In this data release from the ongoing LOw-Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS) we present 120-168 MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44 • 30 and 1h00m +28 • 00 and spanning 4178 and 1457 square degrees respectively. The images were derived from 3,451 hrs (7.6 PB) of LOFAR High Band Antenna data which were corrected for the direction-independent instrumental properties as well as direction-dependent ionospheric distortions during extensive, but fully automated, data processing. A catalogue of 4,396,228 radio sources is derived from our total intensity (Stokes I) maps, where the majority of these have never been detected at radio wavelengths before. At 6 resolution, our full bandwidth Stokes I continuum maps with a central frequency of 144 MHz have: a median rms sensitivity of 83 µJy/beam; a flux density scale accuracy of approximately 10%; an astrometric accuracy of 0.2 ; and we estimate the point-source completeness to be 90% at a peak brightness of 0.8 mJy/beam. By creating three 16 MHz bandwidth images across the band we are able to measure the in-band spectral index of many sources, albeit with an error on the derived spectral index of > ±0.2 which is a consequence of our flux-density scale accuracy and small fractional bandwidth. Our circular polarisation (Stokes V) 20 resolution 120-168 MHz continuum images have a median rms sensitivity of 95 µJy/beam, and we estimate a Stokes I to Stokes V leakage of 0.056%. Our linear polarisation (Stokes Q and Stokes U) image cubes consist of 480 × 97.6 kHz wide planes and have a median rms sensitivity per plane of 10.8 mJy/beam at 4 and 2.2 mJy/beam at 20 ; we estimate the Stokes I to Stokes Q/U leakage to be approximately 0.2%. Here we characterise and publicly release our Stokes I, Q, U and V images in addition to the calibrated uv-data to facilitate the thorough scientific exploitation of this unique dataset.

Context. The interstellar and intra-cluster medium (ICM) in giant elliptical galaxies and clusters of galaxies is often assumed to be in hydrostatic equilibrium. Numerical simulations, however, show that about 5-30% of the pressure in a cluster is provided by turbulence induced by, for example, the central active galactic nucleus (AGN) and merger activity. Aims. We aim to put constraints on the turbulent velocities and the turbulent pressure in the ICM of the giant elliptical galaxies NGC 5044 and NGC 5813 using XMM-Newton reflection grating spectrometer (RGS) observations. Methods. The magnitude of the turbulence is estimated using the Fe xvii lines at 15.01 Å, 17.05 Å, and 17.10 Å in the RGS spectra. At low turbulent velocities, the gas becomes optically thick in the 15.01 Å line due to resonant scattering, while the 17 Å lines remain optically thin. By comparing the (I 17.05 + I 17.10 )/I 15.01 line ratio from RGS with simulated line ratios for different Mach numbers, the level of turbulence is constrained. The measurement is, however, limited by the systematic uncertainty in the line ratio for an optically thin plasma, which is about 20-30%. Results. We find that the (I 17.05 + I 17.10 )/I 15.01 line ratio in NGC 5813 is significantly higher than in NGC 5044. This difference can be explained by a higher level of turbulence in NGC 5044. The best estimates for the turbulent velocities using resonant scattering and upper limits from the line widths, are 320 < V turb < 720 km s −1 for NGC 5044 and 140 < V turb < 540 km s −1 for NGC 5813 at the 90% confidence limit. Conclusions. The high turbulent velocities and the fraction of the turbulent pressure support of >40% in NGC 5044, assuming isotropic turbulence, confirm that it is a highly disturbed system, probably due to an off-axis merger. The turbulent pressure support in NGC 5813 is more modest at 15-45%. The (I 17.05 + I 17.10 )/I 15.01 line ratio in an optically thin plasma, calculated using AtomDB v2.0.1, is 2σ above the ratio measured in NGC 5044, which cannot be explained by resonant scattering. This shows that the discrepancies between theoretical, laboratory, and astrophysical data on Fe xvii lines need to be reduced to improve the accuracy of the determination of turbulent velocities using resonant scattering.

Context. The hot gas in clusters and groups of galaxies is continuously being enriched with metals from supernovae and stars. It is well established that the enrichment of the gas with elements from oxygen to iron is mainly caused by supernova explosions. The origins of nitrogen and carbon are still being debated. Possible candidates include massive, metal-rich stars, early generations of massive stars, intermediate-or low-mass stars and asymptotic giant branch (AGB) stars. Aims. In this paper we accurately determine the metal abundances of the gas in the groups of galaxies NGC 5044 and NGC 5813, and discuss the nature of the objects that create these metals. We mainly focus on carbon and nitrogen. Methods. We use spatially-resolved high-resolution X-ray spectroscopy from XMM-Newton. For the spectral fitting, multitemperature hot gas models are used. Results. The abundance ratios of carbon over oxygen and nitrogen over oxygen that we find are high compared to the ratios in the stars in the disk of our Galaxy. The oxygen and nitrogen abundances we derive are similar to what has been found in earlier work on other giant ellipticals in comparable environments. We show that the iron abundances in both our sources have a gradient along the cross-dispersion direction of the reflection grating spectrometer (RGS). Conclusions. We conclude that it is unlikely that the creation of nitrogen and carbon takes place in massive stars, which end their lives as core-collapse supernovae, enriching the medium with oxygen because oxygen should then also be enhanced. Therefore we favour low-and intermediate-mass stars as sources of these elements. The abundances in the hot gas can be explained best by a 30-40% contribution of type Ia supernovae based on the measured oxygen and iron abundances and under the assumption of a Salpeter initial mass function (IMF).

We describe the APERture Tile In Focus (Apertif) system, a phased array feed (PAF) upgrade of the Westerbork Synthesis Radio Telescope that transforms this telescope into a high-sensitivity, wide-field-of-view L-band imaging and transient survey instrument. Using novel PAF technology, up to 40 partially overlapping beams are formed on the sky simultaneously, significantly increasing the survey speed of the telescope. With this upgraded instrument, an imaging survey covering an area of 2300 deg 2 is being performed that will deliver both continuum and spectral line datasets, of which the first data have been publicly released. In addition, a time domain transient and pulsar survey covering 15 000 deg 2 is in progress. An overview of the Apertif science drivers, hardware, and software of the upgraded telescope is presented, along with its key performance characteristics.

Context. The physics of the coolest phases in the hot intra-cluster medium (ICM) of clusters of galaxies is yet to be fully unveiled. X-ray cavities blown by the central active galactic nucleus (AGN) contain enough energy to heat the surrounding gas and stop cooling, but locally blobs or filaments of gas appear to be able to cool to low temperatures of 10 4 K. In X-rays, however, gas with temperatures lower than 0.5 keV is not observed. Aims. We aim to find spatial and multi-temperature structures in the hot gas of the cooling-core cluster Abell 2052 that contain clues on the physics involved in the heating and cooling of the plasma. Methods. 2D maps of the temperature, entropy, and iron abundance are derived from XMM-Newton data of Abell 2052. For the spectral fitting, we use differential emission measure (DEM) models to account for the multi-temperature structure. Results. About 130 kpc South-West of the central galaxy, we discover a discontinuity in the surface brightness of the hot gas which is consistent with a cold front. Interestingly, the iron abundance jumps from ∼0.75 to ∼0.5 across the front. In a smaller region to the North-West of the central galaxy we find a relatively high contribution of cool 0.5 keV gas, but no X-ray emitting gas is detected below that temperature. However, the region appears to be associated with much cooler Hα filaments in the optical waveband. Conclusions. The elliptical shape of the cold front in the SW of the cluster suggests that the front is caused by sloshing of the hot gas in the clusters gravitational potential. This effect is probably an important mechanism to transport metals from the core region to the outer parts of the cluster. The smooth temperature profile across the sharp jump in the metalicity indicates the presence of heat conduction and the lack of mixing across the discontinuity. The cool blob of gas NW of the central galaxy was probably pushed away from the core and squeezed by the adjacent bubble, where it can cool efficiently and relatively undisturbed by the AGN. Shock induced mixing between the two phases may cause the 0.5 keV gas to cool non-radiatively and explain our non-detection of gas below 0.5 keV.

Context. Apertif is a phased-array feed system for the Westerbork Synthesis Radio Telescope, providing forty instantaneous beams over 300 MHz of bandwidth. A dedicated survey program utilizing this upgrade started on 1 July 2019, with the last observations taken on 28 February 2022. The imaging survey component provides radio continuum, polarization, and spectral line data. Aims. Public release of data is critical for maximizing the legacy of a survey. Toward that end, we describe the release of data products from the first year of survey operations, through 30 June 2020. In particular, we focus on defining quality control metrics for the processed data products. Methods. The Apertif imaging pipeline, Apercal, automatically produces non-primary beam corrected continuum images, polarization images and cubes, and uncleaned spectral line and dirty beam cubes for each beam of an Apertif imaging observation. For this release, processed data products are considered on a beam-by-beam basis within an observation. We validate the continuum images by using metrics that identify deviations from Gaussian noise in the residual images. If the continuum image passes validation, we release all processed data products for a given beam. We apply further validation to the polarization and line data products and provide flags indicating the quality of those data products. Results. We release all raw observational data from the first year of survey observations, for a total of 221 observations of 160 independent target fields, covering approximately one thousand square degrees of sky. Images and cubes are released on a per beam basis, and 3374 beams (of 7640 considered) are released. The median noise in the continuum images is 41.4 uJy beam −1 , with a slightly lower median noise of 36.9 uJy beam −1 in the Stokes V polarization image. The median angular resolution is 11.6 /sin δ. The median noise for all line cubes, with a spectral resolution of 36.6 kHz, is 1.6 mJy beam −1 , corresponding to a 3-σ H i column density sensitivity of 1.8×10 20 atoms cm −2 over 20 km s −1 (for a median angular resolution of 24 ×15 ). Line cubes at lower frequency have slightly higher noise values, consistent with the global RFI environment and overall Apertif system performance. We also provide primary beam images for each individual Apertif compound beam. The data are made accessible using a Virtual Observatory interface and can be queried using a variety of standard tools.

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Large satellite constellations in low-Earth orbit seek to be the infrastructure for global broadband Internet and other telecommunication needs. We briefly review the impacts of satellite constellations on astronomy and show that the Internet service offered by these satellites will primarily target populations where it is unaffordable, not needed, or both. The harm done by tens to hundreds of thousands of low-Earth orbit satellites to astronomy, stargazers worldwide, and the environment is not acceptable. Unified Astronomy Thesaurus concepts: Artificial satellites (68) LOW-EARTH ORBIT SATELLITE CONSTELLATIONS SpaceX's Starlink has launched over 700 low-Earth orbit satellites since May 2019. They plan to offer global broadband Internet with a final "constellation" of 42,000 satellites. Other operators have announced similar plans, and we are witnessing a new era of a sky filled with thousands of low-Earth orbit commercial satellites. Recent studies (McDowell 2020; Hainaut & Williams 2020; Tregloan-Reed et al. 2020) raise concerns about the brightness of these constellations and the detrimental effect of large numbers of satellites to optical astronomy. This is because satellites reflect sunlight even after sunset, and global satellite coverage will cause bright streaks in astronomical images for large portions of the night at ground-based observatories worldwide. Vera C. Rubin Observatory and its Legacy Survey of Space and Time (LSST, Ivezić et al. 2019) will be the optical astronomy facility most severely impacted by satellite constellations due to its wide field of view and large light collecting area. Tyson et al. (2020) estimate a 48,000 satellite constellation will result in at least 30% of LSST images containing a satellite trail. Other optical and near-IR observatories will also be significantly impacted. There are concerns for wavelengths outside optical (e.g., Gallozzi et al. 2020; Massey 2020) as well. For example, satellites directly transmit at 10-30 GHz 1 in bands used for astronomical observations. Depending on satellite transmitter quality, frequencies outside the nominal transmission bands can also be impacted (e.g., Deshpande & Lewis 2019). Unfortunately, the solution is not as simple as moving telescopes into space. Prohibitive costs, inability to maintain instruments in space, constraints imposed by launch vehicle size, the short life expectancy from harsh conditions, and decades to plan and fund large missions make this infeasible. The recent Satellite Constellations 1 Workshop Report (Walker et al. 2020) provides recommendations to mitigate the effects of satellite trails in optical and near-IR images. Final reports from the Conference on Dark and Quiet Skies for Science and Society 2 will soon be sent to the UN Committee on the Peaceful Uses of Outer Space.