Kamlesh Rajpurohit

We identified a peculiar bright and filamentary radio source at a low Galactic latitude ( b ≈ 0.5°) in LOFAR Two-meter Sky Survey data. This source, also detected in previous radio observations, was originally believed to be a pulsar, but was later suggested to lie in a heavily obscured galaxy cluster behind the Galactic plane. In this paper, we characterize the main properties of the host cluster (redshift, mass, temperature, X-ray luminosity, and dynamical status) for the first time, using X-ray observations from Chandra and SRG/eROSITA. In addition, we combined new uGMRT follow-up data with observations from the LOFAR LBA Sky Survey to perform a multifrequency, spatially resolved spectral analysis of the filamentary radio source (VLSS J0318.9+5755, hereafter referred to as the “Treble Clef” due to its morphology). We conclude that this source is a radio phoenix belonging to a massive, merging galaxy cluster in the zone of avoidance. We speculate that its complex morphology is shaped by gas motions generated in the intracluster medium during the ongoing merger, which are also likely responsible for the candidate radio halo tentatively observed in the cluster center. Owing to its highly filamentary morphology, brightness at ≲1 GHz, and extremely steep spectrum, reaching values of α > 4 between 400 and 650 MHz, this source represents an ideal target for high-resolution, very-low-frequency follow-up observations with LOFAR2.0.

We present a detailed, multiband radio continuum study of the compact galaxy group Stephan’s Quintet (HCG 92). We use a combination of new (MeerKAT, uGMRT) and archival (LOFAR, VLA) observations covering the 120 MHz–8 GHz frequency range to examine the radio properties of the group, focusing on the famous radio ridge and surrounding diffuse emission. We find filamentary substructure and branching in the southern half of the ridge, confirm an extension of the ridge to the northwest, and identify for the first time a radio counterpart to the gas bridge linking the ridge and NGC 7319. The northern ridge, northwest extension, and diffuse emission have relatively steep, curved spectra, with a high-frequency spectral index gradient running north–south along the ridge. We show that the ridge emission primarily arises from a single physical mechanism, probably strong ( M ≃ 40 − 100 ) shocks in cold gas, caused by the ∼850–1000 km s ⁻¹ collision between NGC 7318B and tidal gas filaments produced by past galaxy interactions. Synchrotron spectral age estimates suggest the collision began at the north end of the ridge ∼20 Myr ago, and finished only ∼5–6 Myr ago in the south, with the southern end of the shocked ridge likely still within or close to the disk of NGC 7318B. Based on this age gradient we find that the angle between the intruder galaxy’s motion and the tidal filaments was probably only ∼15°, and combining this with the lack of a spectral index gradient in the diffuse radio emission suggests that NGC 7318B’s direction of motion is probably close to the line of sight.

We present the LOFAR Galaxy Cluster Ultra-Deep Field, in which 336 h of LOFAR observations at 120–168 MHz have been collected on the nearby ( z = 0.080) cluster Abell 2255. This massive and merging system is known to host spectacular radio emission from both cluster galaxies and the intracluster medium. Previous LOFAR observations revealed pervasive diffuse synchrotron emission extending from the cluster center to its dynamically active outskirts, tracing relativistic electrons propagating in large-scale magnetic fields. In this work, we present a set of new ultra-deep images at the central frequency of 144 MHz based on the 224 h of data with the best quality, which reach a sensitivity of 24 µJy beam ⁻¹ at 7.1″×4.3″ resolution. These images represent the deepest radio observations of a galaxy cluster obtained to date and provide a glimpse of what should be routinely observed in clusters with SKA-Low in the near future. Using these data, we investigate the topology of the cluster magnetic field out to its virial radius by applying the synchrotron intensity gradient technique. We find that the inferred magnetic field exhibits preferential orientations in distinct regions of the cluster, such as in radio halo extensions (bridges) and in relics, suggesting that the dynamics of the cluster formation process is shaping the large-scale magnetic field. This interpretation is supported by the comparison with the magnetic field orientation obtained from cosmological magnetohydrodynamic simulations. This work provides the first indication of a coherent, large-scale magnetic field topology across an entire galaxy cluster, from the core to the outskirts, and it demonstrates the unique power of ultra-deep, low-frequency observations to trace the structure of cluster magnetic fields on megaparsec scales, thereby probing the magnetization of the large-scale structure of the Universe.

Environmental processes in galaxy clusters play a key role in shaping the gas content and star formation activity of galaxies. While atomic gas is known to be highly susceptible to environmental effects, the response of molecular gas to dense environments remains less well understood. We investigate the molecular and atomic gas properties of galaxies in the A2626 cluster (z = 0.055) in order to understand how environmental processes affect different gas phases. We present an analysis of ten spatially resolved galaxies in A2626 observed as part of the SYMPHANY project. Using CO(2–1) observations from ALMA and SMA together with data from MeerKAT, we examine the interplay between molecular and atomic gas through a joint morphological and kinematic analysis. While the kinematic behaviour of and CO is often similar, the atomic gas is morphologically more extended than the centrally concentrated CO distribution, making it more susceptible to environmental stripping. Nevertheless, we also find evidence for molecular gas asymmetries, disturbed velocity fields, and H_2 deficiencies in some galaxies, indicating that molecular gas reservoirs can also be affected in dense environments. Compared to Virgo, a dynamically unrelaxed, non–cool-core cluster, the relaxed cool-core structure of A2626 likely results in less intense ram-pressure stripping. This may allow galaxies to retain more atomic gas, while local interactions or pre-processing may still influence the molecular phase, producing relatively low deficiencies but higher H_2 deficiencies in A2626 galaxies. This behaviour is also reflected in the higher gas fractions, slightly elevated star formation rates, and shorter depletion timescales (0.3–3 Gyr), implying moderately enhanced star formation efficiency. The absence of a clear correlation between H_2 deficiency and cluster-centric distance or velocity suggests that molecular gas evolution in A2626 may be driven more by early infall or local interactions than by the current location of galaxies within the cluster.

Context. Radio relics are diffuse, extended synchrotron sources located in the outskirts of merging galaxy clusters. Their origin has been linked to shock waves injected into the intracluster medium, but the acceleration mechanism at the shock front remains under debate. Some clusters, such as MACS J1752.0+4440, host a double relics system, with two relics located on opposite sides of the cluster center. Aims. To investigate the acceleration mechanism that generates radio relics, we studied the morphological and spectral properties of double relic system in MACS J1752.0+4440 ( z = 0.366). Methods. We present new wideband radio continuum observations obtained with uGMRT and JVLA, and previously published LOFAR data. We performed a detailed, high-resolution spectral analysis of the double relic system in MACS J1752.0+4440, and we observed and characterized substructures, particularly those of the brighter relic. Results. We find a double-peaked surface brightness and spectral index profile for the NE relic and identify a “bright bar” substructure. Moreover, we observe surprisingly flat integrated spectral indices for both relics, at α _int ^NE = −0.91 ± 0.06 and α _int ^SW = −0.83 ± 0.05. We studied the spatial variation of the spectral index, observing a coherent trend with the observed substructures. We estimate injection Mach numbers of ℳ _NE ^SW = 3.1 ^+0.1 _−0.1 and ℳ _NE ^SW = 3.2 ^+0.1 _−0.1 . We performed a spectral curvature analysis for both relics, generating color-color plots, and we observe two “concave” spectra characterized by positive spectral curvature, in contrast to particle population aging models. Conclusions. The observed properties of the NE relic are not consistent with a simple scenario involving a single shock front and a single particle-injection event. Multiple shock surfaces, reacceleration, and projection effects likely play a role in shaping the morphology of the relic.

The role of cooling gas in triggering active galactic nucleus (AGN) feedback in the centers of galaxy groups and clusters remains a key open question. NGC 5044, the X-ray-brightest galaxy group, hosts the largest known reservoir of molecular gas among cool-core groups and exhibits multiple AGN outbursts, making it an ideal system in which to study AGN feeding. We present new multifrequency Very Long Baseline Array (VLBA) observations of NGC 5044 at 1.4 GHz, 4.9 GHz, and 8.4 GHz, combining continuum imaging with H I spectroscopy. At 1.4 GHz, we recovered the previously known symmetric northeast–southwest jets extending for ∼5.5 pc each, along with evidence for previously undetected, more extended faint emission aligned with the older, kiloparsec-scale outbursts. Comparison of 4.9 GHz and 8.4 GHz data from 2020 and 2024 reveals clear outward proper motion of jet components, yielding an average expansion speed of (0.10 ± 0.02) c and implying a dynamical age of ∼180 yr for the ejection of the parsec-scale jet components. The jet width profile suggests a transition from parabolic to conical collimation at a few ×10 ⁴ Schwarzschild radii. We detect a narrow, redshifted H I absorption line at +264 km s ⁻¹ against the VLBA core, tracing a compact, cold atomic cloud within ∼10–20 pc of the AGN. The close correspondence in velocity with previously detected CO and H I absorption features in Atacama Large Millimeter/submillimeter Array and MeerKAT data, respectively, demonstrates that cold atomic and molecular gas coexists in infalling clouds at parsec scales. Overall, these results provide an unprecedented high-angular-resolution view of AGN cycling, jet growth, and feeding in a galaxy group environment.

We present new MeerKAT 21 cm line observations of the neutral hydrogen gas in the compact galaxy group Stephan’s Quintet. These data provide a significantly improved view of the atomic gas distribution and kinematics in the group. New features include the first detections of H i associated with member galaxies NGC 7319 and NGC 7320C, the identification of a high-velocity H i component associated with SQ-A, and the detection of additional H i at low velocities filling the area of the NGC 7318B disk. We also find H i in the previously detected gas bridge linking NGC 7319 and NGC 7318B, and a new northern bridge linking NGC 7319 to the SQ-A star formation region. We detect H i with a wide range of velocities along the line of sight through the northern half of the famous shock ridge, including in the 6200–6500 km s ⁻¹ velocity range occupied by shocked H α emission. We examine the morphology and velocity structure of the H i and consider the origins of different components, finding some evidence that while the gas associated with NGC 7318B has been disturbed by its collision with the group, it may still retain a component of disk rotation. We find no gaseous connection between the tidal tails and NGC 7320C, but reaffirm the close connection between the shocked gas in the ridge (traced by X-ray, radio continuum, and warm H ₂ emission) and the southern tidal tail. Based on the integrated spectrum, we find a total H i mass in the group of 3.5 ± 0.4 × 10 ¹⁰ M _⊙ , higher than the Very Large Array estimate and comparable to FAST.

We present the third data release of the LOFAR Two-metre Sky Survey (LoTSS-DR3). The survey images cover 88% of the northern sky and were created from 12 950 h of data (18.6 PB) accumulated over 10.5 years. Producing the images took 20 million core hours of processing through direction-independent and direction-dependent calibration pipelines that correct for instrumental effects as well as spatially and temporally varying ionospheric distortions. In our 120–168 MHz continuum mosaic images with an angular resolution of 6″ (9″ below declination 10°) we catalogue 13 667 877 sources, formed from 16 943 656 Gaussian components. The scatter in the astrometric precision approximately follows the expected noise-like behaviour but with an additional systematic component of at least 0.24″ that is likely due to calibration imperfections. The random flux density scale error is 6%, while the systematic offset was previously shown to be within 2%. The median sensitivity of our mosaics is 92 μJy beam ⁻¹ , improving to 68 μJy beam ⁻¹ at high observing elevations, but degrading to 183 μJy beam ⁻¹ at the celestial equator due to station area projection effects. Completeness simulations, accounting for realistic source models, time- and bandwidth-smearing effects, and astrometric errors, indicate that we detect more than 95% of compact sources with integrated flux densities exceeding 9 times the local root mean square (RMS) noise. However, the recovered source counts in a particular integrated flux density bin do not match the injected counts until flux densities exceed 45 times the local RMS noise. The Euclidean-normalised differential source counts derived from the survey constrain the radio source population over five orders of magnitude and are in good agreement with previous deep and wide-area surveys. All data products are publicly available, including catalogues, individual-field Stokes I , Q , U , and V images, mosaicked Stokes I images, and uv data with associated direction-dependent calibration solutions.

Context. Thin, elongated, non-thermal filaments in galaxy clusters and groups are nowadays ubiquitous in sensitive radio maps. Despite the large (and increasing) number of cases, their origin is still unclear. In a previous work, we revealed a wealth of filaments surrounding the main member radio galaxy of Abell 2255: a head-tail named “Original Tailed Radio Galaxy” (Original TRG). We did this using 56 hours of sub-arcsecond resolution LOFAR-VLBI observations at 144 MHz. Aims. In this paper, we aim to further analyze the filaments in Abell 2255 combining LOFAR data with uGMRT (1260 MHz) and VLA (1520 MHz) data to constrain the spectral shape of the filaments. This enables the study of their morphological properties, which is required to understand their origin, at an unprecedentedly high resolution (∼2.3 kpc), crucial for disentangling the different cosmic ray components that populate the Original TRG. Methods. We produced a LOFAR-VLBI map at 1.5″ resolution using the wide-field technique with 56 hours of observations. This was the first time this technique was used for a galaxy cluster, especially for such deep observations. uGMRT and VLA data have been calibrated and imaged to produce spectral index maps and to apply further techniques to extract additional information, such as the radiative ages of the filaments or their equipartition magnetic field. Polarization information was also obtained using VLA through the rotation measure synthesis technique. Results. Thanks to the LOFAR-VLBI wide-field image at 144 MHz, we revealed additional, very steep ( α > 2) filaments beyond those attached to the radio galaxy, extending for around 250 kpc and previously known as the Trail. Combining LOFAR-VLBI with uGMRT and VLA, we found integrated spectral values between 1.1 − 1.7 for the filaments. Spectral analysis revealed also that the Original TRG has a complex structure, showing overlapping features with distinct spectral indices that extend throughout its tail. Polarized emission emerges only from the tail and the brightest part of the filaments, with values up to 22%. Although there is no clear scenario regarding the formation of filaments, we highlight the importance of the Original TRG as the main driver of such structures, even at larger distances from the core.

We present high-frequency, full-polarization Jansky Very Large Array (VLA) radio data for the X band of the radio relic MACS J0717.5+3745. Radio relics trace shock waves in the intracluster medium (ICM) that are produced during mergers. Understanding the physical characteristics of relics is important for determining their nature – whether, for example, they are thermal ICM electrons that are accelerated, or whether they are fossil electrons re-accelerated by a merger event. Radio spectropolarimetric analysis, such as through the Stokes QU-fitting, provides a diagnostic of the nature and structure of the magnetized plasma internal or external to the source, with important implications for theoretical models. The high-frequency polarization analysis presented here shows, for the first time, a change in the magneto-ionic structure compared with that for the low-frequency data available in the literature. These high-frequency, polarized data could also be interpreted with an internal depolarization behaviour, and this new finding may be used to investigate possible particle acceleration mechanisms. If that is true, the change in the behaviour of the polarized signal could be tracing the physical properties of a population of non-thermal particles that are undergoing a re-acceleration in the relic caused by the large-scale internal shocks of active galactic nuclei jet fossil particles ejected from the central Narrow Angle Tail radio galaxy. New upcoming broad-band VLA X- and Ku-band data will clarify this. Finally, we conclude that high-frequency, high-sensitivity, spectropolarimetric radio data should be explored further, as they can effectively trace shock fronts and thereby provide insights into the intrinsic magneto-ionic properties of radio components.