arXiv subjects

Ananda Hota

Publications and source records attributed to Ananda Hota.

At least 19 recordsLinked to original sources

arXiv2026

Tracing the Origin of Circular-Symmetry Diffuse Radio Sources in the Next-Generation SKAO Era

Circularly symmetric diffuse radio sources represent a recently recognized and rare class of extragalactic synchrotron-emitting structures whose physical origin remains uncertain. The currently known population is small, and the limited frequency coverage, sensitivity, and polarimetric information available from existing observations hinder robust discrimination among the various proposed formation scenarios. This chapter presents an overview of four representative circular diffuse sources featuring diverse morphologies, such as a horseshoe-shaped inner ring, circular diffuse emission surrounded by inner S/Z-shaped structures and a double-ring diffuse source. The angular sizes of these sources are in the range of 60-100 arcsec, similar to odd radio circles (ORCs). These systems illustrate the morphological diversity of circular diffuse radio emission and provide valuable clues to the physical processes responsible for their formation. The Square Kilometre Array Observatory (SKAO), with its unprecedented sensitivity, wide instantaneous bandwidth, sub-arcsecond imaging capability, and high-fidelity polarimetric performance, will significantly advance the study of these objects. SKAO surveys are expected to detect a substantially larger population of faint circular diffuse radio sources, enabling the first statistically meaningful studies of their occurrence, environments, and evolution. An expanded sample will allow investigations of their cosmological evolution and help determine whether their origin is linked to AGN feedback processes, episodic jet activity, galaxy interactions, or large-scale structure formation. This chapter summarizes the current observational knowledge of circular diffuse radio sources and describes how next-generation radio facilities, particularly the SKAO, will enable progress in understanding the origin and physical nature of these enigmatic radio structures.

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↗

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 $\alpha_{54}^{144}=1.22\pm0.15$) 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↗

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↗

arXiv2023

RAD@home discovery of a one-sided radio jet hitting the companion galaxy

Minkowski's Object and 'Death Star galaxy' are two of the famous cases of rare instances when a radio jet has been observed to directly hit a neighbouring galaxy. RAD12, the RAD@home citizen science discovery with GMRT being presented here, is not only a new system being added to nearly half a dozen rare cases known so far but also the first case where the neighbouring galaxy is not a minor or dwarf companion but a galaxy bigger than the host of the radio jet. Additionally, the jet appears to be one-sided and the jet after interaction completely stops and forms a bubble inflating laterally which is unlike previous cases of minor deviation or loss of collimation. Since the nature of radio jet-ISM coupling is poorly understood so far, more discovery of objects like RAD12 will be important to the understanding of galaxy evolution through merger and AGN feedback.

astro-ph.GA↗

arXiv2022

RAD@home citizen science discovery of an AGN spewing a large unipolar radio bubble onto its merging companion galaxy

AGN feedback during galaxy merger has been the most favoured model to explain black hole-galaxy co-evolution. However, how the AGN-driven jet/wind/radiation is coupled with the gas of the merging galaxies, which leads to positive feedback, momentarily enhanced star formation, and subsequently negative feedback, a decline in star formation, is poorly understood. Only a few cases are known where the jet and companion galaxy interaction leads to minor off-axis distortions in the jets and enhanced star formation in the gas-rich minor companions. Here, we briefly report one extraordinary case, RAD12, discovered by RAD@home citizen science collaboratory, where for the first time a radio jet-driven bubble ~137 kpc is showing a symmetric reflection after hitting the incoming galaxy which is not a gas-rich minor but a gas-poor early-type galaxy in a major merger. Surprisingly, neither positive feedback nor any radio lobe on the counter jet side, if any, is detected. It is puzzling if RAD12 is a genuine one-sided jet or a case of radio lobe trapped, compressed and re-accelerated by shocks during the merger. This is the first imaging study of RAD12 presenting follow-up with the GMRT, archival MeerKAT radio data and CFHT optical data.

astro-ph.GA↗

arXiv2021

The Sharpest Ultraviolet view of the star formation in an extreme environment of the nearest Jellyfish Galaxy IC 3418

We present the far ultraviolet (FUV) imaging of the nearest Jellyfish or Fireball galaxy IC3418/VCC 1217, in the Virgo cluster of galaxies, using Ultraviolet Imaging Telescope (UVIT) onboard the ASTROSAT satellite. The young star formation observed here in the 17 kpc long turbulent wake of IC3418, due to ram pressure stripping of cold gas surrounded by hot intra-cluster medium, is a unique laboratory that is unavailable in the Milkyway. We have tried to resolve star forming clumps, seen compact to GALEX UV images, using better resolution available with the UVIT and incorporated UV-optical images from Hubble Space Telescope archive. For the first time, we resolve the compact star forming clumps (fireballs) into sub-clumps and subsequently into a possibly dozen isolated stars. We speculate that many of them could be blue supergiant stars which are cousins of SDSS J122952.66+112227.8, the farthest star (~17 Mpc) we had found earlier surrounding one of these compact clumps. We found evidence of star formation rate (4 - 7.4 x 10^-4 M_sun per yr ) in these fireballs, estimated from UVIT flux densities, to be increasing with the distance from the parent galaxy. We propose a new dynamical model in which the stripped gas may be developing vortex street where the vortices grow to compact star forming clumps due to self-gravity. Gravity winning over turbulent force with time or length along the trail can explain the puzzling trend of higher star formation rate and bluer/younger stars observed in fireballs farther away from the parent galaxy.

astro-ph.GA↗

arXiv2018

Rare finding of a 100 kpc large double-lobed radio galaxy hosted in the Narrow Line Seyfert 1 galaxy SDSS J103024.95+551622.7

Among the large varieties of active galactic nuclei (AGN) known, narrow line Seyfert 1 (NLSy1) galaxies are a puzzling class, particularly after the discovery of $γ$-ray emission in a handful of them using observations from the Fermi Gamma-ray Space Telescope. Here, we report the discovery of a rare large double lobed radio source with its radio core associated with a NLSy1 galaxy SDSS J103024.95+551622.7 at z = 0.435. The lobe separation is 116 kpc which is the second largest known projected size among NLSy1 radio sources. This finding is based on the analysis of 1.4 GHz data from the Faint Images of the Radio Sky at Twenty-centimeters (FIRST) archives. Along with the core and edge-brightened lobes we detected significant (30%) fraction of clear diffuse emission showing typical back-flow from FR II radio galaxy lobes. For the source, we estimated a jet power of $3 \times 10^{44}$ erg s$^{-1}$ suggesting that its jet power is similar to that of classical radio galaxies. Emission from the source is also found to be non-variable both in the optical and mid-infrared bands. Identification of more such sources may help to reveal new modes of AGN and understand their role in black hole galaxy evolution.

astro-ph.GA↗

arXiv2016

Tracking galaxy evolution through low frequency radio continuum observations using SKA and Citizen-science Research using Multi-wavelength data

We present a review on galaxy black hole co-evolution through merger, star formation and AGN-jet feedback. We highlight results on transitional galaxies (e.g. NGC1482, NGC6764, NGC3801, Speca, RAD-18 etc.) which has data from Giant Meterwave Radio Telescope (GMRT) and other sub-mm, IR, optical, UV and X-ray telescopes. The `smoking gun' relic-evidences of past AGN-jet feedback which is believed to have quenched star formation in transitional galaxies are still missing. Relic radio lobes, as old as a few hundred Myr, can be best detected at low radio frequencies with the GMRT, LOFAR and in future SKA. However, similar relic evidences of quasar activities, known as `Hanny's Voorwerp' discovered by Galaxy Zoo in optical data, are only around a few tens of thousand years old. More discoveries are needed to match these time-scales with time since the decline of star formation in transitional galaxies. Such faint fuzzy relic emissions in optical and angular-scale sensitive radio interferometric images can be discovered most efficiently by citizen-scientists but with a formal training. We describe RAD@home, the only Indian citizen-science research project in astronomy which takes such a modified approach. We present interesting objects, discovered from the TIFR GMRT Sky Survey (TGSS) by 69 trained citizen-scientists or e-astronomers, like relic radio lobes, episodic radio galaxies, jet-galaxy interaction, bent radio galaxy in filament etc.. This model can provide an equal opportunity of academic-growth to people even in the under-developed regions where we always need to establish our optical and radio telescopes. This can expand the research-activity of city-based research-institutes beyond their four brick walls, and alleviate various socio-economic and geo-political constraints on growth of citizens located in remote areas. #RADatHomeIndia #ABCDresearch

astro-ph.GA↗

arXiv2016

A GMRT Study of Seyfert Galaxies NGC4235 & NGC4594: Evidence of Episodic Activity ?

Low frequency observations at 325 and 610 MHz have been carried out for two "radio-loud" Seyfert galaxies, NGC4235 and NGC4594 (Sombrero galaxy), using the Giant Meterwave Radio Telescope (GMRT). The 610 MHz total intensity and 325-610 MHz spectral index images of NGC4235 tentatively suggest the presence of a "relic" radio lobe, most likely from a previous episode of AGN activity. This makes NGC4235 only the second known Seyfert galaxy after Mrk6 to show signatures of episodic activity. Spitzer and Herschel infrared spectral energy distribution (SED) modelling using the clumpyDREAM code predicts star formation rates (SFR) that are an order of magnitude lower than those required to power the radio lobes in these Seyferts (~0.13-0.23 M_sun/yr compared to the required SFR of ~2.0-2.7 M_sun/yr in NGC4594 and NGC4235, respectively). This finding along with the detection of parsec and sub-kpc radio jets in both Seyfert galaxies, that are roughly along the same position angles as the radio lobes, strongly support the suggestion that Seyfert lobes are AGN-powered. SED modelling supports the "true" type 2 classification of NGC4594: this galaxy lacks significant dust obscuration as well as a prominent broad-line region. Between the two Seyfert galaxies, there is an inverse relation between their radio-loudness and Eddington ratio and a direct relation between their Eddington-scaled jet power and bolometric power.

astro-ph.GA↗

arXiv2014

Mega parsec relativistic jets launched from an accreting supermassive blackhole in an extreme spiral galaxy

Radio galaxy phenomenon is directly connected to mass accreting, spinning supermassive black holes found in the active galactic nuclei (AGN). It is still unclear how the collimated jets of relativistic plasma on hundreds to thousands of kpc scale form, and why nearly always they are launched from the nuclei of bulge dominated elliptical galaxies and not flat spirals. Here we present the discovery of giant radio source J2345-0449 (z=0.0755), a clear and extremely rare counter example where relativistic jets are ejected from a luminous and massive spiral galaxy on scale of ~1.6 Mpc, the largest known so far. Extreme physical properties observed for this bulgeless spiral host, such as its high optical and infra-red luminosity, large dynamical mass, rapid disk rotation, and episodic jet activity are possibly the results of its unusual formation history, which has also assembled, via gas accretion from a disk, its central black hole of mass >2 x 10^8 M_sun. The very high mid-IR luminosity of the galaxy suggests that it is actively forming stars and still building a massive disk. We argue that the launch of these powerful jets is facilitated by an advection dominated, magnetized accretion flow at low Eddington rate onto this unusually massive (for a bulgeless disk galaxy) and possibly fast-spinning central black hole. Therefore, J2345-0449 is an extremely rare, unusual galactic system whose properties challenge the standard paradigms for black hole growth and formation of relativistic jets in disk galaxies. Thus, it provides fundamental insight into accretion disk -- relativistic jet coupling processes.

astro-ph.GA↗

arXiv2014

New results on the exotic galaxy `Speca' and discovering many more Specas with RAD@home network

We present the first report on an innovative new project named "RAD@home", a citizen-science research collaboratory built on free web-services like Facebook, Google, Skype, NASA Skyview, NED, TGSS etc.. This is the first of its kind in India, a zero-funded, zero-infrastructure, human-resource network to educate and directly involve in research, hundreds of science-educated under-graduate population of India, irrespective of their official employment and home-location with in the country. Professional international collaborators are involved in follow up observation and publication of the objects discovered by the collaboratory. We present here ten newly found candidate episodic radio galaxies, already proposed to GMRT, and ten more interesting cases which includes, bent-lobe radio galaxies located in new Mpc-scale filaments, likely tracing cosmological cluster accretion from the cosmic web. Two new Speca-like rare spiral-host large radio galaxies have also been been reported here. Early analyses from our follow up observations with the Subaru and XMM-Newton telescopes have revealed that Speca is likely a new entry to the cluster and is a fast rotating, extremely massive, star forming disk galaxy. Speca-like massive galaxies with giant radio lobes, are possibly remnants of luminous quasars in the early Universe or of first supermassive black holes with in first masssve galaxies. As discoveries of Speca-like galaxies did not require new data from big telescopes, but free archival radio-optical data, these early results demonstrate the discovery potential of RAD@home and how it can help resource-rich professionals, as well as demonstrate a model of academic-growth for resource-poor people in the underdeveloped regions via Internet.

astro-ph.GA↗

arXiv2014

The impact of a young radio galaxy: clues from the cosmic-ray electron population

In the framework of hierarchical structure formation AGN feedback shapes the galaxy luminosity function. Low luminosity, galaxy-scale double radio sources are ideal targets to investigate the interplay between AGN feedback and star formation. We use VLA and BIMA observations to study the radio continuum emission of NGC 3801 between 1.4 and 112.4 GHz. We find a prominent spectral break at ~10 GHz, where the spectrum steepens as expected from cosmic-ray electron (CRe) ageing. Using the equipartition magnetic field and fitting JP models locally we create a spatially resolved map of the spectral age of the CRe population. The spectral age of tau_int = 2.0 +/- 0.2 Myr agrees within a factor of two with the dynamical age of the expanding X-ray emitting shells. The spectral age varies only little across the lobes, requiring an effective mixing process of the CRe such as a convective backflow of magnetized plasma. The jet termination points have a slightly younger CRe spectral age, hinting at in-situ CRe re-acceleration. Our findings support the scenario where the supersonically expanding radio lobes heat the ISM of NGC 3801 via shock waves, and, as their energy is comparable to the energy of the ISM, are clearly able to influence the galaxy's further evolution.

astro-ph.GA↗

arXiv2013

Discovery of a possibly single blue supergiant star in the intra-cluster region of Virgo cluster of galaxies

IC 3418 is a dwarf irregular galaxy falling into the Virgo cluster, and a 17 kpc long trail is seen behind the galaxy, which is considered to have formed due to ram pressure stripping. The trail contains compact knots and diffuse blobs of ultraviolet and blue optical emission and, thus, it is a clear site of recent star formation but in an unusual environment, surrounded by a million degree intra-cluster medium. We report on our optical spectroscopy of a compact source in the trail, SDSS J122952.66+112227.8, and show that the optical spectrum is dominated by emission from a massive blue supergiant star. If confirmed, our report would mark the farthest star with spectroscopic observation. We interpret that a massive O-type star formed in situ in the trail has evolved recently out of the main sequence into this blue supergiant phase, and now lacks any detectable spectral sign of its associated HII region. We argue that turbulence within the ram pressure striped gaseous trail may play a dominant role for the star formation within such trails.

astro-ph.CO↗

arXiv2011

NGC3801 caught in the act: A post-merger starforming early-type galaxy with AGN-jet feedback

In the current models of galaxy formation and evolution, AGN feedback is crucial to reproduce galaxy luminosity function, colour-magnitude relation and M-sigma relation. However, if AGN-feedback can indeed expel and heat up significant amount of cool molecular gas and consequently quench star formation, is yet to be demonstrated observationally. Only in four cases so far (Cen A, NGC 3801, NGC 6764 and Mrk 6), X-ray observations have found evidences of jet-driven shocks heating the ISM. We chose the least-explored galaxy, NGC 3801, and present the first ultraviolet imaging and stellar population analysisis of this galaxy from GALEX data. We find this merger-remnant early-type galaxy to have an intriguing spiral-wisp of young star forming regions (age ranging from 100--500 Myr). Taking clues from dust/PAH, HI and CO emission images we interpret NGC 3801 to have a kinamatically decoupled core or an extremely warped gas disk. From the HST data we also show evidence of ionised gas outflow similar to that observed in HI and molecular gas (CO) data, which may have caused the decline of star formation leading to the red optical colour of the galaxy. However, from these panchromatic data we interpret that the expanding shock shells from the young ($\sim$2.4 million years) radio jets are yet to reach the outer gaseous regions of the galaxy. It seems, we observe this galaxy at a rare stage of its evolutionary sequence where post-merger star formation has already declined and new powerful jet feedback is about to affect the gaseous star forming outer disk within the next 10 Myr, to further transform it into a red-and-dead early-type galaxy.

astro-ph.CO↗

arXiv2011

Discovery of a spiral-host episodic radio-galaxy

We report the discovery of a unique radio galaxy at z=0.137, which could possibly be the second spiral-host large radio galaxy and also the second triple-double episodic radio galaxy. The host galaxy shows signs of recent star formation in the UV but is optically red and is the brightest galaxy of a possible cluster. The outer relic radio lobes of this galaxy, separated by ~1 Mpc, show evidence of spectral flattening and a high fraction of linear polarisation. We interpret that these relic lobes have experienced re-acceleration of particles and compression of the magnetic field due to shocks in the cluster outskirts. From the morphology of the relics and galaxy distribution, we argue that re-acceleration is unlikely to be due to a cluster-cluster merger and suggest the possibility of accretion shocks. The source was identified from SDSS, GALEX, NVSS and FIRST survey data but we also present follow up optical observations with the Lulin telescope and 325 MHz low frequency radio observations with the GMRT. We briefly discuss the scientific potential of this example in understanding the evolution of galaxies and clusters by accretion, mergers, star formation, and AGN feedback.

astro-ph.CO↗