Baryonic feedback remains one of the largest uncertainties in cosmological hydrodynamical simulations, with different prescriptions producing divergent predictions for the fraction of gas expelled from halos, the radial extent of the gas expulsion and the impact on large scale matter clustering. We present the first systematic study of the kinetic Sunyaev-Zel'dovich (kSZ) effect across a wide range of simulations (FLAMINGO, ANTILLES, BAHAMAS, SIMBA, FABLE and their variants), and compare them directly to DESI Year 1 + ACT kSZ measurements. We ensure a like-for-like comparison with observations by developing a robust methodology that accounts for the halo mass selection using galaxy-galaxy lensing, cosmic variance, miscentering and satellites, establishing the kSZ effect as a new benchmark for the simulations. We find that fiducial feedback models are disfavoured by >3 sigma, while simulations with more powerful AGN feedback within the FLAMINGO and BAHAMAS suites, as well as SIMBA, reproduce the observed kSZ signal within <2 sigma. We use the ANTILLES simulation suite to demonstrate that the amplitude of the kSZ effect is a strong predictor of matter power spectrum suppression, competitive with baryon fraction metrics. These results establish the kSZ as a critical probe for evaluating feedback physics and for advancing the fidelity of cosmological simulations.

Continuum emission at 220 and 355 GHz from the prototype Class 0 source VLA 1623 has been detected using the JCMT-CSO interferometer. Gaussian fits to the data place an upper limit of 70 AU on the half-width half-maximum radius of the emission, which implies an upper limit of ~175 AU for the cutoff radius of the circumstellar disk in the system. In the context of existing collapse models, this disk could be magnetically supported on the largest scales and have an age of ~6x10^4 yr, consistent with previous suggestions that Class 0 sources are quite young. The innermost region of the disk within ~6 AU is likely to be in centrifugal support, which is likely large enough to provide a drive for the outflow according to current theoretical models. Alternatively, if 175 AU corresponds to the centrifugal radius of the disk, the age of the system is ~2x10^5 yr, closer to age estimates for Class I sources.

We use data from the DESI Legacy Survey imaging to probe the galaxy density field in tomographic slices covering the redshift range $0

We present the results and conclusions from the citizen science competition `Observing Dark Worlds', where we asked participants to calculate the positions of dark matter halos from 120 catalogues of simulated weak lensing galaxy data, using computational methods. In partnership with Kaggle (this http URL), 357 users participated in the competition which saw 2278 downloads of the data and 3358 submissions. We found that the best algorithms improved on the benchmark code, LENSTOOL by > 30% and could measure the positions of > 3x10^14MSun halos to less than 5'' and < 10^14MSun to within 1'. In this paper, we present a brief overview of the winning algorithms with links to available code. We also discuss the implications of the experiment for future citizen science competitions.

We demonstrate how the two dominant constituents of the Universe, dark energy and dark matter, could possess a large scattering cross-section without considerably impacting observations. Unlike models involving energy exchange between the two fluids, the background cosmology remains unaltered, leaving fewer observational signatures. Following a brief review of the scattering cross-sections between cosmologically significant particles, we explore the implications of an elastic interaction between dark matter and dark energy. The growth of large scale structure is suppressed, yet this effect is found to be weak due to the persistently low dark energy density. Thus we conclude that the dark matter-dark energy cross section may exceed the Thomson cross-section by several orders of magnitude.

We present a comprehensive full-sky 3-dimensional analysis of the weak-lensing fields and their corresponding power spectra. Using the formalism of spin-weight spherical harmonics and spherical Bessel functions, we relate the two-point statistics of the harmonic expansion coefficients of the weak lensing shear and convergence to the power spectrum of the matter density perturbations, and derive small-angle limits. Such a study is relevant in view of the next generation of large-scale weak lensing surveys which will provide distance information about the sources through photometric redshifts. This opens up the possibility of accurate cosmological parameter estimation via weak lensing, with an emphasis on the equation of state of dark energy.

We present a search for luminous, long-duration ambiguous nuclear transients (ANTs) similar to the unprecedented discovery of the extreme, ambiguous event AT2021lwx with a &gt;150\,d rise time and luminosity $10^{45.7}$\,erg\,s$^{-1}$. We use the Lasair transient broker to search Zwicky Transient Facility (ZTF) data for transients lasting more than one year and exhibiting smooth declines. Our search returns 59 events, seven of which we classify as ANTs assumed to be driven by accretion onto supermassive black holes. We propose the remaining 52 are stochastic variability from regular supermassive black hole accretion rather than distinct transients. We supplement the seven ANTs with three nuclear transients in ZTF that fail the light curve selection but have clear single flares and spectra that do not resemble typical AGN. All of these 11 ANTs have a mid-infrared flare from an assumed dust echo, implying the ubiquity of dust around the black holes giving rise to ANTs. No events are more luminous than AT2021lwx, but one (ZTF19aamrjar) has twice the duration and a higher integrated energy release. On the other extreme, ZTF20abodaps reaches a luminosity close to AT2021lwx with a rise time &lt;20\,d and that fades smoothly in &gt;600\,d. We define a portion of rise-time versus flare amplitude space that selects ANTs with $\sim50$ per cent purity against variable active galactic nuclei. We calculate a volumetric rate of $\gtrsim 3\times10^{-11}$\,Mpc$^{-1}$\,yr$^{-1}$, consistent with the events being caused by tidal disruptions of intermediate and high-mass stars.

We present the Herschel Extragalactic Legacy Project (HELP). This project collates, curates, homogenises, and creates derived data products for most of the premium multi-wavelength extragalactic data sets. The sky boundaries for the first data release cover 1270 deg2 defined by the Herschel SPIRE extragalactic survey fields; notably the Herschel Multi-tiered Extragalactic Survey (HerMES) and the Herschel Atlas survey (H-ATLAS). Here, we describe the motivation and principal elements in the design of the project. Guiding principles are transparent or "open" methodologies with care for reproducibility and identification of provenance. A key element of the design focuses around the homogenisation of calibration, meta data and the provision of information required to define the selection of the data for statistical analysis. We apply probabilistic methods that extract information directly from the images at long wavelengths, exploiting the prior information available at shorter wavelengths and providing full posterior distributions rather than maximum likelihood estimates and associated uncertainties as in traditional catalogues. With this project definition paper we provide full access to the first data release of HELP; Data Release 1 (DR1), including a monolithic map of the largest SPIRE extragalactic field at 385 deg2 and 18 million measurements of PACS and SPIRE fluxes. We also provide tools to access and analyse the full HELP database. This new data set includes far-infrared photometry, photometric redshifts, and derived physical properties estimated from modelling the spectral energy distributions.

Recently detected coherent low-frequency radio emission from M dwarf systems shares phenomenological similarities with emission produced by magnetospheric processes from the gas giant planets of our Solar System. Such beamed electron-cyclotron maser emission can be driven by a star-planet interaction or a breakdown in co-rotation between a rotating plasma disk and a stellar magnetosphere. Both models suggest that the radio emission could be periodic. Here we present the longest low-frequency interferometric monitoring campaign of an M dwarf system, composed of twenty-one $\approx$8 hour epochs taken in two series of observing blocks separated by a year. We achieved a total on-source time of 6.5 days. We show that the M dwarf binary CR Draconis has a low-frequency 3$\sigma$ detection rate of 90$^{+5}_{-8}$% when a noise floor of $\approx$0.1 mJy is reached, with a median flux density of 0.92 mJy, consistent circularly polarised handedness, and a median circularly polarised fraction of 66%. We resolve three bright radio bursts in dynamic spectra, revealing the brightest is elliptically polarised, confined to 4 MHz of bandwidth centred on 170 MHz, and reaches a flux density of 205 mJy. The burst structure is mottled, indicating it consists of unresolved sub-bursts. Such a structure shares a striking resemblance with the low-frequency emission from Jupiter. We suggest the near-constant detection of high brightness temperature, highly-circularly-polarised radiation that has a consistent circular polarisation handedness implies the emission is produced via the electron-cyclotron maser instability. Optical photometric data reveal the system has a rotation period of 1.984$\pm$0.003 days. We observe no periodicity in the radio data, but the sampling of our radio observations produces a window function that would hide the near two-day signal.

High-redshift Lyman-alpha blobs (LABs) are an enigmatic class of objects that have been the subject of numerous observational and theoretical investigations. It is of particular interest to determine the dominant power sources for the copious luminosity, as direct emission from HII regions, cooling gas, and fluorescence due to the presence of active galactic nuclei (AGN) can all contribute significantly. In this paper, we present the first theoretical model to consider all of these physical processes in an attempt to develop an evolutionary model for the origin of high-z LABs. This is achieved by combining a series of high-resolution cosmological zoom-in simulations with ionization and Lyman-alpha (Lya) radiative transfer models. We find that massive galaxies display a range of Lya luminosities and spatial extents (which strongly depend on the limiting surface brightness used) over the course of their lives, though regularly exhibit luminosities and sizes consistent with observed LABs. The model LABs are typically powered from a combination of recombination in star-forming galaxies, as well as cooling emission from gas associated with accretion. When AGN are included in the model, the fluorescence caused by AGN-driven ionization can be a significant contributor to the total Lya luminosity as well. We propose that the presence of an AGN may be predicted from the Gini coefficient of the blob's surface brightness. Within our modeled mass range, there are no obvious threshold physical properties that predict appearance of LABs, and only weak correlations of the luminosity with the physical properties of the host galaxy. This is because the emergent Lya luminosity from a system is a complex function of the gas temperature, ionization state, and Lya escape fraction.

We present the Extreme-Horizon (EH) cosmological simulation: EH models galaxy formation with stellar and AGN feedback and uses a very high resolution in the intergalactic and circumgalactic medium. The high resolution in low-density regions results in smaller-size massive galaxies at redshift $z=2$, in better agreement with observations compared to other simulations. This results from the improved modeling of cold gas flows accreting onto galaxies. Besides, the EH simulation forms a population of particularly compact galaxies with stellar masses of $10^{10-11}$\,M$_\sun$ that are reminiscent of observed ultracompact galaxies at $z\simeq2$. These objects form mainly through repeated major mergers of low-mass progenitors, independently of baryonic feedback mechanisms. This formation process can be missed in simulations using a too low resolution in low-density intergalactic regions.

The wide-area component of the LOFAR Two-Metre Sky Survey (LoTSS) is currently the largest radio survey ever carried out, and a large fraction of the 4.5 million radio sources it contains have been optically identified with galaxies or quasars with spectroscopic or photometric redshifts. Identification of radio-luminous AGN from this LoTSS source catalogue is not only important from the point of view of understanding the accretion history of the universe, but also enables a wide range of other science. However, at present the vast majority of the optical identifications lack spectroscopic information or well-sampled spectral energy distributions. We show that colour and absolute magnitude information from the Wide-Field Infrared Survey Explorer (WISE) allows for the robust and efficient selection of radio AGN candidates, generating a radio AGN candidate sample of around 600,000 objects with flux density $> 1.1$ mJy, spanning 144-MHz luminosities between $10^{21}$ and $10^{29}$ W Hz$^{-1}$. We use the catalogue to constrain the total sky density of radio-luminous AGN and the evolution of their luminosity function between $z=0$ and $z\approx 1$, and show that the typical mass of their host galaxies, around $10^{11} M_\odot$, is essentially independent of radio luminosity above around $L_{144} \approx 10^{24}$ W Hz$^{-1}$. Combining with Very Large Array Sky Survey (VLASS) data, we show that the core prominences, radio spectral indices and variability of extended sources from the sample are qualitatively consistent with the expectations from unified models. A catalogue of the radio AGN candidates is released with this paper.

Galaxy cluster Abell 3827 hosts the stellar remnants of four almost equally bright elliptical galaxies within a core of radius 10kpc. Such corrugation of the stellar distribution is very rare, and suggests recent formation by several simultaneous mergers. We map the distribution of associated dark matter, using new Hubble Space Telescope imaging and VLT/MUSE integral field spectroscopy of a gravitationally lensed system threaded through the cluster core. We find that each of the central galaxies retains a dark matter halo, but that (at least) one of these is spatially offset from its stars. The best-constrained offset is 1.62+/-0.48kpc, where the 68% confidence limit includes both statistical error and systematic biases in mass modelling. Such offsets are not seen in field galaxies, but are predicted during the long infall to a cluster, if dark matter self-interactions generate an extra drag force. With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments - but if interpreted solely as evidence for self-interacting dark matter, this offset implies a cross-section sigma/m=(1.7+/-0.7)x10^{-4}cm^2/g x (t/10^9yrs)^{-2}, where t is the infall duration.

We present Spitzer IRAC (3.6, 4.5, 5.8, 8.0 um) and MIPS (24 um) observations of the SCUBA submillimeter sources and uJy VLA radio sources in a 5'x5' area in the Lockman Hole East region. Out of the ~40 SCUBA/VLA sources in the field, Spitzer counterparts were detected for nearly all except for the few low-weight SCUBA detections. We show that the majority (80-90%) of the detected sources are cold (i.e., starburst-like) infrared-luminous galaxies (L_{IR} > 10^{11} Lsun) at redshift 0.5

We present a novel method to identify candidate high redshift quasars (HzQs; ($z\gtrsim5.5$), which are unique probes of supermassive black hole growth in the early Universe, from large area optical/infrared photometric surveys. Using Gaussian Mixture Models to construct likelihoods and incorporate informed priors based on population statistics, our method uses a Bayesian framework to assign posterior probabilities that differentiate between HzQs and contaminating sources. We additionally include deep radio data to obtain informed priors. Using existing HzQ data in the literature, we set a posterior threshold that accepts ${\sim}90\%$ of known HzQs while rejecting $>99\%$ of contaminants such as dwarf stars or lower redshift galaxies. Running the probability selection on test samples of simulated HzQs and contaminants, we find that the efficacy of the probability method is higher than traditional colour cuts, decreasing the fraction of accepted contaminants by 86% while retaining a similar fraction of HzQs. As a test, we apply our method to the Pan-STARRS Data Release 1 (PS1) source catalogue within the HETDEX Spring field area on the sky, covering 400 sq. deg. and coinciding with deep radio data from the LOFAR Two-metre Sky Survey Data Release 1 (LoTSS DR1). From an initial sample of ${\sim}5\times10^5$ sources in PS1, our selection shortlists 251 candidate HzQs, which are further reduced to 63 after visual inspection. Shallow spectroscopic follow-up of 13 high probability HzQs resulted in the confirmation of a previously undiscovered quasar at $z=5.66$ with photometric colours $i-z = 1.4$, lying outside the typically probed regions when selecting HzQs based on colours. This discovery demonstrates the efficacy of our probabilistic HzQ selection method in selecting more complete HzQ samples, which holds promise when employed on large existing and upcoming photometric data sets.

It has been proposed that the cosmological constant $\Lambda$ might be measured from geometric effects on large-scale structure. A positive vacuum density leads to correlation-function contours which are squashed in the radial direction when calculated assuming a matter-dominated model. We show that this effect will be somewhat harder to detect than previous calculations have suggested: the squashing factor is likely to be &lt;1.3, given realistic constraints on the matter contribution to $\Omega$. Moreover, the geometrical distortion risks being confused with the redshift-space distortions caused by the peculiar velocities associated with the growth of galaxy clustering. These depend on the density and bias parameters via the combination $\beta\equiv \Omega^{0.6}/b$, and we show that the main practical effect of a geometrical flattening factor $F$ is to simulate gravitational instability with $\beta_{\rm eff}\simeq 0.5(F-1)$. Nevertheless, with datasets of sufficient size it is possible to distinguish the two effects; we discuss in detail how this should be done. New-generation redshift surveys of galaxies and quasars are potentially capable of detecting a non-zero vacuum density, if it exists at a cosmologically interesting level.

Stellar ejecta gradually enrich the gas out of which subsequent stars form, making the least chemically enriched stellar systems direct fossils of structures formed in the early universe. Although a few hundred stars with metal content below one thousandth of the solar iron content are known in the Galaxy, none of them inhabit globular clusters, some of the oldest known stellar structures. These show metal content of at least ~0.2 percent of the solar metallicity ([Fe/H] > -2.7). This metallicity floor appears universal and it has been proposed that proto-galaxies that merge into the galaxies we observe today were simply not massive enough to form clusters that survived to the present day. Here, we report the discovery of a stellar stream, C-19, whose metallicity is less than 0.05 per cent the solar metallicity ([Fe/H]=-3.38 +/- 0.06 (stat.) +/- 0.20 (syst.)). The low metallicity dispersion and the chemical abundances of the C-19 stars show that this stream is the tidal remnant of the most metal-poor globular cluster ever discovered, and significantly below the purported metallicity floor: clusters with significantly lower metallicities than observed today existed in the past and contributed their stars to the Milky Way halo.

Forthcoming cosmic shear surveys will make precise measurements of the matter density field down to very small scales, scales which are dominated by baryon feedback. The modelling of baryon feedback is crucial to ensure unbiased cosmological parameter constraints; the most efficient approach is to use analytic models, but these are limited by how well they can capture the physics of baryon feedback. We investigate the fitting and residual errors of various baryon feedback models to a suite of hydrodynamic simulations, and propagate these to cosmological parameter constraints for cosmic shear. We present an alternative formalism to binary scale-cuts through the use of a theoretical error covariance, which is a well-motivated alternative using errors in the power spectrum modelling itself. We depart from previous works by modelling baryonic feedback errors directly in the matter power spectrum, which is the natural basis to do so and thus preserves information in the lensing kernels. When including angular multipoles up to $\ell_{\mathrm{max}} = 5000$, and assuming Euclid-like survey properties, we find that even multi-parameter models of baryon feedback can introduce significant levels of bias. In contrast, our theoretical error reduces the bias in $\Omega_{\mathrm{m}}$ and $S_{8}$ to acceptable levels, with only a modest increase in parameter variances. The theoretical error approach bypasses the need to directly determine the per-bin $\ell_{\mathrm{max}}$ values, as it naturally suppresses the biassing small-scale information. We also present a detailed study of how flexible HMCode-2020, a widely-used non-linear and baryonic feedback model, is at fitting a range of hydrodynamical simulations.