MeerKAT’s large number (64) of 13.5 m diameter antennas, spanning 8 km with a densely packed 1 km core, create a powerful instrument for wide-area surveys, with high sensitivity over a wide range of angular scales. The MeerKAT Galaxy Cluster Legacy Survey (MGCLS) is a programme of long-track MeerKAT L-band (900−1670 MHz) observations of 115 galaxy clusters, observed for ∼6−10 h each in full polarisation. The first legacy product data release (DR1), made available with this paper, includes the MeerKAT visibilities, basic image cubes at ∼8″ resolution, and enhanced spectral and polarisation image cubes at ∼8″ and 15″ resolutions. Typical sensitivities for the full-resolution MGCLS image products range from ∼3−5 μJy beam−1. The basic cubes are full-field and span 2° × 2°. The enhanced products consist of the inner 1.2° × 1.2° field of view, corrected for the primary beam. The survey is fully sensitive to structures up to ∼10′ scales, and the wide bandwidth allows spectral and Faraday rotation mapping. Relatively narrow frequency channels (209 kHz) are also used to provide H I mapping in windows of 0 < z < 0.09 and 0.19 < z < 0.48. In this paper, we provide an overview of the survey and the DR1 products, including caveats for usage. We present some initial results from the survey, both for their intrinsic scientific value and to highlight the capabilities for further exploration with these data. These include a primary-beam-corrected compact source catalogue of ∼626 000 sources for the full survey and an optical and infrared cross-matched catalogue for compact sources in the primary-beam-corrected areas of Abell 209 and Abell S295. We examine dust unbiased star-formation rates as a function of cluster-centric radius in Abell 209, extending out to 3.5 R 200. We find no dependence of the star-formation rate on distance from the cluster centre, and we observe a small excess of the radio-to-100 μm flux ratio towards the centre of Abell 209 that may reflect a ram pressure enhancement in the denser environment. We detect diffuse cluster radio emission in 62 of the surveyed systems and present a catalogue of the 99 diffuse cluster emission structures, of which 56 are new. These include mini-halos, halos, relics, and other diffuse structures for which no suitable characterisation currently exists. We highlight some of the radio galaxies that challenge current paradigms, such as trident-shaped structures, jets that remain well collimated far beyond their bending radius, and filamentary features linked to radio galaxies that likely illuminate magnetic flux tubes in the intracluster medium. We also present early results from the H I analysis of four clusters, which show a wide variety of H I mass distributions that reflect both sensitivity and intrinsic cluster effects, and the serendipitous discovery of a group in the foreground of Abell 3365.

Context. The massive young stellar object (MYSO) G358.93−0.03-MM1 showed an extraordinary near-infrared-to (sub-)millimetredark and far-infrared-loud accretion burst, which is closely associated with flares of several class II methanol maser transitions, and, later, a 22 GHz water maser flare. Aims. Water maser flares provide an invaluable insight into ejection events associated with accretion bursts. Although the short timescale of the 22 GHz water maser flare made it impossible to carry out a very long baseline interferometry observation, we could track it with the Karl G. Jansky Very Large Array (VLA). Methods. The evolution of the spatial structure of the 22 GHz water masers and their association with the continuum sources in the region is studied with the VLA during two epochs, pre-and post-H 2 O maser flare. Results. A drastic change in the distribution of the water masers is revealed: in contrast to the four maser groups detected during epoch I, only two newly formed clusters are detected during epoch II. The 22 GHz water masers associated with the bursting source MM1 changed in morphology and emission velocity extent. Conclusions. Clear evidence of the influence of the accretion burst on the ejection from G358.93−0.03-MM1 is presented. The accretion event has also potentially affected a region with a radius of ∼2 ′′ (∼13 500 AU at 6.75 kpc), suppressing water masers associated with other point sources in this region.

Context. Recent studies have shown that 6.7 GHz methanol maser flares can be a powerful tool for verifying the mechanisms of maser production and even the specific signatures of accretion rate changes in the early stages of high-mass star formation. Aims. We characterize the spatial structure and evolution of methanol and water masers during a flare of methanol maser emission at 6.7 GHz in the high-mass young stellar object (HMYSO) G24.33+0.14. Methods. Very Long Baseline Array (VLBA) was used to image the 6.7 and 12.2 GHz methanol and 22.2 GHz water vapor masers at three epochs guided by monitoring the methanol line with the Torun 32m telescope. The 6.7 GHz maser maps were also obtained with the European VLBI Network (EVN) and Long Baseline Array (LBA) during the flare. The Wide-field Infrared Survey Explorer (WISE) data were used to find correlations between the 6.7 GHz maser and infrared (IR) fluxes. Results. The 6.7 GHz methanol maser cloudlets are distributed over ~3500 au, and the morphology of most of them is stable although their brightness varies following the course of the total flux density on a timescale of two months. The 12.2 GHz methanol maser cloudlets cover an area an order of magnitude smaller than that of 6.7 GHz emission, and both transitions emerge from the same masing gas. The 22.2 GHz maser cloudlets lie in the central region and show a systematic increase in brightness and moderate changes in size and orientation, together with the velocity drift of the strongest cloudlet during two months of the Very Long Baseline Interferometry (VLBI) observing period. Time lag estimates imply the propagation of changes in the physical conditions of the masing region with a subluminal speed (~0.3c). A tight correlation of IR (4.6 μm) and 6.7 GHz flux densities is found, supporting the radiative pumping model. Proper motion analysis does not reveal any signs of expansion or inflow of the methanol cloudlets within ~6 mas over ~10 yr. Comparison with the 230 GHz Atacama Large Millimeter Array (ALMA) data indicates that the methanol masers are distributed in the inner part of the rotating disk, whereas the 22.2 GHz emission traces the compact inner component of the bipolar outflow or a jet structure. Conclusions. The maser morphology in the target is remarkably stable over the course of the flare and is similar to the quiescent state, possibly due to less energetic accretion events that can repeat on a timescale of ~8 yr.