arXiv subjects

Pratik Dabhade

Publications and source records attributed to Pratik Dabhade.

At least 19 recordsLinked to original sources

arXiv2026

Tracing the Cosmic web across Cosmic time through SKA observations of radio galaxies

The Square Kilometre Array will transform studies of the cosmic web by tracing radio galaxies (RGs) and star-forming systems across cosmic time with unprecedented sensitivity, angular resolution, frequency coverage, and survey speed. Powered by accreting supermassive black holes, RGs are not only signposts of AGN feedback but also incisive probes of their environments, from dense clusters to the low-density intergalactic medium. Their lobes, magnetic fields, and energy outflows encode the thermal and non-thermal histories of the surrounding gas, offering diagnostics of IGM pressure, particle ageing, and magnetisation over megaparsec scales. With its broad frequency coverage (50 MHz-15 GHz), microJy to sub-microJy continuum sensitivity, and wide field of view, the SKA will detect vast radio-source populations across broad ranges of redshift and environment. Measurements of source size, morphology, spectral ageing, radio power, polarisation, and Faraday rotation will reveal how the environment regulates jet propagation and lobe evolution, how radio plasma heats and magnetises the intracluster and intergalactic media, and how early AGN activity influences galaxy growth and star formation in protoclusters. Combined with host identifications, spectroscopic redshifts, and optical, infrared, X-ray, Sunyaev--Zel'dovich, and cosmic-web catalogues, SKA observations will place RGs within their three-dimensional large-scale environments. This chapter presents a framework for using RGs to trace and probe the cosmic web, from nearby filaments and clusters to high-redshift protoclusters, and to test how environment, magnetic fields, feedback, and gas dynamics shape radio-galaxy evolution, protocluster assembly, and star formation across cosmic time.

astro-ph.GA↗

arXiv2026

Citizen Science Research with the Square Kilometre Array Observatory (SKAO)

Over the past two decades, internet-enabled citizen science research (CSR) has contributed to significant discoveries while involving millions of people in the research process. Our review highlights CSR in extragalactic radio astronomy and emphasises that such approaches will become increasingly relevant across radio astronomy in the era of the Square Kilometre Array (SKA). As astronomical data volumes grow, CSR is converging with Artificial Intelligence and Machine Learning (AI/ML), creating hybrid human-machine frameworks suited to big-data challenges. Two CSR platforms, Radio Galaxy Zoo and RAD@home, demonstrate success: the former excels in large-scale, web-based catalogue creation, while the latter combines structured training with collaborative discovery. Following this, we propose CSR with the SKA, namely SKA@home, with two modes: one purely web-based and the other in collaboratory mode with national training programmes. We argue that CSR can complement, and at times surpass, automated AI/ML pipelines, particularly in identifying rare, intricate, or unexpected features. Illustrative CSR discoveries include an episodic wide-angle-tailed radio galaxy, a jet-galaxy interaction, a collimated synchrotron thread, a twin-ring odd radio circle, and a large-scale shock ahead of a cluster-infalling galaxy. Consistent with the IAU's recognition of CSR as a driver of Astronomy for Development and the United Nations' affirmation of participation in science as a universal human right, both the SKA construction proposal and outreach strategies show commitment to enabling CSR with SKA. The proposed SKA@home would not only enhance the early discovery potential of SKA data but also initiate a deeper and more meaningful connection with society at large.

astro-ph.GA↗

arXiv2026

RAD@home discovery of a bow-and-arrow radio galaxy tracing a ~560 kpc bow-shock structure in a multi-halo environment

We report the RAD@home citizen science discovery of a unique bow-and-arrow-shaped radio galaxy (BAARG; RAD J104501.6+352852, z = 0.159) identified in LoTSS DR2. The source exhibits striking asymmetry: on the western side, a narrow jet feeds a sector-shaped emission region at ~115 kpc, extending backward to form a ~560 kpc arc-like structure; on the eastern side, the jet develops an S-shaped distortion extending to ~250 kpc, followed by a faint, offset tail reaching ~600 kpc. Our analysis shows that the host resides in a dynamically complex, multi-halo environment with nearby cluster-scale systems at similar redshifts. The observed morphology is consistent with interaction between the radio plasma and the surrounding medium, influenced by large-scale environmental gradients and bulk motions. The western structure is consistent with compression of radio plasma near a bow-shock-like feature, possibly linked to supersonic motion of the infalling host galaxy and its circumgalactic medium. This possibly represents one of the first instances in which morphology and environment together suggest signatures of infall- or shock-related processes; surveys such as LoTSS DR3 may reveal similar systems, offering new insights into the interplay between radio galaxies and their large-scale environments.

astro-ph.GA↗

arXiv2026

From DES to KiDS: Domain adaptation for cross-survey detection of low-surface-brightness galaxies

Low-surface-brightness galaxies (LSBGs) are vital for understanding galaxy formation, but their diffuse nature makes them challenging to detect. Upcoming large-scale surveys are expected to uncover large numbers of LSBGs, requiring robust automated methods to identify them across heterogeneous datasets. As a precursor to the Legacy Survey of Space and Time (LSST) and Euclid, we explore domain adaptation techniques for cross-survey LSBG identification. Using models trained on the Dark Energy Survey (DES), we search for LSBGs in the Kilo-Degree Survey Data Release 5 (KiDS DR5). We used an ensemble consisting of one convolutional neural network (CNN) and two transformer models trained on DES cutouts and applied to KiDS DR5 imaging data. Structural parameters were estimated with galfitm, and photometric redshifts and stellar population properties were estimated through spectral energy distribution fitting with CIGALE. We identify 20,180 LSBGs and 434 ultra-diffuse galaxies (UDGs) in KiDS DR5. Their structural parameters are similar to known LSBGs from DES and the Hyper Suprime-Cam SSP Survey (HSC-SSP). The KiDS-LSBGs follow a continuous size-luminosity relation connecting classical dwarf galaxies and UDGs, and their colours are bimodal ($\sim73\%$ blue, $\sim27\%$ red). Cross-matching with spectroscopic and cluster catalogues provides redshifts for 4,913 systems, enabling a systematic characterisation of the star-forming main sequence of LSBGs. Strong environmental trends are evident, with cluster LSBGs and UDGs exhibiting redder colours and reduced star formation compared to non-cluster systems. We demonstrate that domain adaptation enables robust cross-survey LSBG identification with deep learning models, providing a scalable pathway for constructing homogeneous LSBG catalogues for the LSST and Euclid era.

astro-ph.GA↗

arXiv2025

SAGAN-VI: When Jets Meet Filaments -- Environmental Imprints on the Growth of Giant Radio Galaxies

Giant radio galaxies (GRGs) represent the largest individual astrophysical structures, rivalling galaxy clusters in physical extent. Understanding how they attain such scales demands examining their large scale cosmic surroundings, particularly the under explored filament environment. We quantify the three dimensional (3D) distance of GRGs from the nearest filament spine; test how this distance correlates with their growth and formation of different morphological classes; assess whether their radio jets exhibit preferred orientations relative to filament axes; and examine how filament anisotropy from spine to periphery modulates radio morphology. We employed a filament catalogue from the SDSS together with the largest GRG catalogue currently available. For each source, we measured the comoving distance to the nearest filament spine, the projected jet spine orientation angle, and quantified lobe asymmetry via the arm length ratio (ALR). These metrics trace proximity, directionality, and the impact of filamentary environment on morphology. We then compared GRGs with a control sample of small radio galaxies (SRGs) to constrain the environmental factors that regulate the attainment of giant sizes. We validated the robustness of our results via bootstrap resampling and non parametric statistical tests. Our results show that GRGs and SRGs have similar filament occupancy. By contrast, GRGs preferentially display larger alignment angles relative to filament spines, while SRG orientations are consistent with a random distribution. GRGs further show enhanced morphological asymmetry, reflected in lower ALR values than SRGs. Attainment of giant sizes is not governed by proximity to filaments; rather, it correlates with jet filament alignment. Abridged.

astro-ph.GA↗

arXiv2025

A Deep Chandra View of Abell 2597: Bubbles, Shocks, Cold Fueling, and a Plasma Depletion Layer

To examine how AGN feedback shapes the intracluster medium (ICM) and fuels black hole accretion in the cool-core galaxy cluster Abell 2597, we present deep ($\sim$600 ks) Chandra X-ray observations complemented by archival GMRT radio and SINFONI near-infrared data. Radio-mode AGN activity has inflated seven X-ray cavities and driven one to three potential weak shocks ($M \sim 1.05-1.14$) extending to $\sim 150$ kpc, suggesting recurrent outbursts occurring on $\sim 10^7$ year timescales. We also detect a narrow, $\sim$57 kpc X-ray surface brightness deficit-a potential plasma depletion layer-likely shaped by residual sloshing motions that amplified magnetic fields and/or displaced gas within the cluster core. Although the AGN injects $\sim 10^{44}$ erg s$^{-1}$ of energy, comparable to the cluster's cooling luminosity, radiative cooling persists at $\sim$15 M$_{\odot} $yr$^{-1}$, replenishing the billion solar mass cold gas reservoir at the heart of the brightest cluster galaxy. Sustaining this level of activity requires a continuous fuel supply, yet the estimated Bondi accretion power ($\sim 2 \times 10^{43}$ erg s$^{-1}$) falls an order of magnitude short of the observed cavity power, suggesting that "hot" gas fueling is insufficient. Instead, archival ALMA observations continue to support a chaotic cold accretion scenario, where turbulence-driven condensation fuels the AGN at rates exceeding Bondi accretion, sustaining a self-regulated feedback cycle that repeatedly shapes the core of Abell 2597.

astro-ph.GA↗

arXiv2025

RAD@home discovery of extragalactic radio rings and odd radio circles: clues to their origins

We present three rare and striking extragalactic radio sources discovered through visual inspection of low-frequency continuum maps from LoTSS DR2 and TGSS by the RAD@home citizen-science collaboratory. The first, RAD J131346.9+500320, is the first clear Odd Radio Circle (ORC) identified in LoTSS. At photometric $z \sim$ 0.94, it hosts a pair of intersecting rings of ~300 kpc diameter, embedded in diffuse emission extending over ~800 kpc, making it both the most distant and most powerful ORC reported to date. Its steep spectrum $α_{54}^{144}=1.22\pm0.15$) points to a relic synchrotron origin. The second object, RAD J122622.6+640622, is a ~865 kpc giant radio galaxy whose southern jet is abruptly deflected, inflating a ~100 kpc limb-brightened ring, while the northern jet terminates in a compact hotspot-like feature. The third, RAD J142004.0+621715 (~440 kpc), shows a comparable ring at the end of its northern filamentary jet, along with a secondary filament parallel to its southern jet. All three systems lie in $\sim10^{14}M_\odot$ clusters or group-scale haloes, suggesting that environmental density gradients and possible jet-galaxy interactions play a central role in shaping these ring morphologies. These discoveries expand the zoo of extragalactic radio morphologies, highlight the diversity of pathways that can generate ring-like synchrotron structures, and demonstrate the continuing importance of human pattern recognition in identifying rare sources that escape current automated pipelines.

astro-ph.GA↗

arXiv2025

RAD@home citizen science discovery of two spiral galaxies where the 30-220 kpc radio lobes are possibly shaped by ram pressure stripping

We report the RAD@home citizen science discovery of two rare spiral-host radio galaxies (NGC 3898 and WISEA J221656.57-132042434.1 or RAD-"Thumbs up" galaxy), both exhibiting asymmetric radio lobes extending over 30 to 220 kiloparsec scales. We present a multi-wavelength image analysis of these two sources using radio, optical, and ultraviolet data. Both host galaxies are young, star-forming systems with asymmetric or distorted stellar disks. These disks show similarities to those in galaxies undergoing ram pressure stripping, and the radio morphologies resemble those of asymmetric or bent FR-II and wide-angle-tailed radio galaxies. We suggest that non-uniform gas density in the environment surrounding the ram pressure-stripped disks may contribute to the observed asymmetry in the size, shape, and brightness of the bipolar radio lobes. Such environmental effects, when properly accounted for, could help explain many of the non-standard radio morphologies observed in Seyfert galaxies and in recently identified populations of galaxies with galaxy-scale radio jets, now being revealed through deep and sensitive radio surveys with uGMRT, MeerKAT, LOFAR, and, in the future, the SKAO. These findings also underscore the potential of citizen science to complement professional research and data-driven approaches involving machine learning and artificial intelligence in the analysis of complex radio sources.

astro-ph.GA↗

arXiv2025

Search and analysis of giant radio galaxies with associated nuclei (SAGAN) V. Study of giant double-double radio galaxies from LoTSS DR2

To test the hypothesis that megaparsec-scale giant radio galaxies (GRGs) experience multiple epochs of recurrent activity leading to their giant sizes and to understand the nature of double-double radio galaxies (DDRGs), we have built the largest sample of giant DDRGs from the LOFAR Two Metre Sky Survey (LoTSS) data release 2. This sample comprises 111 sources, including 76 newly identified DDRGs, with redshifts ranging from 0.06 to 1.6 and projected sizes between 0.7 Mpc and 3.3 Mpc. We conducted a detailed analysis to characterise their properties, including arm-length ratios, flux density ratios of pairs of lobes, and misalignment angles. These measurements allow us to study the symmetry parameters, which are influenced by the immediate and large-scale environments of DDRGs. Our study shows that based on the observed asymmetries of the inner lobes, the cocoons in which the inner lobes of DDRGs grow are often (~26%) asymmetrically contaminated with surrounding material from the external medium. Our analysis also reveals highly misaligned DDRGs, which could be due to environmental factors and/or changes in the supermassive black hole jet ejection axes. By studying the misalignment angles, we assess the stability of the jets in these systems in relation to their environment. For the first time, we systematically characterised the large-scale environments of DDRGs, identifying their association with dense galaxy clusters and revealing the influence of cluster weather on their morphologies. We discovered a DDRG in a distant galaxy cluster at z ~ 1.4, confirming that dynamic cluster environments can induce significant misalignment in DDRGs, consistent with simulations. Additionally, we identified two gigahertz peaked-spectrum (GPS) candidates in DDRG cores and a potential triple-double candidate, which, if confirmed, would be the fifth known case. Abridged.

astro-ph.GA↗

arXiv2025

Study of giant radio galaxies using spectroscopic observations from the Himalayan Chandra Telescope

We present the results of spectroscopic observations of host galaxies of eleven candidate giant radio galaxies (GRGs), powered by active galactic nuclei (AGNs), conducted with the 2-m Himalayan Chandra Telescope (HCT). The primary aim of these observations, performed with the Hanle Faint Object Spectrograph Camera (HFOSC), was to secure accurate spectroscopic redshifts, enabling precise calculations of their projected linear sizes. Based on these measurements, we confirm all eleven sources as giants, with linear sizes ranging from 0.7 to 2.9 Mpc, including ten GRGs and one giant radio quasar (GRQ). One of the GRGs shows evidence of a potential AGN jet-driven ionized outflow, extending up to $\sim$12 kpc, which, if confirmed, would represent a rarely observed feature. Two of the confirmed GRGs exceed 2 Mpc in size, which are relatively rare examples of GRG. The redshifts of the host galaxies span 0.09323 $\leq$ z $\leq$ 0.41134. Using the obtained spectroscopic data, we characterised their AGN states based on the optical emission line properties. To complement these observations, archival radio and optical survey data were utilised to characterise their large-scale radio morphology and estimate projected linear sizes, arm-length ratios, flux densities, luminosities, and core dominance factors. These results provide new insights into the properties of GRSs and form a critical foundation for further detailed studies of their environments, AGN activity, and evolution using future high-sensitivity optical and radio datasets.

astro-ph.GA↗

arXiv2024

Ten years of searching for relics of AGN jet feedback through RAD@home citizen science

Understanding the evolution of galaxies cannot exclude the important role played by the central supermassive black hole and the circumgalactic medium (CGM). Simulations have strongly suggested the negative feedback of AGN Jet/wind/outflows on the ISM/CGM of a galaxy leading to the eventual decline of star formation. However, no "smoking gun" evidence exists so far where relics of feedback, observed in any band, are consistent with the time scale of a major decline in star formation, in any sample of galaxies. Relics of any AGN-driven outflows will be observed as a faint and fuzzy structure which may be difficult to characterise by automated algorithms but trained citizen scientists can possibly perform better through their intuitive vision with additional heterogeneous data available anywhere on the Internet. RAD@home, launched on 15th April 2013, is not only the first Indian Citizen Science Research (CSR) platform in astronomy but also the only CSR publishing discoveries using any Indian telescope. We briefly report 11 CSR discoveries collected over the last eleven years. While searching for such relics we have spotted cases of offset relic lobes from elliptical and spiral, episodic radio galaxies with overlapping lobes as the host galaxy is in motion, large diffuse spiral-shaped emission, cases of jet-galaxy interaction, kinks and burls on the jets, a collimated synchrotron thread etc. Such exotic sources push the boundaries of our understanding of classical Seyferts and radio galaxies with jets and the process of discovery prepares the next generation for science with the upgraded GMRT and Square Kilometre Array Observatory (SKAO).

astro-ph.GA↗

arXiv2024

Search and analysis of giant radio galaxies with associated nuclei (SAGAN) IV. Interplay with the Supercluster environment

We investigated the prevalence of giant radio galaxies (GRGs), some of the largest structures powered by supermassive black holes, within supercluster environments, and the influence of such environments on their properties. Utilising two large catalogues of superclusters (401) and GRGs (1446), we established the existence of 77 GRGs (5.3%) residing in 64 superclusters (16%) within $\rm 0.05 \leq z \leq 0.42$. Among the 77 GRGs found in superclusters, we identified $\sim$70% as residing within galaxy clusters. Within the subset of GRGs not located in superclusters, which constitutes 94.7% of the sample, a mere 21% are associated with galaxy clusters, while the remaining majority are situated in sparser environments. We examined the influence of differing environments, such as cluster versus non-cluster and supercluster versus non-supercluster regions, on the size of GRGs, while also exploring the driving factors behind their overall growth. Our findings show that the largest GRGs ($\gtrsim$3 Mpc) grow in underdense environments beyond the confines of dense environments. Moreover, we show that $\sim$ 24% of 1446 GRGs reside in galaxy clusters. We conclude that GRGs preferentially grow in sparser regions of the cosmic web and have a significantly larger median size. Finally, we demonstrate the potential of GRGs as astrophysical probes with specific cases where GRGs, exhibiting polarised emissions and located behind superclusters (acting as natural Faraday screens), were used to estimate magnetic field strengths of the supercluster environment at sub-microgauss levels.

astro-ph.GA↗

arXiv2024

Unveiling the Bulge-Disc Structure, AGN Feedback, and Baryon Landscape in a Massive Spiral Galaxy with Mpc-Scale Radio Jets

This study delves into the bulge-disc components and stellar mass distribution in the fast-rotating, highly massive spiral galaxy 2MASX~J23453268-0449256, distinguished by extraordinary radio jets extending to Mpc scales. Using high-resolution multi-wavelength Hubble Space Telescope (HST) observations and multi-parameter panchromatic spectral energy distribution (SED) fitting, we derive estimates for the star formation rate, total baryonic mass in stars, and warm dust properties. Our findings, validated at a spatial resolution of approximately 100 pc, reveal a pseudo-bulge rather than a classical bulge and a small nuclear bar and resonant ring, challenging conventional models of galaxy formation. Additionally, the lack of tidal debris and the highly symmetric spiral arms within a rotationally supported stellar disc indicate a tranquil coevolution of the galactic disc and its supermassive black hole (SMBH). Significantly, the galaxy exhibits suppressed star formation in its center, potentially influenced by feedback from the central accreting SMBH with powerful radio jets. Detailed multi-wavelength studies of potential star-forming gas disclose that, while hot X-ray gas cools down in the galaxy's halo, new stars do not form in the center, likely due to feedback effects. This study raises questions about the efficient fueling and sustained collimated jet ejection activity in J2345-0449, underscoring the imperative need for a comprehensive understanding of its central black hole engine properties, which are presently lacking. The exceptional rarity of galaxies like 2MASX~J23453268-0449256 presents intriguing challenges in unraveling the physical processes responsible for their unique characteristics.

astro-ph.GA↗

arXiv2024

Luminous giants populate the dense Cosmic Web: The radio luminosity-environmental density relation for radio galaxies in action

Giant radio galaxies (GRGs, giant RGs, or giants) are megaparsec-scale, jet-driven outflows from accretion disks of supermassive black holes, and represent the most extreme pathway by which galaxies can impact the Cosmic Web around them. A long-standing but unresolved question is why giants are so much larger than other radio galaxies. It has been proposed that, in addition to having higher jet powers than most RGs, giants might live in especially low-density Cosmic Web environments. In this work, we aim to test this hypothesis by pinpointing Local Universe giants and other RGs in physically principled, Bayesian large-scale structure reconstructions. More specifically, we localised a LOFAR Two-metre Sky Survey (LoTSS) DR2-dominated sample of luminous ($l_ν(ν= 150\ \mathrm{MHz}) \geq 10^{24}\ \mathrm{W\ Hz^{-1}}$) giants and a control sample of LoTSS DR1 RGs, both with spectroscopic redshifts up to $z_\mathrm{max} = 0.16$, in the BORG SDSS Cosmic Web reconstructions. We measured the Cosmic Web density for each RG; for the control sample, we then quantified the relation between RG radio luminosity and Cosmic Web density. With the BORG SDSS tidal tensor, we also measured for each RG whether the gravitational dynamics of its Cosmic Web environment resemble those of clusters, filaments, sheets, or voids. Luminous giants populate large-scale environments that tend to be denser than those of general RGs. This shows that -- at least at high jet powers -- low-density environments are no prerequisite for giant growth. This result is corroborated by gravitational dynamics classification and a cluster catalogue crossmatching analysis. This work presents more than a thousand inferred megaparsec-scale densities around radio galaxies. Our findings are consistent with the view that giants are regular, rather than mechanistically special, members of the radio galaxy population.

astro-ph.CO↗

arXiv2023

Evidence for low power radio jet-ISM interaction at 10 parsec in the dwarf AGN host NGC 4395

Black hole driven outflows in galaxies hosting active galactic nuclei (AGN) may interact with their interstellar medium (ISM) affecting star formation. Such feedback processes, reminiscent of those seen in massive galaxies, have been reported recently in some dwarf galaxies. However, such studies have usually been on kiloparsec and larger scales and our knowledge on the smallest spatial scales to which these feedback processes can operate is unclear. Here we demonstrate radio jet$-$ISM interaction on the scale of an asymmetric triple radio structure of $\sim$ 10 parsec size in NGC 4395. This triple radio structure is seen in the 15 GHz continuum image and the two asymmetric jet-like structures are situated on either side of the radio core that coincides with the optical {\it Gaia} position. The high resolution radio image and the extended [OIII]$λ$5007 emission, indicative of an outflow, are spatially coincident and are consistent with the interpretation of a low power radio jet interacting with the ISM. Modelling of the spectral lines using {\tt MAPPINGS}, and estimation of temperature using optical integral field spectroscopic data suggest shock ionization of the gas. The continuum emission at 237 GHz, though weak, was found to spatially coincide with the AGN. However, the CO(2$-$1) line emission was found to be displaced by around 20 parsec northward of the AGN core. The spatial coincidence of molecular H$_2$$λ$2.4085 along the jet direction, the morphology of ionised [OIII]$λ$5007 and displacement of the CO(2$-$1) emission argues for conditions less favourable for star formation in the central $\sim$ 10 parsec region.

astro-ph.GA↗

arXiv2023

Identification of Superclusters and their Properties in the Sloan Digital Sky Survey Using WHL Cluster Catalog

Superclusters are the largest massive structures in the cosmic web on tens to hundreds of megaparsecs (Mpc) scales. They are the largest assembly of galaxy clusters in the Universe. Apart from a few detailed studies of such structures, their evolutionary mechanism is still an open question. In order to address and answer the relevant questions, a statistically significant, large catalog of superclusters covering a wide range of redshifts and sky areas is essential. Here, we present a large catalog of 662 superclusters identified using a modified $\textit{ Friends of Friends}$ algorithm applied on the WHL (Wen-Han-Liu) cluster catalog within a redshift range of $0.05 \le z \le 0.42$. We name the most massive supercluster at $z \sim 0.25$ as $\textit{Einasto Supercluster}$. We find that the median mass of superclusters is $\sim 5.8 \times 10^{15}$ M$_{\odot}$ and median size $\sim 65$ Mpc. We find that the supercluster environment slightly affects the growth of clusters. We compare the properties of the observed superclusters with the mock superclusters extracted from the Horizon Run 4 cosmological simulation. The properties of superclusters in mocks and observations are in broad agreement. We find that the density contrast of a supercluster is correlated with its maximum extent with a power law index, $α\sim -2$. The phase-space distribution of mock superclusters shows that, on average, $\sim 90\%$ part of a supercluster has a gravitational influence on its constituents. We also show mock halos' average number density and peculiar velocity profiles in and around the superclusters.

astro-ph.CO↗

arXiv2023

A giant radio galaxy with three cycles of episodic jet activity from LoTSS DR2

The excellent sensitivity and optimum resolution of LoTSS DR2 at 144 MHz has enabled us to discover a giant radio galaxy (J1225+4011) with three distinct episodes of jet activity, making it a member of a class of objects called triple-double radio galaxies (TDRGs). The source extends overall up to 1.35 Mpc in projected size, with the second episode extending to 572 kpc, and the inner episode to 118 kpc. J1225+4011 is only the fourth radio source showing a triple-double radio structure. All four sources have overall sizes greater than 700 kpc, making them giants. We also present the LoTSS 144 MHz map of the TDRG J0929+4146 and report its updated size. Lastly, we have summarised and discussed the radio properties of all TDRGs for the first time to understand their growth and evolution. Our observations suggest that the power of their jets may decrease with time.

astro-ph.GA↗

arXiv2023

The 'spectral index-flux density relation' for extragalactic radio sources selected at metre and decametre wavelengths

We use the recent releases of sensitive VLA/LOFAR large-area surveys at 340 MHz and 54 MHz, in conjunction with the 1.4 GHz NVSS, to accurately determine the `spectral index - flux density relation' ($α$ - S) for extragalactic radio sources selected at metre and decametre wavelengths, the latter for the first time. This newly determined $α$ - S$_{\rm 340~MHz}$ relation shows a progressive flattening of $α_{\rm median}$ towards lower flux densities, starting from its steepest value (peak)occurring near S$_{\rm 340~MHz}$ $\sim$ 1-2 Jy. This resolves the controversy extant in the literature since the 1980s. The $α$ - S$_{\rm 54~MHz}$ relation, too, shows a spectral index flattening with decreasing flux density which, however, is significantly milder and the relation is less sharply peaked than that found at 340 MHz. A possible reason for the difference could be that the 54 MHz sample has a distinctly stronger/conspicuous presence ( at $\sim$ 20% level) of very steep spectrum sources having $α_{54}^{1400} <$ -1.3, most of which are probably associated with clusters of galaxies.

astro-ph.GA↗