• Ruta Kale

      Articles written in Journal of Astrophysics and Astronomy

    • Discovery of a Giant Radio Halo in a Massive Merging Cluster at 𝑧 = 0.443

      K. S. Dwarakanath Siddharth Malu Ruta Kale

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      We have discovered a giant radio halo in the massive merging cluster MACSJ0417.5-1154. This cluster, at a redshift of 0.443, is one of the most X-ray luminous galaxy cluster in the MAssive Cluster Survey (MACS) with an X-ray luminosity in the 0.1–2.4 keV band of 2.9 × 1045 erg s-1. Recent observations from GMRT at 230 and 610 MHz have revealed a radio halo of ∼ 1.2 × 0.3 Mpc2 in extent. This halo is elongated along the North-West, similar to the morphology of the X-ray emission from Chandra. The 1400 MHz radio luminosity (𝐿r) of the halo is ∼ 2 × 1025 W Hz-1, in good agreement with the value expected from the 𝐿x - 𝐿r correlation for cluster halos.

    • Double Relics in the Outskirts of A3376: Accretion Flows Meet Merger Shocks?

      Ruta Kale K. S. Dwarakanath Joydeep Bagchi Surajit Paul

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      The case of spectacular ring-like double radio relics in the merging, rich galaxy cluster A3376 is of great interest to study non-thermal phenomena at cluster outskirts.We present the first low frequency (330 and 150 MHz) images of the double relics using the GMRT. With our GMRT 330 MHz map and the VLA 1400 MHz map (Bagchi et al. 2006), we have constructed and analyzed the distribution of spectral indices over the radio relics. We find flat spectral indices at the outer edges of both the relics and a gradual steepening of spectral indices toward the inner regions. This supports the model of outgoing merger shock waves. The eastern relic has a complex morphology and spectral index distribution toward the inner region. This will be discussed in the context of the effect of large-scale accretion flows on the outgoing merger shocks as reported in the recent simulations.

    • A Zoo of Radio Relics: Cluster Cores to Filaments

      Ruta Kale K. S. Dwarakanath

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      Radio relics in galaxy clusters can be electrons accelerated at cluster merger shocks or adiabatically compressed fossil radio cocoons or dying radio galaxies. The spectral evolution of radio relics is affected by the surrounding thermal plasma. We present a low frequency study of three radio relics representing environments of dense cluster core (A4038), cluster outskirts (A1664) and filaments (A786). The properties of the relics are found to be consistent with the effect of confinement by external medium if the effects of projection are ignored.

    • Clusters of Galaxies and the Cosmic Web with Square Kilometre Array

      Ruta Kale K. S. Dwarakanath Dharam Vir Lal Joydeep Bagchi Surajit Paul Siddharth Malu Abhirup Datta Viral Parekh Prateek Sharma Mamta Pandey-Pommier

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      The intra-cluster and inter-galactic media that pervade the large scale structure of the Universe are known to be magnetized at sub-micro Gauss to micro Gauss levels and to contain cosmic rays. The acceleration of cosmic rays and their evolution along with that of magnetic fields in these media is still not well understood. Diffuse radio sources of synchrotron origin associated with the Intra-Cluster Medium (ICM) such as radio halos, relics and mini-halos are direct probes of the underlying mechanisms of cosmic ray acceleration. Observations with radio telescopes such as the Giant Metrewave Radio Telescope, the Very Large Array and the Westerbork Synthesis Radio Telescope have led to the discoveries of about 80 such sources and allowed detailed studies in the frequency range 0.15–1.4 GHz of a few. These studies have revealed scaling relations between the thermal and non-thermal properties of clusters and favour the role of shocks in the formation of radio relics and of turbulent re-acceleration in the formation of radio halos and mini-halos. The radio halos are known to occur in merging clusters and mini-halos are detected in about half of the cool-core clusters. Due to the limitations of current radio telescopes, low mass galaxy clusters and galaxy groups remain unexplored as they are expected to contain much weaker radio sources. Distinguishing between the primary and the secondary models of cosmic ray acceleration mechanisms requires spectral measurements over a wide range of radio frequencies and with high sensitivity. Simulations have also predicted weak diffuse radio sources associated with filaments connecting galaxy clusters. The Square Kilometre Array (SKA) is a next generation radio telescope that will operate in the frequency range of 0.05–20 GHz with unprecedented sensitivities and resolutions. The expected detection limits of SKA will reveal a few hundred to thousand new radio halos, relics and mini-halos providing the first large and comprehensive samples for their study. The wide frequency coverage along with sensitivity to extended structures will be able to constrain the cosmic ray acceleration mechanisms. The higher frequency (>5 GHz) observations will be able to use the Sunyaev–Zel’dovich effect to probe the ICM pressure in addition to tracers such as lobes of head–tail radio sources. The SKA also opens prospects to detect the ‘off-state’ or the lowest level of radio emission from the ICM predicted by the hadronic models and the turbulent re-acceleration models.

    • Active Galactic Nucleus Feedback with the Square Kilometre Array and Implications for Cluster Physics and Cosmology

      Asif Iqbal Ruta Kale Subhabrata Majumdar Biman B. Nath Mahadev Pandge Prateek Sharma Manzoor A. Malik Somak Raychaudhury

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      Active Galactic Nuclei (AGN) feedback is regarded as an important non-gravitational process in galaxy clusters, providing useful constraints on large-scale structure formation. It modifies the structure and energetics of the intra-cluster medium (ICM) and hence its understanding is crucially needed in order to use clusters as high precision cosmological probes. In this context, particularly keeping in mind the upcoming high quality radio data expected from radio surveys like Square Kilometre Array (SKA) with its higher sensitivity, high spatial and spectral resolutions, we review our current understanding of AGN feedback, its cosmological implications and the impact that SKA can have in revolutionizing our understanding of AGN feedback in large-scale structures. Recent developments regarding the AGN outbursts and its possible contribution to excess entropy in the hot atmospheres of groups and clusters, its correlation with the feedback energy in ICM, quenching of cooling flows and the possible connection between cool core clusters and radio mini-halos, are discussed. We describe current major issues regarding modeling of AGN feedback and its impact on the surrounding medium. With regard to the future of AGN feedback studies, we examine the possible breakthroughs that can be expected from SKA observations. In the context of cluster cosmology, for example, we point out the importance of SKA observations for cluster mass calibration by noting that most of z>1 clusters discovered by eROSITA X-ray mission can be expected to be followed up through a 1000 hour SKA1-mid programme. Moreover, approximately 1000 radio mini halos and ∼2500 radio halos at z<0.6 can be potentially detected by SKA1 and SKA2 and used as tracers of galaxy clusters and determination of cluster selection function.

    • Real-time RFI filtering for uGMRT: Overview of the released system and relevance to the SKA


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      Radio Frequency Interference (RFI) of impulsive nature is created by sources like sparking on high-power transmission lines due to gap or corona discharge and automobile sparking, and it affects the entire observing frequency bands of low-frequency radio telescopes. Such RFI is a significant problem at theUpgraded Giant Metrewave Radio Telescope (uGMRT). A real-time RFI filtering scheme has been developed and implemented to mitigate the effect on astronomical observations. The scheme works in real-time on precorrelationdata from each antenna and allows the detection of RFI based on median absolute deviation statistics. The samples are identified as RFI-based on user-defined thresholds and are replaced by digital noise, a constant or zeros. We review the testing and implementation of this system at the uGMRT. We illustrate the effectiveness of the filtering for continuum, spectral line and time-domain data. The real-time filter is released for regular observations in the bands falling in 250–1450 MHz, and recent observing cycles show growing usage. Further,we explain the relevance of the released system to the Square Kilometer Array (SKA) receiver chain and possible ways of implementation to meet the computational requirements.

    • Exploring diffuse radio emission in galaxy clusters and groups with uGMRT and SKA


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      Diffuse radio emission has been detected in a considerable number of galaxy clusters and groups, revealing the presence of pervasive cosmic magnetic fields, and of relativistic particles in the large scale structure of the Universe. Since the radio emission in galaxy systems is faint and its spectrum is steep, itsobservations are largely limited by the instrument sensitivity and frequency of observation, leading to a dearth of information, more so for lower-mass systems. The recent commissioning or upgrade of several large radio telescope arrays, particularly at the low frequency bands (<GHz) is, therefore, a significant step forward. The unprecedented sensitivity of these new instruments, aided by the development of advanced calibration and imaging techniques, have helped in achieving unparalleled image quality and revolutionised the study of cluster-scale radio emission. At the same time, the development of state-of-the-art numerical simulations and the availability of supercomputing facilities have paved the way for high-resolution numerical modelling of radio emission, and the structure of the cosmic magnetic fields, associated with large-scale structures inthe Universe, leading to predictions matching the capabilities of observational facilities. In view of these rapidly-evolving developments in modeling and observations, in this review, we summarise the role of new telescopearrays and the development of advanced imaging techniques and discuss the range of detections of various kinds of cluster radio sources, both in dedicated surveys as well as in numerous individual studies. We pay specific attention to the kinds of diffuse radio structures that have been able to reveal the underlying physics in recent observations. In particular, we discuss observations of large-scale sections of the cosmic web in the form of supercluster filaments, and studies of emission in low-mass systems, such as poor clusters and groups ofgalaxies, and of ultra-steep spectrum sources, the last two being notably aided by low-frequency observations and high sensitivity of the instruments being developed. We also discuss and review the current theoreticalunderstanding of various diffuse radio sources in clusters and the associated magnetic field and polarisation in view of the current observations and simulations. As the statistics of detections improve along with our theoretical understanding, we update the source classification schemes based on the intrinsic properties of these sources. We conclude by summarising the role of the upgraded GMRT (uGMRT) and our expectations from the upcoming Square Kilometre Array (SKA) observatories.

  • Journal of Astrophysics and Astronomy | News

    • Continuous Article Publication

      Posted on January 27, 2016

      Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

    • Editorial Note on Continuous Article Publication

      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

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