We follow up on the surprising recent announcement by Vernstrom et al. (2021, MNRAS) of the detection of the synchrotron cosmic web. We attempt to reproduce their detection with new observations with the Phase II, extended configuration of the Murchison Widefield Array at 118.5 MHz. We reproduce their detection methodology by stacking pairs of nearby luminous red galaxies (LRGs)—used as tracers for clusters and galaxy groups—contained in our low-frequency radio observations. We show that our observations are significantly more sensitive than those used in Vernstrom et al. and that our angular sensitivity is sufficient. And yet, we make no statistically significant detection of excess radio emission along the bridge spanning the LRG pairs. This non-detection is true both for the original LRG pair catalogue as used in Vernstrom et al., as well as for other larger catalogues with modified selection criteria. Finally, we return to the original data sets used in Vernstrom et al., and find that whilst we clearly reproduce the excess X-ray emission from ROSAT, we are not able to reproduce any kind of broad and extended excess intercluster filamentary emission using the original 118.5 MHz MWA survey data. In the interests of understanding this result, as part of this paper we release images of the 14 fields used in this study, the final stacked images, as well as key components of our stacking and modelling code.

Rest-frame mid- to far-infrared (IR) spectroscopy is a powerful tool to study how galaxies formed and evolved, because a major part of their evolution occurs in heavily dust enshrouded environments, especially at the so-called Cosmic Noon ($1< z < 3$). Using the calibrations of IR lines and features, recently updated with Herschel and Spitzer spectroscopy, we predict their expected fluxes with the aim to measure the Star Formation (SF) and the Black Hole Accretion (BHA) rates in intermediate to high redshift galaxies. On the one hand, the recent launch of the James Webb Space Telescope (JWST) offers new mid-IR spectroscopic capabilities that will enable for the first time a detailed investigation of both the SF and the BHA obscured processes as a function of cosmic time. We make an assessment of the spectral lines and features that can be detected by JWST-MIRI in galaxies and active galactic nuclei up to redshift $z \sim 3$. The fine structure lines of [MgIV]4.49 $\unicode{x03BC}\textrm{m}$ and [ArVI]4.53 $\unicode{x03BC}\textrm{m}$ can be used as BHA rate tracers for the $1 \lesssim z \lesssim 3$ range, and we propose the [NeVI]7.65 $\unicode{x03BC}\textrm{m}$ line as the best tracer for $z \lesssim 1.5$. The [ArII]6.98 $\unicode{x03BC}\textrm{m}$ and [ArIII]8.99 $\unicode{x03BC}\textrm{m}$ lines can be used to measure the SF rate at $z \lesssim 3$ and $z \lesssim 2$, respectively, while the stronger [NeII]12.8 $\unicode{x03BC}\textrm{m}$ line exits the JWST-MIRI spectral range above $z \gtrsim 1.2$. At higher redshifts, the PAH features at 6.2 and 7.7 $\unicode{x03BC}\textrm{m}$ can be observed at $z \lesssim 3$ and $z \lesssim 2.7$, respectively. On the other hand, rest-frame far-IR spectroscopic observations of high redshift galaxies ($z \gtrsim 3$) have been collected with the Atacama Large Millimeter Array (ALMA) in the last few years. The observability of far-IR lines from ALMA depends on the observed frequency, due to the significant decrease of the atmospheric transmission at the highest frequencies ($\gtrsim420\,\rm{Hz}$). The [CII]158 $\unicode{x03BC}\textrm{m}$ line is a reliable tracer of the SF rate and can in most cases ($0.9 \lesssim z \lesssim 2$ and $2 \lesssim z \lesssim 9$) be observed. Additionally, we propose the use of the combination of [OIII]88$\,\unicode{x03BC}$m and [OI]145$\,\unicode{x03BC}$m lines as an alternative SF rate tracer, that can be detected above $z \gtrsim 3$. Overall, we emphasize the importance of using multi-feature analysis to measure both BHA and SFR, since individual tracers can be strongly dependent on the local ISM conditions and vary from source to source. However, we conclude that the peak of the obscured SF and BHA activities at Cosmic Noon falls outside the wavelength coverage of facilities currently operating or under development. A new IR space telescope covering the full IR spectral range from about 10 to $300\,\unicode{x03BC}\textrm{m}$ and actively cooled to achieve high sensitivity, will be needed.

We describe the scientific goals and survey design of the First Large Absorption Survey in H i (FLASH), a wide field survey for 21-cm line absorption in neutral atomic hydrogen (H i) at intermediate cosmological redshifts. FLASH will be carried out with the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope and is planned to cover the sky south of $\delta \approx +40\,\deg$ at frequencies between 711.5 and 999.5 MHz. At redshifts between $z = 0.4$ and $1.0$ (look-back times of 4 – 8 Gyr), the H i content of the Universe has been poorly explored due to the difficulty of carrying out radio surveys for faint 21-cm line emission and, at ultra-violet wavelengths, space-borne searches for Damped Lyman-$\alpha$ absorption in quasar spectra. The ASKAP wide field of view and large spectral bandwidth, in combination with a radio-quiet site, will enable a search for absorption lines in the radio spectra of bright continuum sources over 80% of the sky. This survey is expected to detect at least several hundred intervening 21-cm absorbers and will produce an H i-absorption-selected catalogue of galaxies rich in cool, star-forming gas, some of which may be concealed from optical surveys. Likewise, at least several hundred associated 21-cm absorbers are expected to be detected within the host galaxies of radio sources at $0.4 < z < 1.0$, providing valuable kinematical information for models of gas accretion and jet-driven feedback in radio-loud active galactic nuclei. FLASH will also detect OH 18-cm absorbers in diffuse molecular gas, megamaser OH emission, radio recombination lines, and stacked H i emission.

The mobility of lighter species on the surface of interstellar dust grains plays a crucial role in forming simple through complex molecules. Carbon monoxide is one of the most abundant molecules, its surface diffusion on the grain surface is essential to forming many molecules. Recent laboratory experiments found a diverse range of diffusion barriers for CO on the grain surface, their use can significantly impact the abundance of several molecules. The impact of different diffusion barriers of CO, in the astrochemical models, is studied to understand its effect on the abundance of solid CO and the species for which it is a reactant partner. A gas-grain network is used for three different physical conditions; cold core and warm-up models with slow and fast heating rates. Two different ratios (0.3 and 0.5) between diffusion and desorption barrier are utilised for all the species. For each physical condition and ratio, six different models are run by varying diffusion barriers of CO. Solid CO abundance for the models with the lowest diffusion barrier yields less than 0.1% of water ice for cold clouds and a maximum of 0.4% for slow and fast warm-up models. Also, solid $\textrm{CO}_2$ in dense clouds is significantly overproduced (${\sim}140\%$ of water). The abundance of H2CO and $\textrm{CH}_3\textrm{OH}$ showed an opposite trend, and HCOOH, $\textrm{CH}_3\textrm{CHO}$, $\textrm{NH}_2\textrm{CO}$, and $\textrm{CH}_3\textrm{COCH}_3$ are produced in lower quantities for models with low diffusion barriers for CO. Considerable variation in abundance is observed between models with the high and low diffusion barrier. Models with higher diffusion barriers provide a relatively better agreement with the observed abundances when compared with the models having lower diffusion barriers.

This paper is the fourth in a series of low-frequency searches for technosignatures. Using the Murchison Widefield Array over two nights, we integrate 7 h of data toward the Galactic Centre (centred on the position of Sagittarius $\mathrm{A}^{*}$) with a total field-of-view of $200\,\mathrm{deg}^{2}$. We present a targeted search toward 144 exoplanetary systems, at our best yet angular resolution (75 arcsec). This is the first technosignature search at a central frequency of 155 MHz toward the Galactic Centre (our previous central frequencies have been lower). A blind search toward in excess of 3 million stars toward the Galactic Centre and Galactic bulge is also completed, placing an equivalent isotropic power limit $<\!1.1\times10^{19}\,\mathrm{W}$ at the distance to the Galactic Centre. No plausible technosignatures are detected.

Proposed next-generation networks of gravitational-wave observatories include dedicated kilohertz instruments that target neutron star science, such as the proposed Neutron Star Extreme Matter Observatory, NEMO. The original proposal for NEMO highlighted the need for it to exist in a network of gravitational-wave observatories to ensure detection confidence and sky localisation of sources. We show that NEMO-like observatories have significant utility on their own as coincident electromagnetic observations can provide the detection significance and sky localisation. We show that, with a single NEMO-like detector and expected electromagnetic observatories in the late 2020 s and early 2030 s such as the Vera C. Rubin observatory and SVOM, approximately 40% of all binary neutron star mergers detected with gravitational waves could be confidently identified as coincident multimessenger detections. We show that we expect $2^{+10}_{-1}{yr^{-1}}{}$ coincident observations of gravitational-wave mergers with gamma-ray burst prompt emission, $13^{+23}_{-10}{yr^{-1}}{}$ detections with kilonova observations, and $4^{+18}_{-3}{yr^{-1}}{}$ with broadband afterglows and kilonovae, where the uncertainties are 90% confidence intervals arising from uncertainty in current merger-rate estimates. Combined, this implies a coincident detection rate of $14^{+25}_{-11}{yr^{-1}}{}$ out to $300\,\mathrm{Mpc}$. These numbers indicate significant science potential for a single kilohertz gravitational-wave detector operating without a global network of other gravitational-wave observatories.

The distribution of diameters and orbital distances from the parent body of 156 named moons of the planets in the Solar System is not random. All 11 moons with diameters larger than $1\,000\,\mathrm{km}$ are positioned between $400\,000\,\mathrm{km}$ and 4 million km from the parent, whereas the far more numerous small moons are distributed on both sides of this central region and are largely absent from the region in between. This small-satellite ‘exclusion region’ is particularly evident for the gas giants since they have multiple satellites spanning a wide range of distances from the parent. Application of mathematical criteria analogous to those that have been used to help define the ‘gravitational clearing’ of planetary orbits around the Sun suggests that the absence of small satellites in this region around the planets may be a result (atleast in part) of gravitational clearing by the large moons present at these distances from the parent. The most significant exception to the observed diameter-distance distribution—Hyperion, on Saturn—is attributed to its 3:4 orbital resonance with Titan, while other obvious exceptions are the Trojan satellites of Saturn’s moons Tethys and Dione. The smallest satellite diameter that seems necessary for clearing of its ‘sphere of influence’ is around $400\,\mathrm{km}$.

We present the most sensitive and detailed view of the neutral hydrogen (${\rm H\small I}$) emission associated with the Small Magellanic Cloud (SMC), through the combination of data from the Australian Square Kilometre Array Pathfinder (ASKAP) and Parkes (Murriyang), as part of the Galactic Australian Square Kilometre Array Pathfinder (GASKAP) pilot survey. These GASKAP-HI pilot observations, for the first time, reveal ${\rm H\small I}$ in the SMC on similar physical scales as other important tracers of the interstellar medium, such as molecular gas and dust. The resultant image cube possesses an rms noise level of 1.1 K ($1.6\,\mathrm{mJy\ beam}^{-1}$) $\mathrm{per}\ 0.98\,\mathrm{km\ s}^{-1}$ spectral channel with an angular resolution of $30^{\prime\prime}$ (${\sim}10\,\mathrm{pc}$). We discuss the calibration scheme and the custom imaging pipeline that utilises a joint deconvolution approach, efficiently distributed across a computing cluster, to accurately recover the emission extending across the entire ${\sim}25\,\mathrm{deg}^2$ field-of-view. We provide an overview of the data products and characterise several aspects including the noise properties as a function of angular resolution and the represented spatial scales by deriving the global transfer function over the full spectral range. A preliminary spatial power spectrum analysis on individual spectral channels reveals that the power law nature of the density distribution extends down to scales of 10 pc. We highlight the scientific potential of these data by comparing the properties of an outflowing high-velocity cloud with previous ASKAP+Parkes ${\rm H\small I}$ test observations.

Many short gamma-ray bursts (GRBs) originate from binary neutron star mergers, and there are several theories that predict the production of coherent, prompt radio signals either prior, during, or shortly following the merger, as well as persistent pulsar-like emission from the spin-down of a magnetar remnant. Here we present a low frequency (170–200 MHz) search for coherent radio emission associated with nine short GRBs detected by the Swift and/or Fermi satellites using the Murchison Widefield Array (MWA) rapid-response observing mode. The MWA began observing these events within 30–60 s of their high-energy detection, enabling us to capture any dispersion delayed signals emitted by short GRBs for a typical range of redshifts. We conducted transient searches at the GRB positions on timescales of 5 s, 30 s, and 2 min, resulting in the most constraining flux density limits on any associated transient of 0.42, 0.29, and 0.084 Jy, respectively. We also searched for dispersed signals at a temporal and spectral resolution of 0.5 s and 1.28 MHz, but none were detected. However, the fluence limit of 80–100 Jy ms derived for GRB 190627A is the most stringent to date for a short GRB. Assuming the formation of a stable magnetar for this GRB, we compared the fluence and persistent emission limits to short GRB coherent emission models, placing constraints on key parameters including the radio emission efficiency of the nearly merged neutron stars ($\epsilon_r\lesssim10^{-4}$), the fraction of magnetic energy in the GRB jet ($\epsilon_B\lesssim2\times10^{-4}$), and the radio emission efficiency of the magnetar remnant ($\epsilon_r\lesssim10^{-3}$). Comparing the limits derived for our full GRB sample (along with those in the literature) to the same emission models, we demonstrate that our fluence limits only place weak constraints on the prompt emission predicted from the interaction between the relativistic GRB jet and the interstellar medium for a subset of magnetar parameters. However, the 30-min flux density limits were sensitive enough to theoretically detect the persistent radio emission from magnetar remnants up to a redshift of $z\sim0.6$. Our non-detection of this emission could imply that some GRBs in the sample were not genuinely short or did not result from a binary neutron star merger, the GRBs were at high redshifts, these mergers formed atypical magnetars, the radiation beams of the magnetar remnants were pointing away from Earth, or the majority did not form magnetars but rather collapse directly into black holes.

Therecently discovered massive binary system Apep is the most powerful synchrotron emitter among the known Galactic colliding-wind binaries. This makes this particular system of great interest to investigate stellar winds and the non-thermal processes associated with their shocks. This source was detected at various radio bands, and in addition the wind-collision region was resolved by means of very-long baseline interferometric observations. We use a non-thermal emission model for colliding-wind binaries to derive physical properties of this system. The observed morphology in the resolved maps allows us to estimate the system projection angle on the sky to be $\psi \approx 85^\circ$. The observed radio flux densities also allow us to characterise both the intrinsic synchrotron spectrum of the source and its modifications due to free–free absorption in the stellar winds at low frequencies; from this, we derive mass–loss rates of the stars of $\dot{M}_\mathrm{WN} \approx 4\times10^{-5}\;\mathrm{M}_\odot\,\mathrm{yr}^{-1}$ and $\dot{M}_\mathrm{WC} \approx 2.9\times10^{-5}\;\mathrm{M}_\odot\,\mathrm{yr}^{-1}$. Finally, the broadband spectral energy distribution is calculated for different combinations of the remaining free parameters, namely the intensity of the magnetic field and the injected power in non-thermal particles. We show that the degeneracy of these two parameters can be solved with observations in the high-energy domain, most likely in the hard X-rays but also possibly in $\gamma$-rays under favourable conditions.

Recent ground-based deep observations of the Universe have discovered large populations of massive quiescent galaxies at $z\sim3\!-\!5$. With the launch of the James Webb Space Telescope (JWST), the on-board Near-Infrared Spectrograph (NIRSpec) instrument will provide continuous $0.6\!-\!5.3\,\unicode{x03BC}\,\mathrm{m}$ spectroscopic coverage of these galaxies. Here we show that NIRSpec/CLEAR spectroscopy is ideal to probe the completeness of photometrically selected massive quiescent galaxies such as the ones presented by Schreiber et al. (2018b, A&A, 618, A85). Using a subset of the Schreiber et al. (2018b, A&A, 618, A85) sample with deep Keck/MOSFIRE spectroscopy presented by Esdaile J., et al. (2021b, ApJ, 908, L35), we perform a suite of mock JWST/NIRSpec observations to determine optimal observing strategies to efficiently recover the star formation histories (SFHs), element abundances, and kinematics of these massive quiescent galaxies. We find that at $z\sim3$, medium resolution G235M/FL170LP NIRSpec observations could recover element abundances at an accuracy of ${\sim}15\%$, which is comparable to local globular clusters. Mimicking ZFOURGE COSMOS photometry, we perform mock spectrophotometric fitting with Prospector to show that the overall shape of the SFHs of our mock galaxies can be recovered well, albeit with a dependency on the number of non-parametric SFH bins. We show that deep high-resolution G235H/FL170LP integral field spectroscopy with a $S/N\sim7$ per spaxel is required to constrain the rotational properties of our sample at $>\!2\sigma$ confidence. Thus, through optimal grism/filter choices, JWST/NIRSpec slit and integral field spectroscopy observations would provide tight constraints to galaxy evolution in the early Universe.

Most applications of Bayesian Inference for parameter estimation and model selection in astrophysics involve the use of Monte Carlo techniques such as Markov Chain Monte Carlo (MCMC) and nested sampling. However, these techniques are time-consuming and their convergence to the posterior could be difficult to determine. In this study, we advocate variational inference as an alternative to solve the above problems, and demonstrate its usefulness for parameter estimation and model selection in astrophysics. Variational inference converts the inference problem into an optimisation problem by approximating the posterior from a known family of distributions and using Kullback–Leibler divergence to characterise the difference. It takes advantage of fast optimisation techniques, which make it ideal to deal with large datasets and makes it trivial to parallelise on a multicore platform. We also derive a new approximate evidence estimation based on variational posterior, and importance sampling technique called posterior-weighted importance sampling for the calculation of evidence, which is useful to perform Bayesian model selection. As a proof of principle, we apply variational inference to five different problems in astrophysics, where Monte Carlo techniques were previously used. These include assessment of significance of annual modulation in the COSINE-100 dark matter experiment, measuring exoplanet orbital parameters from radial velocity data, tests of periodicities in measurements of Newton’s constant G, assessing the significance of a turnover in the spectral lag data of GRB 160625B, and estimating the mass of a galaxy cluster using weak gravitational lensing. We find that variational inference is much faster than MCMC and nested sampling techniques for most of these problems while providing competitive results. All our analysis codes have been made publicly available.

This review of Aboriginal astronomy and navigation brings together accounts from widely dispersed places in Western Australia, from Noongar Country in the south-west, through to the Eastern Goldfields, the Pilbara, the Kimberley and the Central Deserts. Information for this review has been taken from the literature and non-conventional sources, including artist statements of paintings. The intention for the review is that the scope is traditional, pre-European settlement understandings, but post-settlement records of oral accounts, and later articulation by Aboriginal peoples, are necessarily relied upon. In large part, the Western Australian accounts reflect understandings reported for other states. For example, star maps were used for teaching routes on the ground, but available accounts do not evidence that star maps were used in real-time navigation. The narratives or dreamings that differ most from those of other states explain creation of night-sky objects and landforms on Earth, events including thunder, or they address social behaviour.

We present a software package for single-dish data processing of spacecraft signals observed with VLBI-equipped radio telescopes. The Spacecraft Doppler tracking (SDtracker) software allows one to obtain topocentric frequency detections with a sub-Hz precision and reconstructed and residual phases of the carrier signal of any spacecraft or landing vehicle at any location in the Solar System. These data products are estimated using the ground-based telescope’s highly stable oscillator as a reference, without requiring an a priori model of the spacecraft dynamics nor the downlink transmission carrier frequency. The software has been extensively validated in multiple observing campaigns of various deep space missions and is compatible with the raw sample data acquired by any standard VLBI radio telescope worldwide. In this paper, we report the numerical methodology of SDtracker, the technical operations for deployment and usage, and a summary of use cases and scientific results produced since its initial release.

Mid- and far-infrared (IR) photometric and spectroscopic observations are fundamental to a full understanding of the dust-obscured Universe and the evolution of both star formation and black hole accretion in galaxies. In this work, using the specifications of the SPace Infrared telescope for Cosmology and Astrophysics (SPICA) as a baseline, we investigate the capability to study the dust-obscured Universe of mid- and far-IR photometry at 34 and $70\, {\rm{\mu }}\mathrm{m}$ and low-resolution spectroscopy at $17{-}36\, {\rm{\mu }}\mathrm{m}$ using the state-of-the-art Spectro-Photometric Realisations of Infrared-selected Targets at all-z (Spritz) simulation. This investigation is also compared to the expected performance of the Origins Space Telescope and the Galaxy Evolution Probe. The photometric view of the Universe of a SPICA-like mission could cover not only bright objects (e.g. $L_{IR}>10^{12}\,{\rm L}_{\odot}$) up to ${z}=10$, but also normal galaxies ($L_{IR}<10^{11}\,{\rm L}_{\odot}$) up to $\textit{z}\sim4$. At the same time, the spectroscopic observations of such mission could also allow us to estimate the redshifts and study the physical properties for thousands of star-forming galaxies and active galactic nuclei by observing the polycyclic aromatic hydrocarbons and a large set of IR nebular emission lines. In this way, a cold, 2.5-m size space telescope with spectro-photometric capability analogous to SPICA, could provide us with a complete three-dimensional (i.e. images and integrated spectra) view of the dust-obscured Universe and the physics governing galaxy evolution up to $\textit{z}\sim4$.

The Rapid ASKAP Continuum Survey (RACS) is the first large sky survey using the Australian Square Kilometre Array Pathfinder (ASKAP), covering the sky south of $+41^\circ$ declination. With ASKAP’s large, instantaneous field of view, ${\sim}31\,\mathrm{deg}^2$, RACS observed the entire sky at a central frequency of 887.5 MHz using 903 individual pointings with 15 minute observations. This has resulted in the deepest radio survey of the full Southern sky to date at these frequencies. In this paper, we present the first Stokes I catalogue derived from the RACS survey. This catalogue was assembled from 799 tiles that could be convolved to a common resolution of $25^{\prime\prime}$, covering a large contiguous region in the declination range $\delta=-80^{\circ}$ to $+30^\circ$. The catalogue provides an important tool for both the preparation of future ASKAP surveys and for scientific research. It consists of $\sim$2.1 million sources and excludes the $|b|<5^{\circ}$ region around the Galactic plane. This provides a first extragalactic catalogue with ASKAP covering the majority of the sky ($\delta<+30^{\circ}$). We describe the methods to obtain this catalogue from the initial RACS observations and discuss the verification of the data, to highlight its quality. Using simulations, we find this catalogue detects 95% of point sources at an integrated flux density of $\sim$5 mJy. Assuming a typical sky source distribution model, this suggests an overall 95% point source completeness at an integrated flux density $\sim$3 mJy. The catalogue will be available through the CSIRO ASKAP Science Data Archive (CASDA).

At relatively high frequencies, highly sensitive grating sidelobes occur in the primary beam patterns of low frequency aperture arrays (LFAA) such as the Murchison Widefield Array (MWA). This occurs when the observing wavelength becomes comparable to the dipole separation for LFAA tiles, which for the MWA occurs at ${\sim}300$ MHz. The presence of these grating sidelobes has made calibration and image processing for 300 MHz MWA observations difficult. This work presents a new calibration and imaging strategy which employs existing techniques to process two example 300 MHz MWA observations. Observations are initially calibrated using a new 300 MHz sky-model which has been interpolated from low frequency and high frequency all-sky surveys. Using this 300 MHz model in conjunction with the accurate MWA tile primary beam model, we perform sky-model calibration for the two example observations. After initial calibration a self-calibration loop is performed by all-sky imaging each observation. We mask the main lobe of the all-sky image, and perform a sky-subtraction by estimating the masked image visibilities. We then image the main lobe of the sky-subtracted visibilities, which results in high dynamic range images of the two example observations. These images have been convolved with a Gaussian to a resolution of $2.4$ arcminutes, with a maximum sensitivity of ${{\sim}}31\,\textrm{mJy/beam}$. The calibration and imaging strategy demonstrated in this work opens the door to performing science at 300 MHz with the MWA, which was previously an inaccessible domain. With this paper we release the code described below and the cross-matched catalogue along with the code to produce a sky-model in the range 70–1 400 MHz.

The cosmic evolution of the chemical elements from the Big Bang to the present time is driven by nuclear fusion reactions inside stars and stellar explosions. A cycle of matter recurrently re-processes metal-enriched stellar ejecta into the next generation of stars. The study of cosmic nucleosynthesis and this matter cycle requires the understanding of the physics of nuclear reactions, of the conditions at which the nuclear reactions are activated inside the stars and stellar explosions, of the stellar ejection mechanisms through winds and explosions, and of the transport of the ejecta towards the next cycle, from hot plasma to cold, star-forming gas. Due to the long timescales of stellar evolution, and because of the infrequent occurrence of stellar explosions, observational studies are challenging, as they have biases in time and space as well as different sensitivities related to the various astronomical methods. Here, we describe in detail the astrophysical and nuclear-physical processes involved in creating two radioactive isotopes useful in such studies, $^{26}\mathrm{Al}$ and $^{60}\mathrm{Fe}$. Due to their radioactive lifetime of the order of a million years, these isotopes are suitable to characterise simultaneously the processes of nuclear fusion reactions and of interstellar transport. We describe and discuss the nuclear reactions involved in the production and destruction of $^{26}\mathrm{Al}$ and $^{60}\mathrm{Fe}$, the key characteristics of the stellar sites of their nucleosynthesis and their interstellar journey after ejection from the nucleosynthesis sites. This allows us to connect the theoretical astrophysical aspects to the variety of astronomical messengers presented here, from stardust and cosmic-ray composition measurements, through observation of $\gamma$ rays produced by radioactivity, to material deposited in deep-sea ocean crusts and to the inferred composition of the first solids that have formed in the Solar System. We show that considering measurements of the isotopic ratio of $^{26}\mathrm{Al}$ to $^{60}\mathrm{Fe}$ eliminate some of the unknowns when interpreting astronomical results, and discuss the lessons learned from these two isotopes on cosmic chemical evolution. This review paper has emerged from an ISSI-BJ Team project in 2017–2019, bringing together nuclear physicists, astronomers, and astrophysicists in this inter-disciplinary discussion.

Curiously, our Universe was born in a low entropy state, with abundant free energy to power stars and life. The form that this free energy takes is usually thought to be gravitational: the Universe is almost perfectly smooth, and so can produce sources of energy as matter collapses under gravity. It has recently been argued that a more important source of low-entropy energy is nuclear: the Universe expands too fast to remain in nuclear statistical equilibrium, effectively shutting off nucleosynthesis in the first few minutes, providing leftover hydrogen as fuel for stars. Here, we fill in the astrophysical details of this scenario and seek the conditions under which a Universe will emerge from early nucleosynthesis as almost-purely iron. In so doing, we identify a hitherto-overlooked character in the story of the origin of the second law: matter–antimatter asymmetry.

The RAD@home RGB-maker Tool is a python-based web application that enables citizen science research through collaboration using open-source technology. The tool fetches FITS image data from NASA SkyView and generates false colour images in Red-Green-Blue channels with contour. This helps in the basic multi-wavelength understanding and characterization of extragalactic objects, and further analysis along with reporting of potential new discoveries in a uniform format. Students from Universities or science-graduate citizens gain skills in RGB-C image analysis and identify unique features in objects via either one-day online-weekend events or week-long in-person training. Trained citizen scientists in turn are part of rare discoveries such as the jet-galaxy interaction system RAD12. The tool has been successfully used by thousands of citizen scientists in India since its launch on 26th January 2021 and has demonstrated the potential during IAU symposium 375 to be part of citizen science efforts with international participants.