive repeaters with FAST offers a very powerful avenue towards detection. Finally, we discuss the broad scientific potential of such detections, including constraints on extragalactic Hi spin temperatures, a means to physically probe the environment surrounding the progenitor, and a path towards disentangling host galaxy contributions to dispersion and scattering.

We present the Galactic Plane Monitor (GPM), a dedicated low-frequency radio transient survey conducted with the Murchison Widefield Array at 200 MHz. The GPM targets an underexplored region of transient parameter space spanning timescales from seconds to weeks, where recent discoveries of unusual Galactic radio sources—including long-period transients and other coherent emitters—suggest a rich and largely uncharted population. Leveraging the MWA’s wide field of view, we carried out 72 observing epochs between June 2022 and March 2025, searching ∼ 5,000 sq. deg. of the Southern Galactic plane. We developed a multi-stage processing pipeline to search for variability across a broad range of timescales, from intra-snapshot (4 s to minutes), to inter-snapshot (minutes to hours), and inter-epoch (days to years). This approach combines image differencing, statistical variability metrics, and visual inspection to robustly identify transient candidates while mitigating the effects of ionospheric variability and imaging artefacts. In parallel, we produced deep mosaics using a “lucky imaging” strategy to maximise sensitivity to faint persistent sources. We present the detections of five long-period radio transients, seven eclipsing binary millisecond pulsars, seven intrinsically variable pulsars, and the detection of refractive interstellar scintillation and night-time interplanetary scintillation. The GPM represents a key step toward fully mapping the dynamic low-frequency radio sky and informs the design of next-generation transient surveys with the SKA Observatory.

Radio searches of unassociated gamma-ray sources in the Fermi-LAT catalogues have led to the discoveries of around a fifth of all known millisecond pulsars (MSPs). These searches have almost exclusively been performed at radio frequencies above $300\,\mathrm{MHz}$, where dispersion and scattering in the interstellar medium are less significant. We report on a shallow survey for pulsars targeting 308 unassociated Fermi-LAT sources in archival Murchison Widefield Array (MWA) observations from the Southern-sky MWA Rapid Two-metre (SMART) pulsar survey at 154 MHz. The searches were performed using a new pipeline that implements semi-coherent dispersion removal, enabling greater sensitivities to MSPs than is possible with fully incoherent dispersion removal (e.g. 2–3 times better sensitivity for dispersion measures between $20\text{--}40\,\mathrm{cm}^{-3}\,\mathrm{pc}$). No new pulsars were identified in the survey, which we attribute to insufficient sensitivity. We estimate flux density limits of approximately $30\text{--}220\,\mathrm{mJy}$ at $154\,\mathrm{MHz}$ ($0.7\text{--}5.2\,\mathrm{mJy}$ at $1.4\,\mathrm{GHz}$) for a spin period of $2\,\mathrm{ms}$ and a duty cycle of 28%. The MWA Phase III will increase the number of detectable gamma-ray pulsars by $\sim$30% and enable longer targeted observations with the real-time beamformer. The search pipeline will also be useful for searches of supernova remnants, globular clusters, and pulsar candidates identified in imaging surveys, all of which will help to inform the significance of future surveys with SKA-Low.

The stellar initial mass function (IMF) is a fundamental ingredient in galaxy evolution, linking observed integrated light to stellar masses, star-formation rates, and chemical enrichment histories. Constraining the full IMF shape beyond the Milky Way remains challenging, as most studies focus either on the low-mass end of quiescent galaxies or the high-mass end of star-forming galaxies. Here, we present the first simultaneous analysis of both ends of the IMF in 214 star-forming galaxies from the Hector survey ($z \sim$ 0.01–0.07). We estimate the low-mass end slope ($\alpha_{\mathrm{low}}$) using a stellar population approach that fits IMF-sensitive absorption features with extended star formation histories, while the high-mass end slope ($\alpha_{\mathrm{high}}$) is derived via the Kennicutt diagnostic, which compares the observed H$\alpha$ equivalent width and $g-r$ colour with stellar population synthesis model predictions. We find substantial diversity in IMF shapes, with galaxies spanning combinations of bottom-heavy/light and top-heavy/light slopes. A weak but statistically robust correlation between the low- and high-mass IMF slopes is observed, but partial correlation analysis indicates that this apparent link is largely driven by their mutual dependence on stellar mass and metallicity. Both IMF slopes show significant correlations with stellar mass, star formation activity (traced by H$\alpha$ luminosity and surface density), and stellar metallicity ([M/H]). In general, higher stellar mass, stronger star formation activity, and higher metallicity are associated with both bottom-heavy and top-heavy IMFs. We find that the full IMF shape seems to be modulated by total stellar mass. Partial correlation analysis reveals that $\alpha_{\mathrm{low}}$ is primarily driven by [M/H], whereas $\alpha_{\mathrm{high}}$ is mainly linked to stellar mass and recent star formation. Because $\alpha_{\mathrm{low}}$ traces the IMF over long-term averages and $\alpha_{\mathrm{high}}$ captures only recent ($\lesssim10$ Myr) star formation, the processes shaping each end likely occur over different and possibly decoupled timescales. Our findings challenge the universality of the IMF and emphasise the need for galaxy evolution and stellar population models to incorporate a flexible IMF prescription. Accounting for these variations is essential to build an IMF-consistent picture of galaxy evolution across cosmic time.

We present 29 successfully recovered Civ time lags in Active Galactic Nuclei from the complete Dark Energy Survey Reverberation Mapping campaign. The AGN in this sample span a redshift range of $1.9\lt z\lt 3.5$. We successfully measure the velocity dispersion from the Civ spectral linewidth for 25 of these 29 sources, and use these to calculate new high-redshift black hole mass estimates, finding masses between 0.8 and 1.3 billion solar masses. We also identify a selection effect due to the duration of the survey that can impact the radius-luminosity relation derived from this and other (high-redshift) data. This paper represents the culmination of the OzDES Civ campaign.

Over a hundred fast radio burst (FRB) host galaxies have now been identified, enabling both comparisons of host redshift with FRB dispersion measure to study the cosmological distribution of ionised gas, and analyses of host properties in order to identify FRB progenitors. The standard method for determining the most likely FRB host galaxy in an optical image is the Bayesian framework Probabilistic Association of Transients to their Hosts (PATH), which accounts for uncertainties in the radio localisation, and simplified prior distributions on the host being observable. In this work we extend PATH, incorporating physically-motivated priors that are based on expectations about FRB host galaxy magnitudes. We develop three different models for the apparent r-band magnitude distribution based on an FRB’s expected host galaxy redshift, P(mr|z) and combine these with expectations for redshift based on an FRB’s dispersion measure, P(z|DM). We fit the parameters of these prior models using host galaxy candidates for 32 FRBs detected by the Australian SKA Pathfinder (ASKAP) in incoherent sum (ICS) mode by the Commensal Real-time ASKAP Fast Transients (CRAFT) survey.

Employing PATH with the new priors on the host magnitudes, we find increased confidence in the most probable hosts of all ASKAP ICS FRB host galaxies. All three models predict similar distributions of FRB host magnitudes at low redshift (z ∼ 0.1), and we confirm previous results that the true FRB host galaxy distribution is fainter than expected for a star-formation-weighted distribution (p-value of 0.12%). However, a mass-weighted distribution provides an even worse fit (p-value of 10−9). Tests against more FRBs in the z > 0.5 range, where the models differ, and extensions of the models to account for e.g. host metallicity, may help to resolve these uncertainties in the FRB host distribution.

We investigate how galaxy evolution varies with environment in the nearby Universe by comparing an ‘average’ reference volume in the Southern Galactic Pole (SGP) dataset to the Nexus region, a dynamically assembling superstructure centred on the Abell 4038 galaxy cluster. Environmental effects are quantified using the quenched fraction (fQ) and the mean and scatter of the specific star formation rate (sSFR) for star-forming galaxies, measured as functions of stellar mass and improved group-scale halo mass estimates. Trends are characterised using logistic fits for fQand power-law fits for mean log sSFR. After decoupling stellar and halo mass dependencies, we find that fQ increases with stellar mass in both field and group environments, while group galaxies show an additional dependence on halo mass. Relative to the SGP baseline, the Nexus exhibits systematic differences consistent with enhanced heterogeneity in accretion histories and pre-processing within a forming superstructure. For star-forming galaxies, mean log sSFR declines strongly with stellar mass and is further suppressed in group-scale haloes, whereas the scatter in log sSFR depends primarily on stellar mass and only weakly on halo mass. Most differences remain within current uncertainties. Within the Nexus, splitting the sample by projected distance from Abell 4038 reveals systematic variations in both quenching and star-forming properties, largely driven by changes in the sampled halo mass function. A projected phase-space analysis shows higher fQ in regions associated with earlier infall, linking quenching to orbital history; however, this trend is strongly stellar-mass dependent and is absent for low-mass galaxies ($\log M_{stellar} \lt 10$). These results demonstrate that galaxy evolution depends jointly on stellar mass, halo mass, and location within the surrounding large-scale structure.

Over the last decade, the Australian Dark Energy (OzDES) collaboration has used Reverberation Mapping to measure the masses of high redshift supermassive black holes. Here we present the final review and analysis of this OzDES reverberation mapping campaign. These observations use $6-7$ years of photometric and spectroscopic observations of 735 Active Galactic Nuclei (AGN) in the redshift range $z\in [0.13, 3.85]$ and bolometric luminosity range $\log_{10}(L_{\mathrm{bol}})\in [44.3, 47.5] \; \mathrm{erg/s}$. Both photometry and spectra are observed in visible wavelengths, allowing for the physical scale of the AGN broad line region to be estimated from reverberations of the H$\beta$, MgII and CIV emission lines. We successfully use reverberation mapping to constrain the masses of 62 super-massive black holes, and combine with existing data to fit a power law to the lag-luminosity relation for the H$\beta$ and MgII lines with a scatter of $\sim0.25$ dex, the tightest yet identified, fit specifically for consistency with high redshift AGN. We fit a similarly constrained relation for CIV, resolving a tension with the low luminosity literature AGN by accounting for selection effects arising from finite survey length. We also examine the impact of emission line width and luminosity (related to accretion rate) in reducing the scatter of these scaling relationships and find no significant improvement over the lag-only approach for any of the three lines. Using these relations, we further estimate the masses and accretion rates of 246 AGN with single epoch methods. We also use these relations to estimate the relative sizes of the H$\beta$, MgII and CIV emitting regions, and find evidence that the MgII emission may occur further out than H$\beta$. In short, we provide a comprehensive benchmark of high redshift AGN reverberation mapping at the close of this most recent generation of surveys, including light curves, time-delays, and a set of significantly improved radius-luminosity relations for use with high-redshift populations.

The primordial helium mass fraction, $Y_{p}$, is a key observational pillar of Big Bang nucleosynthesis and a sensitive probe of early-Universe physics. Over the past several decades, numerous observational $Y_{p}$ determinations have been published using a wide range of astrophysical tracers and cosmological techniques. Although recent measurements exhibit striking convergence and increasingly small uncertainties, the statistical and historical context of this consensus has not been examined systematically. Here, we compile and analyse a comprehensive dataset of observational $Y_{p}$ determinations published between the late-1960s and 2022. The final sample comprises 143 reported values spanning multiple tracers. We find clear evidence for long-term convergence in published $Y_{p}$ values, punctuated by statistically significant change points in the mid-2000s and early 2010s. Careful examination reveals that many extragalactic Hii-region determinations are not fully independent, relying on re-analyses or partial reuse of a limited number of observational datasets. This reduces the effective number of independent constraints and provides important context for interpreting the precision of recent results. Our findings do not challenge the overall consistency of modern $Y_{p}$ determinations with standard cosmology, but they underscore the importance of accounting for data dependence, methodological homogeneity and historical evolution when synthesising measurements.

Techniques using multiple tracers of the large scale structure of the universe show great promise for examining the fundamentals of our Universe’s cosmology. Such techniques rely on the different relationship between the overdensity of tracers and the broader matter overdensity, enabling cosmic-variance-free tests of primordial non-Gaussianity in the initial curvature perturbations. There is a great opportunity for current and future all-sky extra-galactic radio surveys to make use of this technique to test for non-Gaussianity at a precision greater than existing all-sky constraints from the cosmic microwave background. To realise this goal, there is a need for accurate simulations. Previous radio galaxy simulations have either been realistic but covering only a small area (and so unhelpful for cosmological forecasts), or all-sky dark matter only cosmological simulations but having no connection to a real radio galaxy population. In this study, we use the FLAMINGO suite of cosmological surveys, as well as the matching of dark matter halos to radio galaxy population, to create an accurate sky simulation in order to examine the feasibility of multi-tracer techniques. We present an analysis of the clustering (with a bias model for the simulation), as well as redshift distributions, source counts and radio luminosity functions, and discuss future work on non-Gaussianity detection.