Galaxy evolution is profoundly shaped by intricate internal and external mechanisms that regulate the baryon cycle and star formation activity. To characterize the role of these processes as a function of galaxy environment, we present a theoretical framework based on the GAlaxy Evolution and Assembly (GAEA) semi-analytic model. We extracted portions of simulated volumes that include isolated galaxies, pairs, group, and filament members at z ∼ 0, specifically avoiding massive clusters. Galaxies were classified using both intrinsic (halo-based) and observational (2D projected) parameterizations, reconstructing their environmental histories from z = 2 and identifying mergers, tidal interactions, ram pressure stripping (RPS), and starvation. GAEA predictions show that 2D information biases environment definitions, decreasing isolated and group fractions, while doubling pairs. More than half of galaxies remain unaffected by the investigated processes since z = 2. Among the galaxies affected by external mechanisms, mergers dominate at high stellar masses (40−60% at log(M*/M⊙) > 10.5). Tidal interactions are less frequent and their incidence increases with stellar mass (up to 20%). RPS dominates in groups and filaments at intermediate masses (∼50%), while starvation ranges from 20 to 30%. The incidence of the different mechanisms depends strongly on both mass and environment, although their imprints on global properties (e.g., colors, gas fractions, sizes) are often subtle. Quenched fractions rise steadily from isolated galaxies to groups. Distinct evolutionary pathways emerge: at low masses (log(M*/M⊙) < 9.5), galaxies in groups and filaments exhibit a faster mass growth than galaxies in other environments, especially those undergoing starvation, mergers, and (to a lesser extent) RPS. The differences are less significant when moving to higher masses, where no clear dependence on any physical mechanism emerges, despite the fact that at these masses, a clear star formation suppression is evident in mergers and starved galaxies. This theoretical investigation provides essential context for the recently initiated multiwavelength program Mechanisms Affecting Galaxies Nearby and Environmental Trends (MAGNET), introduced here for the first time.

The evolution of galaxies in clusters is driven by their interaction with the environment, which deeply affects and alters the properties of the multi-phase interstellar medium. Here we make use of MeerKAT observations to study the properties of the neutral and nonthermal interstellar medium, traced respectively by the HI 21 cm line and by the radio continuum emission at 1.4 GHz, out to about twice the virial radius of three local (z≃0.04) galaxy clusters, namely IIZW108, A4059 and A3558, with M_200≃2-9×10^14 M_⊙. We assemble a sample of 61 galaxies, including the so-called jellyfish and unwinding galaxies, detected in both HI and radio continuum, and derive their optical properties from IR-to-UV photometry using ancillary data. We find that cluster galaxies in our sample are on average ∼2.7 times more star-forming than galaxies with the same stellar and HI mass in the field, indicating that the HI + radio-continuum selection has intercepted galaxies at the very early stages of their environmental processing when the external pressure has not yet removed the gas, but the resulting fast compression has enhanced both the star formation and the radio continuum luminosity. The study also reveals that galaxies at the initial stage can features a radio luminosity excess due to the old relativistic electrons permeating the interstellar medium. We show that unwinding galaxies are characterized by high stellar and HI masses, arguing that their peculiar morphology may results from their large baryonic mass. Finally, via the stacking analysis of 677 optically-selected cluster members we quantitatively show that cluster galaxies are more HI-poor than in the field, and the deficiency steadily grows approaching the cluster center where galaxies have, on average, a factor ∼3× less HI mass than those in the field.

[ABRIDGED] Galaxy evolution is shaped by internal and external mechanisms that regulate the baryon cycle and star formation activity. We present a theoretical framework based on the GAlaxy Evolution and Assembly (GAEA) semi-analytic model. We extracted portions of simulated volumes that include isolated galaxies, pairs, group, and filament members at z 0, specifically avoiding massive clusters. Galaxies were classified using both intrinsic (halo-based) and observational (2D projected) parameterizations, reconstructing their environmental histories from z = 2 and identifying mergers, tidal interactions, ram pressure stripping (RPS), and starvation. 2D information decreases isolated and group fractions while doubles pairs. More than half of galaxies remain unaffected by the investigated processes since z = 2. Among affected galaxies, mergers dominate at high stellar masses (40-60

This chapter introduces the contributions of the HI galaxy science in this volume reviewing the latest developments and urgent questions in HI galaxy science, providing guiding principles for a layered set of future key science projects. The key science will include: a complete censuses of HI morphologies and kinematics at sub-kpc and 1 km/s resolution within and around galaxies in the nearby Universe; a measurement of the cosmic HI mass density and HI mass function evolution at least up to z 1; an improved understanding of the Universe at z>1, particularly the balance between cold molecular and cool atomic gas. We also provide a view of the synergistic multi-wavelength surveys available in 2028+ in the southern hemisphere. This effort will improve our understanding of the baryon cycle across a significant fraction of the cosmic history, including the processes of gas accretion, consumption and removal as well as AGN and star formation feedback. Based on these science goals, the earlier proposed three-tiered survey strategy remains, but survey parameters and predictions are adjusted according to AA* and AA4 developments. This chapter is an update of the earlier "Advancing Astrophysics with the Square Kilometre Array" chapter 'HI Science with the SKA' by Staveley-Smith Oosterloo.

All gas-rich galaxies in cluster environments are expected to experience ram-pressure stripping from the intracluster medium. However, only a fraction of these develop ongoing star formation in their stripped tail, becoming the so-called jellyfish galaxies. In this work we provide observational evidence that magnetic fields can signal differences in extraplanar star formation, and we explore the physical conditions that lead to the formation of a jellyfish galaxy. We first focus on JO147, a jellyfish galaxy that features weak star formation activity in its tail. Using MeerKAT radio continuum observations, we discovered polarized emission only in a small fraction of its tail, with an average fraction of 10%, and a low Mach number, & Mscr; = 1.3 - 1.6, suggesting a possible association between magnetic field draping, shock compression of the gas, and extraplanar star formation activity. We then tested this scenario in a sample of 17 jellyfish galaxies from the GASP project. We combined dynamical models for their orbits within the host clusters with realistic cluster temperature profiles to infer their Mach number, and we found a positive correlation between it and the star formation activity in their tail. We conclude that supersonic motion is a necessary condition for triggering star formation in the stripped tails of jellyfish galaxies. Our findings provide empirical evidence that the critical factor preventing evaporation of the stripped gas is the shock compression induced by the supersonic motion through the cluster. This process likely enhances the magnetic field surrounding the galaxy and the properties of the stripped material.

Large-scale magnetic fields in galaxy clusters can influence their physics and the evolution of cluster-embedded galaxies. These properties remain poorly constrained due to a historical lack of high-sensitivity and high-resolution spectropolarimetric data. Thanks to the advent of the Square Kilometre Array pathfinders and precursors, this situation is now dramatically changing. Using the densest rotation measure (RM) grid produced to date from broadband spectropolarimetric data within the MeerKAT Fornax Survey (508 sources over 6.35 deg(2); presented in a previous paper), we aim to study the Fornax cluster's magnetic field in detail. We compared the RM grid properties with numerical simulations to constrain the strength and the structure of the intracluster magnetic field. We modelled the magnetic field power spectrum with a power law, and find a slope of 2.7(-0.4)(+0.2), fluctuating between minimum and a maximum scales of 1.01(-0.02)(+0.01) and 15(-2)(+9) kpc, respectively. It has a central strength of 5.0(-0.4)(+0.3) mu G, decreasing with the thermal plasma density according to a power-law exponent eta = 1.6(-0.5)(+0.3), the highest value to date in large-scale systems. By analysing a sample of 17 galaxy clusters and groups with magnetic field estimates from the literature, we observe longer autocorrelation lengths in the case of massive merging clusters and lower values for relaxed clusters and low-mass clusters or galaxy groups. We also observe a systematic increase in the central magnetic field strength as a function of central density, B-0 proportional to n(0)((0.38 +/- 0.14)). We argue that the steepening of the Fornax cluster's magnetic field profile and its relatively high central strength could indicate a recent re-amplification at the centre due to the extended central radio galaxy. The sample analysis supports the proposed scenario; however, more detailed magnetic field studies conducted using consistent modelling on larger samples are needed to better understand magnetisation in clusters and groups.

We present the first observational hints of the severe removal of both molecular and HI gas from the dwarf galaxy VCC 1249. This extreme stripping event is thought to be driven by the combined effects of tidal interaction and ram pressure. Using deep CO (2-1) observations from the James Clerk Maxwell Telescope (JCMT), we obtained marginal CO detections in three regions within the stripped HI tail, with molecular masses of ∼10^5 to 10^6M_⊙, comparable to typical masses of giant molecular clouds. In contrast, we did not find CO emission within the stellar disk of VCC 1249. This indicates the severe removal of cold gas, which likely caused the sudden cessation of star formation in the galaxy. This identifies VCC 1249 as a unique laboratory for witnessing the rapid, environmentally-driven quenching of a dwarf galaxy. Our findings provide a critical observational link between gas removal mechanisms and the dramatic phase transition of cluster dwarfs from star-forming to quiescent systems.

Active Galactic Nuclei (AGN) are key drivers of galaxy evolution, triggered by cold gas accreting onto a super-massive black hole. However, the processes regulating this gas accretion (feeding) and how AGN alter the interstellar medium to affect star formation (feedback) remain poorly understood. A major observational challenge is the vast range of spatial scales involved: AGN fuelling and jet-ejection occur over the sub-pc scales, while AGN feedback shocks and heats the ISM preventing star formation over the galactic and circum-galactic scales. Moreover, it is unclear how short stochastic AGN episodes are connected with the long timescales of gas accretion and star formation. In this manuscript, we illustrate how SKAO will provide the unprecedented opportunity to solve the observational limitations of AGN feeding and feedback studies by observing hundreds of nearby AGN down to low radio powers (10^21 W Hz^-1). Simultaneous SKA-Low and Mid observations of nearby galaxies will trace the thermal emission associated with star formation and AGN feedback and the synchrotron emission of their jets of relativistic plasma. These broad-band radio observations enable the detailed characterisation of the AGN duty-cycle, unravelling the time-scales of the nuclear activities. Reaching in 10 hours neutral atomic hydrogen (HI) column density sensitivities ∼ 10^19 cm^-2 at arcsecond resolution, SKA AA4 observations will trace the typical low column density of HI gas in AGN inflows and outflows, to understand the impact AGN feedback over the full galaxy and trace fuelling processes from the environment onto the SMBH. Combining SKA with mm, sub-mm and optical Integral Field Spectrographic observations at comparable arcsecond resolution will provide an exhaustive understanding of the link between multi-phase AGN feeding and feedback processes and star formation.

Satellite galaxies in clusters are significantly more likely to be red and passive than similar mass galaxies in the field. This fact is known as the environmental quenching of galaxy star formation, which is believed to be driven by ram pressure stripping (RPS). The large velocity differences between the infalling galaxies and the intracluster medium (ICM) result in a strong ram pressure on their interstellar medium (ISM), which can strip it from the stellar disk. The stripped ISM can be studied at various wavelengths, including the radio band, thanks to the synchrotron emission produced by the magnetic fields and relativistic electrons embedded in them. This emission is typically steep-spectrum and thus best observed at low frequencies. Thus, continuum studies of the RPS effect are currently mostly carried out with LOFAR, limiting them to the northern hemisphere. SKA-Low will permit us to extend them to the southern sky, where they will synergize with the southern observatories and the upcoming ELT. Lastly, the sub-arcsecond resolution provided by SKA-Mid will facilitate the exploration of the polarization and filamentary structure of RPS radio tails and allow us to detect them up to z≃0.5, advancing our understanding of the impact of RPS on satellite galaxies in clusters and groups.

As modern astronomy confronts unprecedented data volumes, automated pipelines and machine-learning techniques have become essential for processing and analysis. As these workflows grow more complex, astronomers also require input and inspection tools that can keep pace. To address challenges in navigating multidimensional datasets for quality control and scientific interpretation, we present the immersive Data Visualisation Interactive Explorer (iDaVIE), a virtual reality (VR) software suite developed in collaboration with the astronomy community. iDaVIE enables users to import and render large 3D data cubes within a VR environment, offering real-time tools for selection, cropping, catalogue overlays, and exporting results back into existing pipelines. Built on the Unity engine and SteamVR, the system uses custom plug-ins for efficient data parsing, downsampling, and statistical calculations. The software has already been integrated into workflows such as verifying HI data cubes from MeerKAT, ASKAP, and APERTIF, refining detection masks, and identifying new sources. Its intuitive interface aims to reduce the cognitive load associated with higher-dimensional data, allowing researchers to focus more directly on scientific goals. As an open-source, scalable, and adaptable platform, iDaVIE supports continued development and integration with other tools. Version 1.0 marks a significant milestone, with planned enhancements including subcube loading, advanced rendering modes, video-generation scripts, and collaborative capabilities. By pairing immersive visualisation with robust interaction tools, iDaVIE seeks to transform how researchers engage with complex datasets and enhance productivity in the era of big data.

We present MeerKAT H I observations of ESO 137-001, a quintessential jellyfish galaxy with long multi-phase tails formed due to the interaction with the intra-cluster medium of its host galaxy cluster, ACO 3627. Our observations reveal the presence of H I in both the disc and outer regions of the galaxy for the first time, with a total H I mass of (3.5 +/- 0.4)x108 M circle dot. ESO 137-001 is at an advanced stage of gas stripping; it is extremely H I deficient and seems to have lost 90% of its initial H I mass; about 2/3 of the surviving H I is found at larger radius than expected for a normal H I disc and forms similar to 40 kpc tail coincident with the tail detected at other wavelengths. Only similar to 10% of the surviving H I is still found within the stellar disc, consistent with the expectation of an outside-in truncation due to ram pressure. Similarly to other jellyfish galaxies, ESO137-001 has a high star formation rate for the low amount of H I detected. We measure an H I depletion time of 0.29 Gyr. However, when taking into account the total gas (H I + H2) content, the depletion time is consistent with typical values measured in nearby spiral galaxies. This suggests that ESO 137-001 is at its current stage of ram pressure interaction characterised by an efficient H I stripping, rather than an enhanced conversion of H I to H2, which was recently observed in some other jellyfish galaxies.

Using the Square Kilometre Array (SKA) mid precursor MeerKAT, we acquired broadband spectro-polarimetric data in the context of the MeerKAT Fornax Survey to study the Fornax cluster’s magnetic fields in detail by building the densest rotation measure (RM) grid to date. Here, we present the survey, the analysis, and a discussion of the RM grid properties. We analyzed a circular region centered on the Fornax cluster center with a radius of ∼1.4°; that is, ∼0.73 R vir . The mosaics have a resolution of 13″ and cover the frequencies between 900 MHz and 1.4 GHz, reaching an average noise of 16 μJy beam −1 in total intensity and 3 μJy beam −1 in the Q and U Stokes images. With these data, we detected 508 polarized sources over an area of ∼6.35 deg 2 corresponding to a density of ∼80 polarized sources/deg 2 . This is the densest RM grid ever built. Of the polarized sources, five are cluster sources. Excluding the cluster sources, we built the Euclidean-normalized differential source counts in polarization and we went a factor of ten deeper than previous surveys. We tentatively detect for the first time an increment in the differential source counts at low polarized flux densities; that is, ∼9 μJy at 1.4 GHz. The average degree of polarization of about 3–4% suggests that the sub-μJansky population is not dominated by star-forming galaxies, typically showing a degree of polarization lower than 1%. The majority of the polarized sources are Faraday simple; in other words, their polarization plane rotates linearly with the wavelength squared. The RM shows the typical decrement going from the center to the outskirts of the Fornax cluster. However, interesting features are observed both in the RM grid and in the RM radial profiles across different directions. A combination of the cluster physics and large-scale structure filaments surrounding the Fornax cluster could explain the RM characteristics.

We obtain CO(1-0) molecular gas measurements with the Australia Telescope Compact Array on a sample of 43 spectroscopically confirmed H alpha emitters in the Spiderweb protocluster at z = 2.16 and investigate the relation between their star formation activities and cold gas reservoirs as a function of environment. We achieve a CO(1-0) detection rate of similar to 23 +/- 12% with ten dual CO(1-0) and H alpha detections within our sample at 10 < log M-*/M-circle dot < 11.5. In addition, we obtain upper limits for the remaining sources. In terms of total gas fractions (F-gas), we find our sample is divided into two different regimes mediated by a steep transition at log M-*/M-circle dot approximate to 10.5. Galaxies below that threshold have gas fractions that in some cases are close to unity, indicating that their gas reservoir has been replenished by inflows from the cosmic web. However, objects at log M-*/M-circle dot > 10.5 display significantly lower gas fractions than their lower stellar mass counterparts and are dominated (12 out of 20) by objects hosting an active galactic nucleus (AGN). Stacking results yield F-gas approximate to 0.55 for massive emitters excluding AGN, and F-gas approximate to 0.35 when examining only AGN candidates. Furthermore, depletion times of our sample show that most H alpha emitters at z = 2.16 will become passive by 1 < z < 1.6, concurrently with the surge and dominance of the red sequence in the most massive clusters. Our environmental analyses suggest that galaxies residing in the outskirts of the protocluster have larger molecular-to-stellar mass ratios and lower star formation efficiencies than galaxies residing in the core. However, star formation across the protocluster structure remains consistent with the main sequence, indicating that galaxy evolution is primarily driven by the depletion of the gas reservoir towards the inner regions. We discuss the relative importance of inflow and outflow processes in regulating star formation during the early phases of cluster assembly and conclude that a combination of feedback and overconsumption may be responsible for the rapid cold gas depletion these objects endure.

We present the full sample of 76 galaxies in 39 galaxy cluster fields at z = 0.04 to 0.07 observed with VLT/MUSE by the GAs Stripping Phenomena in galaxies (GASP) survey. Most of them (64) were observed as possible ram pressure stripped galaxies (stripping candidates) based on optical B-band images, while the remaining 12 were a control sample of both star-forming and passive galaxies. Based on spatially resolved ionized gas and stellar kinematics, we assessed the physical origin of the gas asymmetries and find that 89% of the stripping candidates are confirmed by the VLT/MUSE data. In addition, three of the four star-forming galaxies in the control sample also show signs of ram pressure. These control galaxies display a ring of unusual emission line ratios, which we observe in field galaxies as well, possibly originating from the interaction with a hotter surrounding medium. The stripped galaxies are classified into various classes corresponding to different degrees of stripping, from the weakest to the strongest and most extreme (jellyfish galaxies) stripping, as well as truncated gas disks with gas remaining only in the galaxy center. Our results show that selecting cluster stripping candidates based on optical imaging yields a sample that is indeed largely dominated by galaxies affected by ram pressure at different stages and stripping strength, though some contamination is present mostly due to tidal processes. Strong ram pressure cases are found in galaxies over the whole range of stellar masses studied (10(9)-10(11.5) M-circle dot), both in low-mass and high-mass clusters (cluster velocity dispersions sigma = 500-1100 km s(-1)). We examine the possible connection between the progressive stages of stripping, up to the phase of a truncated gas disk, and the subsequent complete stripping of gas. We discuss the incompleteness intrinsic to this and other methods of selection to obtain a complete census of ram pressure stripping in clusters.

The Virgo cluster is the closest richest nearby galaxy cluster. It is in the formation process, with a number of sub-clusters undergoing merging and interactions. Although a great laboratory to study galaxy evolution and cluster formation, its large apparent size and the severe dynamic range limitations due to the presence of the bright radio source Virgo A (M 87) reduced the ability of past wide-area radio surveys to image the region with high sensitivity and fidelity. In this paper we describe the "Virgo Cluster multi-Telescope Observations in Radio of Interacting galaxies and AGN" (ViCTORIA) project. The survey and its data reduction strategy are designed to mitigate the challenges of this field and deliver: images from 42 MHz to 1.7 GHz frequencies of the Virgo cluster, about 60 times deeper than existing data, in full polarisation, and including a blind HI survey that aims at mapping seven times more galaxies than previous experiments and without selection biases. Data have been collected with the Low-Frequency Array (LOFAR) and with MeerKAT in L-band, including polarisation and enough frequency resolution to conduct local HI studies. At the distance of Virgo, current radio instruments have the resolution to probe scales of 500 pc and the sensitivity to study dwarf galaxies, the most fragile systems given their shallow gravitational potential wells, making Virgo a unique laboratory to study galaxy evolution and AGN feedback in a rich environment. In this work, we present some preliminary results, including high resolution images of the radio emission surrounding M 87, that show that the lobes are filled with filamentary structures. The combination of the presented radio surveys with state-of-the-art optical, UV, X-ray surveys will massively increase the scientific output from the studies of the Virgo cluster, making the ViCTORIA Project's legacy value outstanding.

We present the SARAO MeerKAT Galactic Plane Survey (SMGPS), a 1.3 GHz continuum survey of almost half of the Galactic Plane (251 degrees <= l <= 358 degrees and 2 degrees <= l <= 61. at vertical bar b vertical bar = 1 degrees.5). SMGPS is the largest, most sensitive, and highest angular resolution 1 GHz survey of the plane yet carried out, with an angular resolution of 8 arcsec and a broad-band root-mean-square sensitivity of similar to 10-20 mu Jy beam(-1). Here, we describe the first publicly available data release from SMGPS which comprises data cubes of frequency-resolved images over 908-1656 MHz, power-law fits to the images, and broad-band zeroth moment integrated intensity images. A thorough assessment of the data quality and guidance for future usage of the data products are given. Finally, we discuss the tremendous potential of SMGPS by showcasing highlights of the Galactic and extragalactic science that it permits. These highlights include the discovery of a new population of non-thermal radio filaments; identification of new candidate supernova remnants, pulsar wind nebulae and planetary nebulae; improved radio/mid-infrared classification of rare luminous blue variables and discovery of associated extended radio nebulae; new radio stars identified by Bayesian cross-matching techniques; the realization that many of the largest radio-quiet Wide-field Infrared Survey Explorer (WISE) H II region candidates are not true H II regions; and a large sample of previously undiscovered background H I galaxies in the Zone of Avoidance.

The presence of dense, neutral hydrogen clouds in the hot, diffuse intragroup and intracluster (IC) medium is an important clue to the physical processes controlling the survival of cold gas and sheds light on cosmological baryon flows in massive halos. Advances in numerical modeling and observational surveys mean that theory and observational comparisons are now possible. In this paper, we use the high-resolution TNG50 cosmological simulation to study the H i distribution in seven halos with masses similar to the Fornax galaxy cluster. Adopting observational sensitivities similar to the MeerKAT Fornax Survey (MFS), an ongoing H i survey that will probe to column densities of 10 18 cm −2 , we find that Fornax-like TNG50 halos have an extended distribution of neutral hydrogen clouds. Within 1 R vir , we predict the MFS will observe a total H i covering fraction of ∼12% (mean value) for 10 kpc pixels and 6% for 2 kpc pixels. If we restrict this to gas more than 10 half-mass radii from galaxies, the mean values only decrease mildly, to 10% (4%) for 10 (2) kpc pixels (albeit with significant halo-to-halo spread). Although there are large amounts of H i outside of galaxies, the gas seems to be associated with satellites, judging both by the visual inspection of projections and by comparison of the line of sight velocities of galaxies and IC H i .

We combine new and archival MUSE observations with data from the MeerKAT Fornax Survey and the ALMA Fornax Cluster Survey to study the ionised, atomic, and molecular gas in six gas-rich dwarf galaxies in the Fornax cluster in detail. We compare the distributions and velocity fields of the three gas phases with each other, with MUSE white-light images, and with the stellar velocity fields. Additionally, we derive the resolved molecular Kennicutt-Schmidt relation for each object, and compare these with existing relations for field galaxies and for the Fornax and Virgo clusters. Finally, we explore global measurements such as gas deficiencies and star formation rates to paint as complete a picture of their evolutionary state as possible. We find that all six gas-rich dwarf galaxies have very disturbed ISM, with all three gas phases being irregular both in terms of spatial distribution and velocity field. Most objects lie well below the Kennicutt-Schmidt relations from the literature. Furthermore, they are quite deficient in HI (with def_HI between 1 and 2 dex), and moderately deficient in H2 (with def_H2 between 0 and 1), suggesting that, while both cold gas phases are affected simultaneously, HI is removed in significant quantities before H2. We suggest that these dwarfs are on their first infall into the cluster, and are in the process of transitioning from star-forming to passive. A combination of tidal interactions, mergers/pre-processing, and ram pressure stripping is likely responsible for these transformations.

ABSTRACT WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William Herschel Telescope, saw first light in late 2022. WEAVE comprises a new 2-deg field-of-view prime-focus corrector system, a nearly 1000-multiplex fibre positioner, 20 individually deployable ‘mini’ integral field units (IFUs), and a single large IFU. These fibre systems feed a dual-beam spectrograph covering the wavelength range 366–959 nm at R ∼ 5000, or two shorter ranges at $R\sim 20\, 000$. After summarizing the design and implementation of WEAVE and its data systems, we present the organization, science drivers, and design of a five- to seven-year programme of eight individual surveys to: (i) study our Galaxy’s origins by completing Gaia’s phase-space information, providing metallicities to its limiting magnitude for ∼3 million stars and detailed abundances for ∼1.5 million brighter field and open-cluster stars; (ii) survey ∼0.4 million Galactic-plane OBA stars, young stellar objects, and nearby gas to understand the evolution of young stars and their environments; (iii) perform an extensive spectral survey of white dwarfs; (iv) survey ∼400 neutral-hydrogen-selected galaxies with the IFUs; (v) study properties and kinematics of stellar populations and ionized gas in z < 0.5 cluster galaxies; (vi) survey stellar populations and kinematics in ${\sim} 25\, 000$ field galaxies at 0.3 ≲ z ≲ 0.7; (vii) study the cosmic evolution of accretion and star formation using >1 million spectra of LOFAR-selected radio sources; and (viii) trace structures using intergalactic/circumgalactic gas at z > 2. Finally, we describe the WEAVE Operational Rehearsals using the WEAVE Simulator.

Abstract With nearly half of galaxies in the local Universe residing in groups or clusters, understanding galaxy evolution in dense environments is key to understanding galaxy evolution as a whole. Environmental mechanisms that quench star formation in group and cluster galaxies, first and foremost affect the (cold) ISM. With the emergence of facilities such as as MeerKAT and ALMA, we are now able to study both the extended Hi emission, a well-known tracer of environmental processes; as well as the molecular gas, the direct fuel for star formation, in unprecedented detail. In these proceedings we present results of resolved studies comparing the molecular and atomic gas contents of galaxies in nearby clusters, with a focus on the “Virgo Environment Traced in CO” (VERTICO) survey, and our study showing evidence of truncated molecular gas discs in Hi deficient galaxies. Furthermore, we will highlight ongoing observational efforts to increase our understanding of the effects of environment on the cold gas in galaxies, both using the MeerKAT telescope as well as future prospects with the SKA.