Simona Giacintucci

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 results from a deep MeerKAT 1.28 GHz observation of A520, a galaxy cluster in the middle of a merger and a treasure trove of merger-generated phenomena. MeerKAT’s combination of high angular resolution and sensitivity to extended radio emission lets us uncover unprecedented detail of the cluster’s giant halo and puzzling features beyond the halo. We find that the main radio halo is sharply bounded along the direction of the NE–SW merger axis by the two X-ray shock fronts. In the perpendicular direction, the halo wings smoothly span over 2 Mpc, with the spectrum steepening in the outskirts. We detect faint radio emission upstream of the main (SW) bow shock; the emissivity jump across this shock can be used to constrain the physical mechanisms at work there. We discover a tail of radio emission extending past the NE shock to the gas-poor, high-entropy “dark subcluster” in the NE outskirts. The tail and subcluster have a higher radio/X-ray brightness ratio than the main halo’s, suggesting efficient reacceleration of cosmic rays. We also discover two peripheral radio relics beyond the cluster virial radius, and a faint, ∼2 Mpc long, broad radio bridge connecting the dark subcluster to one of the relics along the direction perpendicular to the merger axis. The spectral slopes of the dark subcluster and the bridge are consistent with that of the main halo. This bridge and both relics may be caused by another giant merger-generated shock front at the virial radius.

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 new Very Long Baseline Array (VLBA) imaging of a megahertz peaked-spectrum source that was found using commensal low-frequency data taken with the Karl G. Jansky Very Large Array (VLA). The source, J0330–2730, was identified in multiepoch data from the VLA Low-band Ionosphere and Transient Experiment (VLITE). VLITE continuously collects low-frequency data at 340 MHz during regular VLA observations. Our analysis of the VLITE light curve demonstrates that J0330–2730 has significant 340 MHz flux variability at the ∼20% level over a timescale of approximately 1 yr. Our VLBA images reveal a resolved, double-lobed morphology with a projected linear size of 64 pc. We consider plausible mechanisms that could explain the observed 340 MHz variability and the source properties on milliarcsecond scales. We rule out variable Doppler boosting and conclude that refractive interstellar scintillation or variable free–free absorption are the most likely explanations. We argue that the properties of J0330–2730 are consistent with the class of compact symmetric objects and consider the evolutionary stage of the source. The extent of the resolved lobes revealed by the VLBA is significantly smaller than predictions based on the turnover–size relation for a standard synchrotron self-absorbed jet model. We discuss possible explanations for the departure from the turnover–size relation, including jet formation by a transient phenomenon such as a tidal disruption event or a “frustrated jet” impeded by the presence of dense gas or a high-pressure environment. This study highlights the potential of VLITE for the identification of compact and young radio sources.

Context. Faraday rotation measure (RM) grids provide a sensitive means to trace magnetized plasma across a wide range of cosmic environments. Aims. We study the RM signal from the Shapley Supercluster core (SSC) in order to constrain the magnetic field properties of its gas. The SSC region consists of two galaxy clusters, A3558 and A3562, and two galaxy groups between them, at z ≃ 0.048. Methods. We combined RM Grid data with thermal Sunyaev-Zeldovich effect data, obtained from the POlarisation Sky Survey of the Universe’s Magnetism (POSSUM) pilot survey, and Planck, respectively. To robustly determine the gas density, its magnetic field properties, and their correlation | B | ∝ n _e ^η , we studied the RM scatter in the SSC region (𝔖 _RM ) and its behavior as a function of distance to the nearest cluster and/or group ( d _nrst ). We compared observational results with semi-analytic Gaussian random field models and more realistic cosmological magnetohydrodynamical (MHD) simulations. Results. With a sky-density of 36 RMs/deg ² , we detect an excess RM scatter of 30.5 ± 4.6 rad/m ² in the SSC region. When we compare with models, we find an average magnetic field strength of ∼1−3 μG (in the groups and clusters). The 𝔖 _RM ( d _nrst ) profile, derived from data ranging from ∼0.3−1.8 r ₅₀₀ for all objects, is systematically flatter than expected compared to the models, with η < 0.5 being favored. Despite this discrepancy, we find that cosmological MHD simulations matched to the SSC structure most closely align with scenarios where the magnetic field is amplified by the turbulent velocity ( v _turb ) in the intercluster regions B _ℱ ∝ n _e ^1/2 v _turb on scales d _nrst ≲ 0.8. Conclusions. The dense RM grid and precision provided by POSSUM allows us to probe magnetized gas in the SSC clusters and groups on scales within and beyond their r ₅₀₀ . Flatter-than-expected RM scatter profiles reveal a significant challenge in reconciling observations with even the most realistic predictions from cosmological MHD simulations in the outskirts of interacting clusters.

We investigate the spatial relationships between multiphase gas components and supermassive black hole (SMBH) activity in a sample of 25 cool-core galaxy groups and clusters. Using high-angular-resolution observations from Chandra, the Very Large Telescope/Multi Unit Spectroscopic Explorer, and the Very Long Baseline Array (VLBA), we robustly locate the positions, respectively, of the X-ray peak of the intracluster medium, of the H α peak of the warm ionized gas, and of the SMBH radio core, on parsec scales. We identify spatial offsets between the X-ray peak of the hot gas and the SMBH in 80% of the systems, with an average displacement of 〈 Δ X-ray SMBH 〉 = 4.8 kpc (dispersion of 3.8 kpc). In contrast, the peak of warm ionized gas traced by H α exhibits a much smaller offset ( 〈 Δ H α SMBH 〉 = 0.6 kpc; dispersion of 1.4 kpc) and a lower incidence of displacement (15%). Our findings suggest that hot-gas sloshing primarily drives the observed spatial offsets, with active galactic nucleus (AGN)–driven uplift contributing in some systems. Importantly, systems with H α– SMBH offsets of ≥1 kpc uniformly lack detectable radio cores on VLBA scales, with upper limits on the 5 GHz power of P _5GHz ​​​​​ ≤ 10 ^21–22 W Hz ⁻¹ , while those without such offsets exhibit radio-powerful AGNs with parsec-scale radio emission up to P _5GHz ∼ 10 ^24–25 W Hz ⁻¹ . This correlation indicates that centrally concentrated warm gas is critical for sustaining radio-loud SMBH activity, possibly supporting scenarios of cold-mode accretion. Overall, our results highlight the importance of high-angular-resolution multiwavelength observations for understanding the interplay between multiphase gas cooling and AGN fueling in central galaxies.