Context. The evolution of galaxies is shaped by both internal processes and their external environments. Galaxy clusters and their surroundings provide ideal laboratories to study these effects, particularly with respect to mechanisms such as quenching and morphological transformation. The Chilean Cluster galaxy Evolution Survey (CHANCES) Low-z sub-survey is part of the CHileAN Cluster galaxy Evolution Survey, a 4MOST community survey designed to uncover the relationship between the formation and evolution of galaxies and hierarchical structure formation as it happens, through deep and wide multi-object spectroscopy. Aims. We present the target selection strategy followed to select galaxy cluster candidate members for the CHANCES Low-z sub-survey. The selection was focussed in and around 50 clusters and two superclusters at z < 0.07, out to (5 × R200) and down to mr = 20.4. Methods. Combining public photometric redshift estimates from the DESI Legacy Imaging Survey and T80S/S-PLUS iDR5 with custom photometric redshifts, we were able to identify likely galaxy cluster candidate members, whose photometric redshifts were consistent with being located at the known redshift of the cluster. We measured the average deviations of their photometric redshifts with respect to the spectroscopic redshift measurements, σNMAD. We tested various selection parameters to maximise completeness, while maintaining purity. Results. We successfully compiled our CHANCES-low-redshift catalogues, split into three different sub-surveys: Low-z bright (mr < 18.5), Low-z faint (18.5 ≤ mr < 20.4), and Low-z faint supplementary. To do so, we selected ≳500 000 galaxy cluster candidate members and included confirmed spectroscopic galaxy cluster members, from which we expect to obtain 4MOST low-resolution (R ∼ 6500) spectra for ∼320 000 galaxies. Conclusions. The CHANCES Low-z target catalogues form a statistically robust sample for spectroscopic follow-up, enabling studies of galaxy evolution and environmental effects in nearby cluster and supercluster environments.

We investigate how different galaxy formation models impact the stellar properties of brightest group galaxies (BGGs) in four cosmological simulations: R omulus , S imba , S imba -C, and O bsidian . The stellar masses, specific star formation rates, and mass-weighted stellar ages of the simulated BGGs are analyzed alongside those of observed BGGs from X-ray-selected galaxy groups in the Cosmic Evolution Survey (COSMOS) field. We find that the global properties and underlying evolutionary pathways of simulated BGG populations are strongly impacted by the strength and mechanism of their respective active galactic nucleus (AGN) feedback models, which play a critical role in regulating the growth of massive galaxies. O bsidian ’s sophisticated three-regime AGN feedback model achieves the highest overall agreement with COSMOS observations, matching stellar property distributions, quenched fractions, and the evolution of star formation in increasingly massive systems. We find evidence suggesting that BGG populations of O bsidian and COSMOS undergo a gradual decline in star formation with stellar mass, in contrast to S imba and S imba -C, which display rapid quenching linked to the onset of powerful AGN jet feedback. By comparison, R omulus produces highly star-forming, under-quenched BGGs due to the inefficiency of its thermal AGN feedback in preventing cooling flows from fuelling BGG growth. The success of the O bsidian simulation demonstrates the importance of physically motivated subgrid prescriptions for realistically capturing the processes that shape BGGs and their dynamic group environments.

Context. Galaxy clusters grow through the accretion of galaxies from groups, filaments, and other clusters. During this process, galaxies may undergo pre-processing in lower density environments, where galaxy-galaxy mergers and other interactions can significantly alter their properties prior to cluster infall. Aims. We investigate the role of galaxy mergers in the pre-processing of galaxies prior to cluster infall by studying the spatial distribution of galaxy mergers across the cosmic web. Methods. We used a sample of 43 922 galaxies being targeted by the 4MOST CHANCES survey in and around 33 low-redshift clusters (z < 0.07). Using Zoobot, a deep-learning framework trained on Galaxy Zoo data, we identified 698 galaxy mergers. We measured their distances to cosmic web filaments and compared those results to those of non-merging galaxies. Results. We find that galaxy mergers are significantly closer to filaments than the non-merging galaxy population, with this trend being strongest beyond the cluster virial radius. This suggests that filaments provide conditions conducive to mergers, possibly moderating relative velocities and enhancing gas availability. Conclusions. Our findings support a scenario in which filaments play a key role in transforming (pre-processing) galaxies by promoting mergers before entering the cluster cores where star formation is quenched.

Aims. We investigate the redshift evolution and group richness dependence of the quiescent galaxy fraction and red sequence (RS) parameters in COSMOS galaxy groups, spanning a wide redshift range, from z = 0 to z = 3.7. Methods. We analyzed the deep and well-characterized sample of groups recently detected with the AMICO algorithm in the COSMOS(-Web) field. Our study of the quiescent galaxy population is based on a machine-learning classification tool based on rest-frame magnitudes. The algorithm learns from several traditional methods to estimate the probability of a galaxy being quiescent, achieving high precision and recall. Starting from this classification, we computed quiescent galaxy fractions within groups via two methods: one based on the membership probabilities provided by AMICO, which rely on an analytical model, and another using a model-independent technique. We then detected the RS by estimating the ridgeline position using probability-weighted photometric data, followed by σ clipping to remove outliers. This analysis was performed using both rest-frame magnitudes and observer-frame magnitudes with rest-frame matching. We compared the results from both approaches and investigated the redshift and richness dependence of the RS parameters. Results. We found that the quiescent galaxy population in groups builds up steadily from z = 1.5 − 2 across all richnesses, with faster and earlier growth in the richest groups. The first galaxies settle onto the RS ridgeline by z ∼ 2, consistent with current evolutionary scenarios. Notably, we reported a rare protocluster core hosting quiescent galaxies at z = 3.4, potentially one of the most distant early RSs observed. Extending our study to X-ray properties, we found that X-ray faint groups have, on average, lower quiescent fractions than X-ray bright ones, likely reflecting their typical location in filaments where pre-processing is lower. Leveraging the broad wavelength coverage of COSMOS2025, we traced RS evolution using observed and rest-frame colors over ∼12 Gyr, finding no significant trends in either the slope or the scatter of the ridgeline.

Context. The evolution of galaxies is shaped by both internal processes and their external environments. Galaxy clusters and their surroundings provide ideal laboratories to study these effects, particularly with respect to mechanisms such as quenching and morphological transformation. The Chilean Cluster galaxy Evolution Survey (CHANCES) Low-z sub-survey is part of the CHileAN Cluster galaxy Evolution Survey, a 4MOST community survey designed to uncover the relationship between the formation and evolution of galaxies and hierarchical structure formation as it happens, through deep and wide multi-object spectroscopy. Aims. We present the target selection strategy followed to select galaxy cluster candidate members for the CHANCES Low-z sub-survey. The selection was focussed in and around 50 clusters and two superclusters at z < 0.07, out to (5 × R200) and down to mr = 20.4. Methods. Combining public photometric redshift estimates from the DESI Legacy Imaging Survey and T80S/S-PLUS iDR5 with custom photometric redshifts, we were able to identify likely galaxy cluster candidate members, whose photometric redshifts were consistent with being located at the known redshift of the cluster. We measured the average deviations of their photometric redshifts with respect to the spectroscopic redshift measurements, σNMAD. We tested various selection parameters to maximise completeness, while maintaining purity. Results. We successfully compiled our CHANCES-low-redshift catalogues, split into three different sub-surveys: Low-z bright (mr < 18.5), Low-z faint (18.5 ≤ mr < 20.4), and Low-z faint supplementary. To do so, we selected ≳500 000 galaxy cluster candidate members and included confirmed spectroscopic galaxy cluster members, from which we expect to obtain 4MOST low-resolution (R ∼ 6500) spectra for ∼320 000 galaxies. Conclusions. The CHANCES Low-z target catalogues form a statistically robust sample for spectroscopic follow-up, enabling studies of galaxy evolution and environmental effects in nearby cluster and supercluster environments.

We present the Flagship galaxy mock, a simulated catalogue of billions of galaxies designed to support the scientific exploitation of the Euclid mission. Euclid is a medium-class mission of the European Space Agency optimised to determine the properties of dark matter and dark energy on the largest scales of the Universe. It probes structure formation over more than 10 billion years primarily from the combination of weak gravitational lensing and galaxy clustering data. The breath of Euclid's data will also foster a wide variety of scientific analyses. The Flagship simulation was developed to provide a realistic approximation to the galaxies that will be observed by Euclid and used in its scientific analyses. We ran a state-of-the-art N-body simulation with four trillion particles, producing a lightcone on the fly. From the dark matter particles, we produced a catalogue of 16 billion haloes in one octant of the sky in the lightcone up to redshift z=3. We then populated these haloes with mock galaxies using a halo occupation distribution and abundance matching approach, calibrating the free parameters of the galaxy mock against observed correlations and other basic galaxy properties. Modelled galaxy properties include luminosity and flux in several bands, redshifts, positions and velocities, spectral energy distributions, shapes and sizes, stellar masses, star formation rates, metallicities, emission line fluxes, and lensing properties. We selected a final sample of 3.4 billion galaxies with a magnitude cut of H_E<26, where we are complete. We have performed a comprehensive set of validation tests to check the similarity to observational data and theoretical models. In particular, our catalogue is able to closely reproduce the main characteristics of the weak lensing and galaxy clustering samples to be used in the mission's main cosmological analysis. (abridged)

Context. In a hierarchical Lambda CDM Universe, cosmic filaments serve as the primary channels for matter accretion into galaxy clusters, influencing the shape of their dark matter halos. Aims. We investigate whether the elongation of galaxy clusters correlates with the orientation of surrounding filaments, providing the first observational test of this relationship in large supercluster regions. Methods. We identified and characterized cosmic filaments in two dimensions within the two superclusters that are part of the low-redshift sub-survey of the Chilean Cluster Galaxy Evolution Survey (CHANCES): the Shapley supercluster and the Horologium-Reticulum supercluster. We analyzed the alignment between filament directions-traced by galaxy distributions-and the triaxiality of cluster gravitational potentials-traced by X-ray emission-using publicly available optical and X-ray data. Results We have found that most (82%) of the X-ray clusters are associated with and interconnected by the optically detected filaments. The clusters-filaments alignment analysis shows that the elongation of most clusters is well aligned with nearby filaments, providing observational confirmation of theoretical predictions, with the alignment progressively reducing at larger cluster-centric distances (> 1.6r(200)). Conclusions. Overall, our results support the notion that filaments are the main source of galaxy accretion at redshift below 0.1, and additionally provide evidence that matter accretion through filaments shapes the gravitational potential of galaxy clusters. We propose this measurement as a simple observational proxy to determine the direction of accretion in clusters, which is key to understanding both galaxy evolution and the merger history of galaxy clusters.

Massive protoclusters at z similar to 1.5 - 4, the peak of the cosmic star formation history, are key to understanding the formation mechanisms of massive galaxies in today's clusters. However, studies of protoclusters at these high redshifts remain limited, primarily due to small sample sizes and heterogeneous selection criteria. For this work, we conducted a systematic investigation of the star formation and cold gas properties of member galaxies of eight massive protoclusters in the COSMOS field, using the statistical and homogeneously selected sample from the Noema formIng Cluster survEy (NICE). Our analysis reveals a steep increase in the star formation rates per halo mass (Sigma(SFR)/M-halo) with redshifts in these intensively star-forming protoclusters, reaching values one to two orders of magnitude higher than those observed in the field at z > 2. We further show that instead of an enhancement of starbursts, this increase is largely driven by the concentration of massive and gas-rich star-forming galaxies in the protocluster cores. The member galaxies still generally follow the same star-forming main sequence as in the field, with a moderate enhancement at the low-mass end. Notably, the most massive protocluster galaxies (M-star > 8x10(10) M-circle dot) exhibit higher mu(gas) and tau(gas) than their field counterparts, while remaining on the star-forming main sequence. These gas-rich, massive, and star-forming galaxies are predominantly concentrated in the protocluster cores and are likely progenitors of massive ellipticals in the center of today's clusters. These results suggest that the formation of massive galaxies in such environments is sustained by substantial gas reservoirs, which in turn support persistent star formation and drive early mass assembly in forming cluster cores.

[Abridged] To understand the role of the galaxy group environment on galaxy evolution, we present a study of radio luminosity functions (RLFs) of group galaxies based on the VLA-COSMOS 3 GHz Large Project. The radio-selected sample of 7826 COSMOS galaxies with robust optical/near-infrared counterparts, excellent photometric coverage, and the COSMOS X-ray galaxy groups (M_200c > 10^13.5 M_sun) enables us to construct the RLF of group galaxies (GGs) and their contribution to the total RLF since z 2.3. Using the Markov chain Monte Carlo algorithm, we fit a redshift-dependent pure luminosity evolution model and a linear and power-law model to the luminosity functions. We compare it with past RLF studies from VLA-COSMOS on individual populations of radio-selected star-forming galaxies (SFGs) and galaxies hosting active galactic nuclei (AGN). These populations are classified based on the presence or absence of a radio excess concerning the star-formation rates derived from the infrared emission. We find that the density of radio galaxies in groups is low compared to the field at z 2 down to z 1.25, followed by a sharp increase at z 1 by a factor of 6, and then a smooth decline towards low redshifts. This trend is caused by both decrease in the volume abundance of massive groups at high-z and the changes in the halo occupation of radio AGN, which are found by other studies to reside at smaller halo mass groups. This indicates that the bulk of high-z log10(M_200c/M_sun) > 13.5 groups must have been forming recently, and so the cooling has not been established as yet. The slope of the GG RLF is flatter compared to the field, with excess at high radio luminosities. The evolution in the GG RLF is driven mainly by satellite galaxies in groups. A drop in occurrence of AGN in groups at z > 1 by a factor of 6, manifests an important detail on the processes governing galaxy evolution.

Aims. We analyze the clustering of galaxy clusters in a large contiguous sample, the Constrain Dark Energy with X-ray (CODEX) sample. We construct a likelihood for cosmological parameters by comparing the measured clustering signal and a theoretical prediction, and use this to obtain parameter constraints. Methods. We measured the three multipole moments (monopole, quadrupole, and hexadecapole, & ell;=0,2,4) of the power spectrum of a subset of the CODEX clusters. To fully model cluster clustering, we also determined the expected clustering bias of the sample using estimates for the cluster masses and a mass-to-bias model calibrated using N-body simulations. We estimated the covariance matrix of the measured power spectrum multipoles using a set of simulated dark-matter halo catalogs. Combining all these ingredients, we performed a Markov chain Monte Carlo sampling of cosmological parameters Omega(m) and sigma(8 )to obtain their posterior. Results. We found the CODEX clustering signal to be consistent with an earlier X-ray selected cluster sample, the REFLEX II sample. We also found that the measured power spectrum multipoles are compatible with the predicted, bias-scaled linear matter power spectrum when the cosmological parameters determined by the Planck satellite are assumed. Furthermore, we found the marginalized parameter constraints of Omega(m )= 0.24(-0.04)(+0.06) and sigma(8 )= 1.13(-0.24)(+0.43). The full 2D posterior is consistent, for example, with the Planck cosmology within the 68% confidence region.

We study the constraint on f(R) gravity that can be obtained by photometric primary probes of the Euclid mission. Our focus is the dependence of the constraint on the theoretical modelling of the nonlinear matter power spectrum. In the Hu-Sawicki f(R) gravity model, we consider four different predictions for the ratio between the power spectrum in f(R) and that in Lambda cold dark matter (Lambda CDM): a fitting formula, the halo model reaction approach, ReACT, and two emulators based on dark matter only N-body simulations, FORGE and e-Mantis. These predictions are added to the MontePython implementation to predict the angular power spectra for weak lensing (WL), photometric galaxy clustering, and their cross-correlation. By running Markov chain Monte Carlo, we compare constraints on parameters and investigate the bias of the recovered f(R) parameter if the data are created by a different model. For the pessimistic setting of WL, one-dimensional bias for the f(R) parameter, log10|fR0|, is found to be 0.5 sigma when FORGE is used to create the synthetic data with log10|fR0| = -5.301 and fitted by e-Mantis. The impact of baryonic physics on WL is studied by using a baryonification emulator, BCemu. For the optimistic setting, the f(R) parameter and two main baryonic parameters are well constrained despite the degeneracies among these parameters. However, the difference in the nonlinear dark matter prediction can be compensated for the adjustment of baryonic parameters, and the one-dimensional marginalised constraint on log10|fR0| is biased. This bias can be avoided in the pessimistic setting at the expense of weaker constraints. For the pessimistic setting, using the Lambda CDM synthetic data for WL, we obtain the prior-independent upper limit of log10|fR0| < -5.6. Finally, we implement a method to include theoretical errors to avoid the bias due to inaccuracies in the nonlinear matter power spectrum prediction.

Galaxy groups with M_tot≲ 10^14 M_⊙ and up to a few tens of members are the most common galaxy environment, marking the transition between field and massive clusters. Identifying groups plays a crucial role in understanding structure formation and galaxy evolution. Modern deep surveys allow us to build well-characterized samples of groups up to the regime where structures were taking shape. We aimed to build the largest deep catalog of galaxy groups to date over the COSMOS-Web field effective area of 0.45 deg^2, leveraging the deep high quality data of the new COSMOS-Web photometric catalog resulted from the James Webb Space Telescope observations of the COSMOS-Web field. We performed the group search with the AMICO algorithm, a linear matched filter based on an analytical model for the group signal. AMICO has already been tested in wide and deep field surveys, including COSMOS data up to z=2. In this work, we tested the algorithm performances at even higher redshift and searched for protocluster cores at z>2. We compiled a list of known protoclusters in COSMOS at 2 ≤ z ≤ 3.7, matched them with our detections and studied the clustering of the detected cores. We estimated purity and completeness of our sample by creating data-driven mocks with the SinFoniA code and linked signal-to-noise to purity. We detected 1678 groups in the COSMOS-Web field up to z=3.7, including lists of members extending nearly two magnitudes deeper than the previous AMICO-COSMOS catalog. 756 groups were detected with purity of 80%. More than 500 groups have their redshift confirmed by assigning spectroscopic counterparts. This group catalog offers a unique opportunity to explore galaxy evolution in different environments spanning ∼12 Gyr and to study groups, from the least rich population to the formation of the most massive clusters.

Galaxy clusters are crucial to understanding the role of the environment in galaxy evolution. However, due to their rarity, only a limited number of clusters have been identified at z greater than or similar to 2. In this paper, we report a discovery of seven cluster candidates with massive quiescent galaxies at z similar to 2 in the 3.5 deg(2) area of the XMM Large Scale Structure field, roughly doubling the known cluster sample at this frontier redshift if confirmed. We construct a photometric redshift catalog based on deep (i similar to 26, K-s similar to 24) multiwavelength photometry from the u* band to the K band gathered from the Hyper Suprime-Cam Subaru Strategic Program and other collaborative/public surveys. We adopt a Gaussian kernel density estimate with two different spatial scales (10'' and 60'') to draw a density map of massive (log(M & lowast;/M circle dot)>10.5) and quiescent (log(sSFR[yr-1])<-10) galaxies at z similar to 2. Then, we identify seven prominent overdensities. These candidates show clear red sequences in color-magnitude diagrams (z - H versus H). Moreover, one of them shows an extended X-ray emission with L-X = (1.46 +/- 0.35) x 10(44) erg s(-1), suggesting its virialized nature. There is no clear evidence of enhancement nor suppression of the star formation rate of the main-sequence galaxies in the clusters. We find that cluster galaxies have a higher fraction of transition population with -10.5

Context. Galaxy groups with total masses below similar to 10(14 )M(circle dot) and up to a few tens of members are the most common galaxy environment, marking the transition between the field and the most massive galaxy clusters. In this framework, identifying and studying groups plays a crucial role in understanding structure formation and galaxy evolution. Despite the challenges in detecting such relatively small structures, modern deep surveys allow us to build well-characterized samples of galaxy groups up to the regime where the structures we observe today were taking shape. Aims. We aim to build the largest deep catalog of galaxy groups to date over the COSMOS-Web field effective area of 0.45 deg(2). Methods. We leveraged the deep imaging, high resolution, and high-quality photometry from the James Webb Space Telescope observations of the COSMOS-Web field. We used the recent COSMOS-Web photometric catalog with sky position, photometric redshift, and magnitude in a reference band for each selected galaxy. We performed the group search with the Adaptive Matched Identifier of Clustered Objects (AMICO) algorithm, a linear matched filter based on an analytical model for the cluster/group signal. This algorithm has already been tested in wide and deep field surveys, including a successful application to COSMOS data up to z = 2. In this work, we tested the algorithm's performances at even higher redshift and searched for protocluster cores and groups at z > 2. To benchmark this relatively unexplored regime, we compiled a list of known protoclusters in COSMOS at 2 <= z <= 3.7 and matched them with our detections. We studied the spatial connection between detected cores through a clustering analysis. We estimated the purity and the completeness of our group sample by creating data-driven mocks via a Monte Carlo approach with the SinFoniA code and linked signal-to-noise to purity levels to define desired purity thresholds. Results. We detected 1678 groups in the COSMOS-Web field up to z = 3.7 with a purity level of similar to 77%, providing a deep catalog of galaxy members that extends nearly two magnitudes deeper than the previous application of AMICO to COSMOS. Around 670 groups have been detected with a purity of 90%. Our catalog includes more than 850 groups whose photometric redshift was confirmed by assigning robust spectroscopic counterparts. Conclusions. This catalog of galaxy groups is the largest ultra-deep group sample built on JWST observations so far and offers a unique opportunity to explore several aspects of galaxy evolution in different environments spanning similar to 12 Gyr and study groups themselves, from the least rich population of groups to the formation of the most massive clusters.

The Near-Infrared Spectrometer and Photometer (NISP) on board the satellite provides multiband photometry and $R slitless grism spectroscopy in the 950--2020\,nm wavelength range. In this reference article, we illuminate the background of NISP's functional and calibration requirements, describe the instrument's integral components, and provide all its key properties. We also sketch the processes needed to understand how NISP operates and is calibrated as well as its technical potentials and limitations. Links to articles providing more details and the technical background are included. The NISP's 16 H2RG detectors with a plate scale of $ deliver a field of view of 0.57\,deg$^2$. In photometric mode, NISP reaches a limiting magnitude of sim \,24.5\,AB\,mag in three photometric exposures of about 100\,s in exposure time for point sources and with a S/N of five. For spectroscopy, NISP's point-source sensitivity is a SNR = 3.5 detection of an emission line with flux sim \,$2 $ integrated over two resolution elements of 13.4\ in 3times 560\,s grism exposures at 1.6\ (redshifted Halpha ). Our calibration includes on-ground and in-flight characterisation and monitoring of the pixel-based detector baseline, dark current, non-linearity, and sensitivity to guarantee a relative photometric accuracy better than 1.5 and a relative spectrophotometry better than 0.7. The wavelength calibration must be accurate to 5\ or better. The NISP is the state-of-the-art instrument in the near-infrared for all science beyond small areas available from HST and JWST -- and it represents an enormous advance from any existing instrumentation due to its combination of field size and high throughput of telescope and instrument. During six-year survey covering 14\,000\,deg$^2$ of extragalactic sky, NISP will be the backbone in determining distances of more than a billion galaxies. Its near-infrared data will become a rich reference imaging and spectroscopy data set for the coming decades.

The near-infrared calibration unit (NI-CU) on board NISP is the first astronomical calibration lamp based on LED to be operated in space. is a mission in ESA's Cosmic Vision 2015--2025 framework to explore the dark universe and provide a next-level characterisation of the nature of gravitation, dark matter, and dark energy. Calibrating photometric and spectrometric measurements of galaxies to better than 1.5 accuracy in a survey homogeneously mapping sim \,14\,000\,deg$^2$ of extragalactic sky requires a very detailed characterisation of NIR detector properties as well as constant monitoring of them in flight. To cover two of the main contributions -- relative pixel-to-pixel sensitivity and non-linearity characteristics -- and to support other calibration activities, NI-CU was designed to provide spatially approximately homogeneous ($<$\,12 variations) and temporally stable illumination (0.1--0.2 over 1200\,s) over the NISP detector plane with minimal power consumption and energy dissipation. NI-CU covers the spectral range sim \,nm -- at cryo-operating temperature -- at five fixed independent wavelengths to capture wavelength-dependent behaviour of the detectors, with fluence over a dynamic range of gtrsim \,100 from sim $. For this functionality, NI-CU is based on LED . We describe the rationale behind the decision and design process, the challenges in sourcing the right LED and the qualification process and lessons learned. We also provide a description of the completed NI-CU, its capabilities, and performance as well as its limits. NI-CU has been integrated into NISP and the satellite, and since launch in July 2023, it has started supporting survey operations.

The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015-2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14,000 deg^2 of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high-level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance.

The Athena mission entered a redefinition phase in July 2022, driven by the imperative to reduce the mission cost at completion for the European Space Agency below an acceptable target, while maintaining the flagship nature of its science return. This notably called for a complete redesign of the X-ray Integral Field Unit (X-IFU) cryogenic architecture towards a simpler active cooling chain. Passive cooling via successive radiative panels at spacecraft level is now used to provide a 50 K thermal environment to an X-IFU owned cryostat. 4.5 K cooling is achieved via a single remote active cryocooler unit, while a multi-stage Adiabatic Demagnetization Refrigerator ensures heat lift down to the 50 mK required by the detectors. Amidst these changes, the core concept of the readout chain remains robust, employing Transition Edge Sensor microcalorimeters and a SQUID-based Time-Division Multiplexing scheme. Noteworthy is the introduction of a slower pixel. This enables an increase in the multiplexing factor (from 34 to 48) without compromising the instrument energy resolution, hence keeping significant system margins to the new 4 eV resolution requirement. This allows reducing the number of channels by more than a factor two, and thus the resource demands on the system, while keeping a 4' field of view (compared to 5' before). In this article, we will give an overview of this new architecture, before detailing its anticipated performances. Finally, we will present the new X-IFU schedule, with its short term focus on demonstration activities towards a mission adoption in early 2027.

Scaling relations between galaxy cluster properties are crucial for understanding cosmology and baryonic physics. Rigorous calibration of the M – L _X relation, employing weak lensing mass and consistent statistical methodology, is challenging due to heterogeneous cluster samples. The release of LEGACY imaging data introduced the possibility of unifying the cluster selection. We present the all-sky extension of the CODEX catalog based on LEGACY data and introduce a Bayesian framework for calibrating the X-ray luminosity–mass relation, derived for 100 clusters with weak lensing mass measurements. Using the X-ray luminosity estimates for those clusters from ROSAT All-Sky Survey (RASS) data, we perform a power-law fit to the M – L _X relation. Furthermore, taking advantage of the recently released eROSITA data (eRASS1), we assess the impact of point source contamination on cluster fluxes for 42 clusters in the eRASS1 footprint. The RASS fit yields a slope of β = 0.75 ± 0.09, 1.7 σ lower than the best self-similar prediction, with marginal evidence for the redshift evolution of the normalization ( γ = 0.65 ± 0.43). As for the eRASS1 analysis, the slope is substantially steeper, β = 1.11 ± 0.15, and in further agreement with the prediction of self-similarity. No additional evolution is also seen ( γ = 0.004 ± 0.790). While our results provide the practical means for cosmological studies of both RASS and eRASS data, the link to cluster physics is much cleaner after the cluster flux contamination is reduced. We also analyzed the impact of the selection function on calibration, finding that its full modeling is essential.

To date, galaxy image simulations for weak lensing surveys usually approximate the light profiles of all galaxies as a single or double Sérsic profile, neglecting the influence of galaxy substructures and morphologies deviating from such a simplified parametric characterisation. While this approximation may be sufficient for previous data sets, the stringent cosmic shear calibration requirements and the high quality of the data in the upcoming survey demand a consideration of the effects that realistic galaxy substructures and irregular shapes have on shear measurement biases. Here we present a novel deep learning-based method to create such simulated galaxies directly from Hubble Space Telescope (HST) data. We first build and validate a convolutional neural network based on the wavelet scattering transform to learn noise-free representations independent of the point-spread function (PSF) of HST galaxy images. These can be injected into simulations of images from optical instrument VIS without introducing noise correlations during PSF convolution or shearing. Then, we demonstrate the generation of new galaxy images by sampling from the model randomly as well as conditionally. In the latter case, we fine-tune the interpolation between latent space vectors of sample galaxies to directly obtain new realistic objects following a specific Sérsic index and half-light radius distribution. Furthermore, we show that the distribution of galaxy structural and morphological parameters of our generative model matches the distribution of the input HST training data, proving the capability of the model to produce realistic shapes. Next, we quantify the cosmic shear bias from complex galaxy shapes in simulations by comparing the shear measurement biases between a sample of model objects and their best-fit double-Sérsic counterparts, thereby creating two separate branches that only differ in the complexity of their shapes. Using the Kaiser, Squires, and Broadhurst shape measurement algorithm, we find a multiplicative bias difference between these branches with realistic morphologies and parametric profiles on the order of $(6.9± 0.6) $ for a realistic magnitude-Sérsic index distribution. Moreover, we find clear detection bias differences between full image scenes simulated with parametric and realistic galaxies, leading to a bias difference of $(4.0± 0.9) $ independent of the shape measurement method. This makes complex morphology relevant for stage IV weak lensing surveys, exceeding the full error budget of the Euclid Wide Survey (Δμ_