arXiv subjects

Ruta Kale

Publications and source records attributed to Ruta Kale.

At least 19 recordsLinked to original sources

arXiv2026

Multi-Wavelength Signatures of a Giant Cometary Radio Halo in MACSJ0417-1154

Galaxy clusters hosting diffuse non-thermal radio emission offer direct insight into plasma processes of the intracluster medium (ICM). We present the first multi-frequency study of the radio halo in MACSJ0417 (z = 0.445) using uGMRT (300-850 MHz), MeerKAT (900-1670 MHz), and archival \textit{XMM-Newton} data. The halo extends to $\sim$1.75 Mpc at 400 MHz, while two candidate relics (R1 and R2) are detected at 2.9 Mpc. The integrated spectra follow single power-laws with spectral indices $\alpha \simeq -1.3$ for the halo and $\alpha \simeq -1.6$ for the relics. Sensitive uGMRT imaging reveals a radio surface brightness edge $\sim$43$''$ SE of the cluster centre, which coincides with an X-ray discontinuity. Resolved spectral maps (400--1280 MHz) show significant fluctuations and a clear radial steepening of the spectral index. X-ray analysis reveals an elongated SE-NW morphology and high-temperature regions ($\sim$11 keV) along this axis. A strong radio and X-ray surface brightness correlation is found (correlation coefficient $\sim$ 0.85), with the correlation slope evolving from sublinear at 400 MHz to linear at 1280 MHz. These results, together with the spectral properties, support the turbulent re-acceleration model and point to inhomogeneous ICM conditions. The pure hadronic model is excluded owing to unrealistic energy requirements for cosmic-ray protons. We propose that MACSJ0417 is undergoing a minor off-axis dissociative merger (mass ratio $\sim$6:1) along the SE-NW axis, which has preserved its cool core while driving turbulence that powers the giant radio halo.

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arXiv2026

Probing High-redshift Intracluster Medium Using SKA

Galaxy clusters host vast reservoirs of magnetised plasma in their intracluster medium (ICM), where turbulence and shocks generated during structure formation sometimes give rise to diffuse synchrotron emission in the form of radio halos and relics. However, the origin and amplification of the magnetic field in the ICM remains among the least understood problems, particularly at high redshift, where direct detections are scarce. Recent observations of clusters at z $\gtrsim 0.6$ have revealed radio powers of the diffuse sources comparable to those of nearby systems, implying that efficient magnetic amplification suppressed inverse Compton losses. Such rapid growth of the magnetic fields after a few Gyrs of the Big Bang challenges standard dynamo timescales, and it raises fundamental questions about the mechanisms that strengthened cluster magnetic fields. In this chapter, we highlight the massive merging system El Gordo (ACT-CL J0102$-$4915, z$\sim$0.87) as a unique testbed for investigating theoretical models under extreme conditions. Proposing as a SKA science verification target, El Gordo will allow detailed studies of the ICM magnetic field structure, strength through broadband continuum and polarisation measurements with SKA-Low and SKA-Mid, across staged deployments starting from AA0.5 to AA4, reaching a physical resolution of $\sim$ 20-30 kpc at our redshift of interest. We further outline how upcoming SKA surveys will build statistical samples of high-redshift clusters, enabling tests of different models of magnetic fields and cosmic rays in the formation of large-scale structure.

astro-ph.CO↗

arXiv2026

Probing Merger Shocks in Galaxy Clusters in the SKA Era

Galaxy cluster mergers represent the most energetic phenomena in the Universe since the Big Bang releasing gravitational potential energy of $\sim 10^{63-64}$ erg, injecting turbulence and driving shocks through the intracluster medium (ICM). These merger shocks can accelerate cosmic ray electrons and compress magnetic fields, sometimes producing Mpc-scale synchrotron radio structures known as radio relics. Radio relics are powerful tracers of merger dynamics, particle acceleration, and magnetic field evolution, yet fundamental questions about the underlying physics and their time evolution remain unresolved. In this chapter, we review the current understanding of cluster merger shocks and their radio signatures, presenting the observational evidence from the SKA pathfinders and precursors, linking radio relics to shocks alongside outstanding theoretical challenges. We also consider related shock-influenced phenomena: radio phoenices from revived fossil AGN plasma and Gently Re-Energised Tails. Further, we outline the directions of investigation using the sensitivities of the SKA-Low and Mid complemented with X-ray observations that will allow us to make significant progress in understanding the cluster merger shocks. Detailed studies of individual targets in continuum and polarization and studies of populations of relics using wide surveys will both provide insights to the micro-physics and cosmic evolution of merger shocks. We present SKAO capabilities across staged deployments starting from AA0.5 to AA4 and identify science verification targets that will illuminate the physics of cluster merger shocks in the SKA era

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arXiv2026

uGMRT and MeerKAT observation of RXCJ0232-4420: a quiet cluster with a giant radio halo

Giant radio halos are the Mpc-scale extended sources associated with the merging clusters, while the mini-halos are preferentially associated with cool-core clusters. Both trace the ICM plasma physical process, and recent low-frequency observations increasingly blur the distinction between the two classes. We present the first multi-frequency spectral analysis of the galaxy cluster RXCJ0232--4420, hosting a cool core, using uGMRT (400 and 650 MHz) and MeerKAT (1283 MHz) observations. The central radio emission extends beyond $\sim 1$ Mpc at all frequencies, confirming it as a giant radio halo. One candidate relic (in the east) has also been detected, with an extent of $\sim 300$ kpc. The integrated spectral indices of halo and candidate east relic are $\alpha = -1.17 \pm 0.17$, and $\alpha = -0.85 \pm 0.17$, respectively. The resolved spectral map of the halo is mostly uniform ($-1.0$ to $-1.3$) and does not show any radial steepening. The radio surface brightness profile is well modelled by a single exponential law, with the e-folding radius constant across frequencies. The radio halo emission is morphologically well correlated with the thermal emission. Point-to-point radio-X-ray correlation analysis gives a sublinear relationship (slope $\sim 0.80$), with no frequency evolution. The presence of Mpc-scale emission in the cool-core cluster shows that such emission can arise in dynamically intermediate systems. Our results demonstrate that merger-driven turbulence, even from minor disturbances, can sustain cluster-wide particle re-acceleration without destroying the cool core.

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arXiv2026

Wiggling Through the ICM: Multi-Resolution Radio Imaging of a Tailed Radio Galaxy in MACS J1354.6+7715

Tailed radio galaxies are powerful tracers of interactions between active galactic nuclei (AGN) and the intracluster medium (ICM), providing unique insights into cluster dynamics. We present LOw Frequency ARray (LOFAR) 144 MHz and uGMRT 400 MHz observations of the cluster MACS J1354.6+7715 (z = 0.3967) to investigate the radio emission associated with its member galaxies and the cluster environment. The dominant tailed radio galaxy in the cluster exhibits a sharply bent tail extending over approximately 300 kpc, with the spectral index steepening from approximately -0.46 +/- 0.21 near the AGN core to approximately -2.43 +/- 0.30 in the outermost regions. Synchrotron modelling of the tail yields a radiative age of 150 +/- 10 Myr, implying a galaxy velocity of 1956 +/- 130 km s^-1, which is of order ~ 0.9 times the escape velocity. We find no evidence of relics or halos in our radio images, and the X-ray morphology from Chandra appears relatively undisturbed, suggesting that the system is a pre-merging candidate. Our results indicate that the radio galaxy is undergoing its first infall into the cluster, providing an excellent laboratory for studying the impact of the ICM on AGN activity and galaxy evolution, and demonstrating how multi-frequency radio observations of tailed galaxies can uniquely probe both AGN lifecycles and the early stages of cluster assembly.

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arXiv2026

Evidence of sloshing-driven mini-halo formation in the cool-core cluster RXCJ1558.3-1410

Radio mini-halos are perplexing features, typically hosted by X-ray cool-core galaxy clusters. Understanding the connection between thermal X-ray and non-thermal radio emission is key to uncovering their origin. Here, we present a multiwavelength study of the cool-core cluster RXCJ1558.3-1410 using archival Chandra X-ray and wideband uGMRT radio data (Bands 3, 4 and 5). Our improved analysis confirms a previously known X-ray cavity at $\sim$36 kpc south-east of the cluster centre and we report a new cavity at $\sim$42 kpc to the north-west. These cavities suggest that the AGN provides mechanical power of $\sim$$6.0 \times 10^{44}$ erg s$^{-1}$, sufficient to offset radiative cooling in the ICM. We also detect a sharp surface brightness edge at $\sim$72 kpc south-east of the centre, characterised by a temperature jump and pressure continuity, consistent with a cold front, likely caused by gas sloshing from a minor merger. Our uGMRT images reveals an interesting diffuse emission surrounding the brightest cluster galaxy (BCG), with its edge spatially coinciding with the sloshing cold front and roughly with the cooling radius. Furthermore, a low star formation rate and uniform metal abundance up to the sloshing edge are consistent with the earlier findings of suppression of star formation and metallicity homogenisation by mixing core gas through sloshing. Finally, the spatial correlation between the mini-halo and the observed X-ray features indicates that ICM sloshing, rather than AGN feedback, plays a dominant role in powering the proposed radio mini-halo emission.

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arXiv2026

Investigating quenching in Recently Quenched Elliptical galaxies with HI studies

Recently Quenched Elliptical galaxies (RQEs) represent a critical phase in the transition from star-forming to quiescent galaxies. However, the mechanisms driving their quenching remain elusive. We conduct a multi-wavelength analysis of 155 RQEs, along with their precursors (preRQEs) and descendants (postRQEs), focusing on their neutral hydrogen (HI) content and star formation properties. Contrary to conventional quenching models emphasizing gas depletion, RQEs retain substantial HI reservoirs ($f_{\text{gas}} \geq 17\%$), suggesting that quenching is not primarily driven by gas exhaustion. We identify a critical halo mass threshold at $\log M_{\text{halo}} = 12.1 M_{\odot}$, delineating different evolutionary pathways for RQEs. This threshold aligns with the transition from cold-mode to hot-mode gas accretion in theoretical models. RQEs in lower-mass halos ($\log M_{\text{halo}} < 12.1 M_{\odot}$) likely experience rapid quenching, possibly initiated by major mergers, followed by brief AGN activity and sustained LINER emission. We propose two evolutionary pathways: (a) rapid quenching via major mergers followed by AGN/LINER activity and passive evolution, and (b) rapid quenching followed by rejuvenation through minor mergers before evolving into more massive, long-term quenched ellipticals. These results challenge the conventional understanding of galaxy quenching, especially in low-density environments where RQEs typically reside. Our findings suggest that while RQEs may follow a rapid quenching pathway, their evolution is influenced by interactions between gas accretion modes, feedback mechanisms, and environmental factors. Future observations with advanced radio interferometers like SKA will be crucial for elucidating the quenching mechanisms in RQEs and their role in galaxy evolution.

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arXiv2025

Deep insights into Abell 2163: unveiling the treasure trove of ICM plasma physics

Nonthermal emission observed in galaxy clusters provides a direct probe into the plasma physics of the intra-cluster medium (ICM) under extreme conditions. We report the first detailed analysis of the giant radio halo in the merging galaxy cluster Abell 2163, using upgraded Giant Metrewave Radio Telescope (uGMRT) and Very Large Array (VLA) observations. Combining radio data (300-1400 MHz) with archival X-ray data offers a unique opportunity to study the complex ICM physics of the cluster. The sensitive uGMRT observations map the halo emission for the first time out to an extent of ~3.3 Mpc, up to $r_{500}$, and also effectively recover other diffuse sources. The radio surface brightness profile is well fitted with an exponential function up to r$_{500}$, with an evolution of e-folding radius over frequencies (larger at low frequencies). The spatially resolved spectral index map reveals fluctuations and outward radial steepening of the average spectral index. Radio and X-ray surface brightness are well correlated, with a correlation slope of $\sim$0.70 for the halo, and $\sim$0.40 for the ridge. The correlation slope varies from cluster centre to outskirts, suggesting the magnetic field and thermal gas density scaling relation (B $\propto n_{e}^{0.5}$) should be reassessed, provided that the re-acceleration efficiency is constant. We propose that diffuse lobes at the periphery could serve as a reservoir for seed electrons, behind the radio halo emission, with an estimated acceleration efficiency reaching $\sim$0.1% in the external regions. Additionally, a major E-W merger is suggested, leading to turbulence in the ICM and generating the halo.

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arXiv2025

Investigation of the Axe-shaped Radio Galaxy J1051+5523 with uGMRT

We present a multi-frequency study of the bent-tail radio galaxy J1051+5523, located in the galaxy cluster WHL J105147.4+552309. This wide-angle tail (WAT) galaxy exhibits a complex radio morphology, characterised by a right-angled bend in the northern jet, which resembles an axe, and multiple kinks in the southern jet, as observed in the deep uGMRT radio observations. The radio power of J1051+5523 at 150 MHz is estimated to be $2.91 \times 10^{25}\,\mathrm{W\,Hz^{-1}}$, placing it in the transition zone between FRI and FRII radio galaxies. The spectral index map reveals a flat core and relatively flat lobes, which may indicate ongoing particle acceleration or a relatively young population of relativistic electrons in the lobes. Further, we estimate the equipartition magnetic fields, and spectral ages of the northern and southern lobes to be approximately 150 Myr and 153 Myr, respectively, suggesting a long-lived radio source with sustained AGN activity. A relative velocity of 278 $\pm$ 2643 $\mathrm{km\,s^{-1}}$ is obtained for the host galaxy. Due to the large uncertainty associated with the relative velocity estimates, the contribution of ram pressure to the jet bending remains inconclusive. The low mass of the host cluster ($\sim 2 \times 10^{14}\,M_\odot$) and the lack of diffuse X-ray emission indicate a reduced likelihood of major mergers, but minor mergers or interactions remain possible. We propose that the observed WAT morphology of J1051+5523 is likely shaped by a combination of ram pressure and/or buoyant forces within the cluster environment.

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arXiv2025

A Linearly Polarized Merger Shock Down to 550 MHz: A uGMRT Study of the Merging Cluster Abell 746

Radio relics, arc-like polarized sources with highly aligned magnetic fields, are typically found on the outskirts of merging galaxy clusters. The magneto-ionic media responsible for the significant coherence observed in radio relics remain poorly understood. Low-frequency measurements of radio relics are essential for constraining depolarization models, which provide crucial insights into the magnetic field distribution. However, these measurements are challenging due to the emission properties and interferometer systematics. We have detected polarization signals from the northwest radio relic in Abell 746 at 650 MHz with the upgraded Giant Meterwave Radio Telescope, marking the first-ever detection of polarisation from radio relics below 1 GHz. At this frequency, the average Rotation Measure (RM) corrected magnetic fields align well with shock radio emission, typical of radio relics. The fractional polarization at 650 MHz is $\sim 18\pm4 \%$. Our results indicate that a single internal depolarization model cannot explain the observed depolarization spectra, suggesting a non-uniform magnetic field distribution or complex contribution of different polarized regions in the radio relic. Our detection of polarization signals at 650 MHz reveals critical insights into radio relic magnetic field structures, offering a low-frequency approach to understanding ICM magnetic fields in merging galaxy clusters.

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arXiv2025

A Multi-frequency Study of the Candidate Double-Double Radio Galaxy J2349-0003 with a Possible Misalignment

We present a multi-frequency analysis of the candidate double-double radio galaxy (DDRG) J2349-0003, exhibiting a possible lobe misalignment. High-resolution uGMRT observations at Bands 3 and 4 reveal a complex radio morphology featuring a pair of inner and outer lobes, and the radio core, while the Band 5 image detects the core and the compact components. The positioning of both pairs of lobes with the central core supports its classification as a DDRG. Spectral age estimates for the inner and outer lobes indicate two distinct episodes of active galactic nucleus (AGN) activity interspaced by a short quiescent phase. The possible compact steep spectrum nature of the core, together with its concave spectral curvature, suggests ongoing or recent jet activity, suggesting the possibility that J2349-0003 may be a candidate triple-double radio galaxy. With a projected linear size of 1.08 Mpc, J2349-0003 is classified as a giant radio galaxy (GRG), although its moderate radio power (~10^24 W/Hz) suggests a sparse surrounding environment. Arm-length (R_theta) and flux density ratios (R_S) indicates environmental influences on source symmetry. The observed lobe misalignment and the presence of nearby galaxies in the optical image suggest that merger-driven processes may have played a key role in shaping the source's evolution.

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arXiv2025

Real-time Broadband RFI Excision for the Upgraded GMRT

A real-time broadband RFI mitigation system has been developed for the Upgraded GMRT (uGMRT). Here, we report the engineering, imaging, and beam/pulsar results from the test observations in all the bands of the uGMRT during the monsoon of 2018 and 2019. Broadband power-line RFI is almost always present in uGMRT bands 2 (120 - 240 MHz), 3 (300 - 500 MHz) and 4 (550 - 850 MHz), but the monsoon period is chosen to understand the performance in the worst RFI situations arising due to an increase in the wind-speed and moisture.

astro-ph.IM↗

arXiv2025

MeerKAT HI observations of Low Surface Brightness/Ultradiffuse Galaxy Candidates Projected around Two Southern Loose Groups

A large catalogue of low surface brightness galaxies (LSBGs) from the Dark Energy Survey showed significant clustering around nearby galaxy groups and clusters. Using the HIPASS survey, we tried to determine the redshift of a sub-sample of these LSBGs and determine whether they were members of the groups they were projected near, but this was hampered by HIPASS's high spectral rms. This letter reports on MeerKAT H I observations to determine the redshifts of 52 LSBG candidates projected in the vicinity of two groups from our previous HIPASS study. The main goal is to investigate and ascertain whether these LSBGs are genuine group members. H I was detected with MeerKAT and redshifts were determined for only five of the 52 candidates within a velocity range of $\pm$ 2500 km/s of their respective group velocities. All five H I detections were blue LSBGs and two of them were confirmed to be ultradiffuse galaxies (UDGs). Both these UDGs were group members, while the other three detections were either foreground or background galaxies. In this letter we explore scenarios that can explain the 90% non-detection. MeerKAT's excellent sensitivity allows us to conclude that the majority of the non-detected candidates, particularly the blue galaxies, are not group members but lie at higher redshifts. However, this still leaves the open question as why Tanoglidis LSBG candidates, in particular the red ones, appear to be clustered in projection around nearby groups.

astro-ph.CO↗

arXiv2025

Discovery of a radio megahalo in the cluster PLCKG287.0+32.9 using the uGMRT

We report the discovery of a radio megahalo in the merging cluster PLCKG287.0+32.9, based on upgraded Giant Metrewave Radio telescope (uGMRT) observations at frequencies 300-850 MHz. The sensitive radio observations provide a new window to study the complex physics occurring in this system. Apart from significant detections of the known diffuse radio emission in the cluster, we detect the central diffuse emission to a much larger extent of $\sim$ 3.2 Mpc, reaching the R$_{500}$ of the cluster. The radial surface brightness profile shows a distinct flattening beyond $\sim$0.5R$_{500}$, dividing the emission into inner and outer components. This outer envelope shows a steep spectral index ($\lesssim$ -1.5) and, emissivity $\sim 20$ times lower than the inner component, confirming the megahalo characteristics. The radial profile of the spectral index also distinguishes the steep spectrum megahalo emission. Our observational results align with recent numerical simulations, showing megahalo emission oriented along the merger axis and the re-acceleration of electrons driven by late-stage merger-induced turbulence. This is the first detection of a radio megahalo at a frequency higher than the LOFAR 144 MHz, opening the possibilities for more discoveries and spectral studies to understand their origin.

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arXiv2025

Real-time RFI Excision Techniques and their Limitations

Contemporary real-time RFI mitigation is carried out at different stages primarily using regulatory and technical approaches. Regulatory approaches include spectrum management, radio quiet zones, and ensuring protection from self-generated RFI. The technical approaches include mitigation RFI in the analog and RF frontend systems, digital signal processing systems, and offline systems. As is known, the signal received by a radio telescope is a combination of the contributions from the astronomical signal and a combination of system and sky background noise. RFI has an additive effect on the signal received by the radio telescope. The key distinguishing properties of RFI are that it is generally stronger than the signal and non-Gaussian. The capability of signal processing receiver systems has grown manifold with the advent of high-speed signal processing platforms like Field Programmable Gate Arrays (FPGA)and Graphics Processing Unit (GPU). This has enabled the development of different signal-processing techniques for real-time RFI mitigation algorithms. This document provides an overview of contemporary techniques while focusing on the implementation of the same in specific radio telescopes.

astro-ph.IM↗

arXiv2024

An upgraded GMRT and MeerKAT study of radio relics in the low mass merging cluster PSZ2 G200.95-28.16

Diffuse radio sources known as radio relics are direct tracers of shocks in the outskirts of merging galaxy clusters. PSZ2 G200.95-28.16, a low-mass merging cluster($\textrm{M}_{500} = (2.7 \pm 0.2) \times 10^{14}~\mathrm{M}_{\odot}$) features a prominent radio relic, first identified by Kale et al. 2017. We name this relic as the Seahorse. The MeerKAT Galaxy Cluster Legacy Survey has confirmed two additional radio relics, R2 and R3 in this cluster. We present new observations of this cluster with the Upgraded GMRT at 400 and 650 MHz paired with the Chandra X-ray data. The largest linear sizes for the three relics are~1.53 Mpc, 1.12~kpc, and 340~kpc. All three radio relics are polarized at 1283~MHz. Assuming the diffusive shock acceleration model, the spectral indices of the relics imply shock Mach Numbers of $3.1 \pm 0.8$ and $2.8 \pm 0.9$ for the Seahorse and R2, respectively. The Chandra X-ray surface brightness map shows two prominent subclusters, but the relics are not perpendicular to the likely merger axis as typically observed; no shocks are detected at the locations of the relics. We discuss the possible merger scenarios in light of the low mass of the cluster and the radio and X-ray properties of the relics. The relic R2 follows the correlation known in the radio relic power and cluster mass plane, but the Seahorse and R3 relics are outliers. We have also discovered a radio ring in our 650~MHz uGMRT image that could be an Odd radio circle candidate.

astro-ph.GA↗

arXiv2024

The radio halo in PLCKESZ G171.94 $-$ 40.65: Beacon of merging activity

We present the first multi-frequency analysis of the candidate ultra-steep spectrum radio halo in the galaxy cluster PLCKESZ G171.94$-$40.65, using the upgraded Giant Metrewave Radio telescope (uGMRT; 400 MHz), and Karl G. Jansky Very Large Array (JVLA; 1-2 GHz) observations. Our radio data have been complemented with archival \textit{Chandra} X-ray observations to provide a crucial insight into the complex intracluster medium (ICM) physics, happening at large scales. We detect the radio halo emission to the extent of $\sim$ 1.5 Mpc at 400 MHz, significantly larger than previously reported, along with five tailed galaxies in the central region. We also report the discovery of an unknown diffuse source 'U', at the cluster periphery, with an extent of 300 kpc. Using the available observations, we have found that the radio spectrum of the halo is well-fitted with a single power law, having a spectral index of $-1.36 \pm 0.05$, indicating that it is not an ultra-steep spectrum radio halo. Our low-resolution (25$''$) resolved spectral map shows an overall uniform spectral index, with some patches of fluctuations. The X-ray and radio surface brightness are morphologically co-spatial, with a slight extension along the northwest-southeast direction, seen in both maps. The radio and X-ray surface brightness indicates strong positive correlations, with sub-linear correlation slopes ($\sim$ 0.71). Multiple tailed galaxies and the radio halo indicate a high dynamical activity at the cluster central region.

astro-ph.CO↗

arXiv2023

A new enigmatic radio relic in the low mass cluster Abell 2108

We report the discovery of a radio relic in the northeastern periphery of the cluster Abell 2108 (A2108). A2108 is a part of the uGMRT LOw-MAss Galaxy Cluster Survey (GLOMACS), where our main aim is to search for diffuse radio emission signatures in very sparsely explored low-mass galaxy clusters using uGMRT band-3 (central frequency 400 MHz). We used our uGMRT band-3 data along with the existing archival band-3 uGMRT data to improve image sensitivity. Along with the previously reported southwestern relic, the discovery of the new relic makes A2108 one of the few low-mass clusters hosting double relics. The new relic spans over a region of 610 kpc $\times$ 310 kpc and, interestingly, differs considerably in size and morphology from the other relic. With XMM-Newton science archive data, we also report the tentative detection of a mildly supersonic shock of Mach number $\mathcal{M}_\mathrm{SB}=1.42$ and $\mathcal{M}_\mathrm{T} = 1.43$ from the surface brightness and temperature discontinuities, respectively near this newly found relic. Both the relics in A2108 are found to be significantly under-luminous compared to other double relic systems in the mass-luminosity plane. Though mild supersonic shocks resulting from an off-axis merger could have influenced their origin, we hypothesize that further local environments have played a crucial role in shaping their morphology.

astro-ph.HE↗