arXiv subjects

Dharam V. Lal

Publications and source records attributed to Dharam V. Lal.

At least 19 recordsLinked to original sources

arXiv2026

A new look at old devils. II: New insights on classical radio galaxies from MeerKAT and uGMRT

This paper presents a detailed morphological and spectral analysis of a sample of ten radio galaxies observed with the MeerKAT in L-band ($856\, \text{-}\,1712\ \mathrm{MHz}$) and the upgraded Giant Metrewave Radio Telescope in Band-4 ($550\,\text{-}\,850\ \mathrm{MHz}$). Our main goals are to revisit the current classification scheme for classical radio galaxies and study the properties of new radio features in jets, lobes and hot spots, which are becoming increasingly numerous due to the sensitivity and imaging capabilities of current radio interferometers, and suggest previous unexplored interaction mechanisms between the radio plasma and the external medium. The sample from the 4C catalogue includes FR I, FR II, wide-angle and narrow-angle tailed sources as well as FR 0 radio galaxies from FR0CAT, with redshifts ranging from $0.04\,\text{-}\,0.20$. The high angular resolution, $\sim4^{\prime\prime}\text{-}10^{\prime\prime}$ total intensity images are presented, revealing complex structures in the jets, lobes and hotspots of these sources. The integrated spectra of the sources, constructed using flux density measurements from our observations and archival data, reveal spectral breaks and slopes indicative of radiative ageing, with ages spanning $\sim40\,\text{-}\,242\ \mathrm{Myr}$. While the sample broadly confirms the classical FR classification as a useful first-order scheme, the substructures within the radio emission of sources highlight the need for models that incorporate environmental complexity and episodic jet activity to fully describe radio-galaxy evolution.

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arXiv2026

Unveil the nature of JWST-AGN and Little Red Dots with SKAO continuum surveys

The advent of JWST has revealed a large population of AGN at $z>4$, which are $\sim1$ dex more abundant than previously expected, including also the enigmatic population of Little Red Dots (LRDs). Remarkably, the vast majority of JWST-discovered AGN and LRDs are not detected in X-rays, and most of them also show faint rest-frame UV continua and faint high-ionization emission lines, as well as unusually faint emission in the Mid and Far infrared. Recent studies investigating their radio properties have reported no significant detections, even in deep stacking analyses, reaching sensitivities of 0.5-0.1 $μ$Jy at $z\sim 5-6$, corresponding to $L_{R}\lesssim 10^{39}\rm \ erg\ s^{-1}$. While these non-detections may be consistent with a standard radio-quiet nature, some results suggest that the radio emission might instead be significantly suppressed by other physical phenomena. Three main scenarios have been proposed in the literature to explain the physical properties of these objects across the electromagnetic spectrum: Compton-thick absorption by a broad-line region with high covering-factor, intrinsically weak emission driven by high accretion rates, or the presence of a cocoon of dense ionized gas that produces strong scattering effects. The unprecedented sensitivity of SKAO will enable the detection of the radio emission of these AGN in all three cases. Because each scenario is expected to produce distinct radio signatures, future SKAO continuum surveys will be able to distinguish between them, uncovering the physical processes responsible for their peculiar properties. Observations spanning a wide range of integration times (1-1000 hours) and frequencies with SKA-Mid and SKA-Low (0.2-11 GHz) will allow us to characterize these objects from the local Universe to high redshift, investigate possible radio variability, and test alternative scenarios to black hole accretion.

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arXiv2026

Opening new parameter space windows on galaxy/AGN co-evolution with SKA radio continuum surveys

In this chapter we provide an overview of the science enabled by the SKAO, focusing on galaxy/AGN co-evolution studies. In particular we discuss a number of radio continuum `reference' surveys with the SKAO, highlighting the role they can play in advancing this research field with respect to the pre-SKAO era. Alongside well-explored scenarios for wedding cake-like, tiered extragalactic surveys at specific frequencies, we also address the scope for complementary efforts to obtain deep multi-frequency imaging over parts of (an) extragalactic field(s). In addition to providing key information on the physical properties of the emitting sources, such multi-frequency imaging will make important contributions to the calibration of observables from surveys with sparser radio spectral coverage. In this context, we explore possible pathways that can fully exploit the SKAO from initial (AA*) to baseline capabilities (AA4). Finally, we highlight observational synergies with other major facilities -- for wide field and targeted follow-up science -- that will be operational in the 2030s, and for which joint coverage of extragalactic fields will generate significant legacy value

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arXiv2026

Radio emission from ultra-diffuse galaxies residing in galaxy clusters

Ultra-diffuse galaxies (UDGs), defined by their extremely low surface brightness ($g$-band $μ\gtrsim 24$ mag arcsec$^{-2}$) and large effective radii (3--10 arcsec), remain one of the most puzzling galaxy populations in the nearby Universe \citep{vanDokkum2015}. Predominantly found in dense environments, UDGs in the Coma cluster show a preferential alignment of their major axes toward the cluster centre, suggesting strong environmental influence on their formation and evolution. Using high-sensitivity, low-frequency radio data from the upgraded Giant Metrewave Radio Telescope (GMRT), a pathfinder instrument we examined all 854 UDGs cataloged in Coma cluster \citep{Yagi2016}. Despite the unprecedented depth of these observations, no individual detections were made. A median stacking analysis in the upgraded GMRT band-3 achieved a 5 $\times$ \textsc{rms} upper limit $\simeq$1.5~$μ$Jy, providing the most stringent constraint yet on the average (mean) radio emission from UDGs, corresponding to star-formation rates $\lesssim$10$^{-3}$~M$_\odot$~yr$^{-1}$ for Coma-cluster-like systems and $\lesssim$10$^{-1}$~M$_\odot$~yr$^{-1}$ at $z \sim 0.05$. Looking ahead, the Square Kilometre Array (SKA) will transform the study of such faint galaxies. While the early AA$^\star$ configuration will deliver sensitivities comparable to the upgraded GMRT, the AA4 design baseline will achieve sub-$μ$Jy \textsc{rms} levels at matched frequencies ($ν\sim 200$~MHz--1.4~GHz), enabling detections of UDGs with star formation rates as low as 10$^{-4}$--10$^{-3}$~M$_\odot$~yr$^{-1}$ within Virgo and Coma distances. Such capabilities will allow robust discrimination between quenched, dark-matter-dominated systems and those sustaining weak residual star formation or low-luminosity nuclear activity.

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arXiv2026

The Radio--X-ray Correlation of High-Redshift AGN: A Numerical Study of Inverse-Compton Scattering of the CMB Photons in Relativistic Jets

Relativistic jets from active galactic nuclei are expected to exhibit strong redshift evolution in their radiative output due to the increasing energy density of the cosmic microwave background (CMB). We investigate the role of inverse Compton (IC) scattering of CMB photons in regulating the radio and X-ray emission from large-scale jets using three-dimensional relativistic magnetohydrodynamic simulations coupled with a hybrid Eulerian-Lagrangian particle framework. By keeping the jet dynamics and ambient medium properties fixed across redshifts, we are able to isolate the impact of the cosmological evolution of the CMB on the jet radiation. From our simulations, we construct synthetic spectral energy distributions and intensity maps considering synchrotron and IC/CMB losses along with particle acceleration from shocks. We are able to reproduce the weak redshift dependence of radio luminosity and the strong enhancement of X-ray emission toward high redshift that is observed in radio-loud quasars. At high redshift, the X-ray luminosity follows the expected $(1+z)^4$ scaling, confirming IC/CMB as the dominant mechanism driving the X-ray enhancement. The resulting X-ray-to-radio flux ratio increases systematically with redshift and is consistent with observational constraints. Finally, we show that slower jets exhibit a stronger redshift evolution of the X-ray enhancement than faster jets, highlighting the critical role of jet propagation length scales and particle energy evolution. The simulations also naturally reproduce the steepening of the radio spectral index with redshift - the $α$-$z$ relation - thus providing a unified framework that allows to interpret the multiwavelength properties of high-redshift radio sources.

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arXiv2026

Metal enrichment in the galaxy group IC 1262

We present a new metal enrichment analysis of a unique galaxy group IC 1262 using archival Chandra and GMRT observations, focusing on metal transport via radio jet, sloshing cold fronts, and shock front. This group shows two sloshing cold fronts along the east and north-west direction which is nearly orthogonal to the north - south orientated radio jet. We report discontinuities in the metallicity at the location of previously detected cold fronts, a more prominent one towards the eastern direction. In addition, the gas inside the cold fronts is 45$\pm$8 per cent more enriched than the gas outside the cold front, suggesting the role of sloshing in transporting metals through the IGrM. We also confirm the presence of a previously reported shock front with higher significance and with greater details. Across this shock, we detect a significant metallicity drop from 0.45$\pm$0.05 $Z_{\odot}$ to 0.22$\pm$0.04 $Z_{\odot}$, located at a projected distance of 78$\pm$2 kpc in the southern direction. The shock could potentially account for the region of gas enrichment seen in the abundance map and profile, which could be the result of a non-Maxwellian electron distribution in its vicinity. This should be considered a contributing factor rather than the sole cause of the observed discontinuity in the abundance. Furthermore, our spectral analysis reveals two temperature X-ray gas preferentially aligned with the radio-jet axis, indicating a possible influence of radio AGN activity on the surrounding gas.

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arXiv2025

The superMIGHTEE project: MeerKAT and GMRT Together to Unveil the Deep Radio Sky

An international team of researchers has come together to undertake an ultra-broadband exploration of the deep radio sky. The superMIGHTEE project combines data from the MIGHTEE project, using the precursor Square Kilometre Array (SKA) MeerKAT telescope in South Africa, with observations from the upgraded Giant Metrewave Radio Telescope (uGMRT) in India to produce deep images at several $μ$Jy sensitivity over a frequency range of 200 MHz--2.5 GHz, with an angular resolution of a few arcseconds. This paper describes the initial superMIGHTEE uGMRT data release, comprising total intensity continuum images covering a total of 9.9 deg$^2$ at 650 MHz and 6.9 deg$^2$ at 400 MHz in the XMM-LSS, COSMOS, and E-CDFS deep fields. The associated radio source catalogs include 27,101 sources at 650 MHz and 10,946 sources at 400 MHz. The redshift distribution of the sources extends to $z\sim4$ with a median value of $z=1$. An overview of the broadband spectra of the sources, in combination with the MeerKAT MIGHTEE 1280 MHz data, reveals a clear change in spectral properties at the transition from an active galactic nuclei-dominated population to a population dominated by star-forming galaxies at flux densities of a few mJy. At higher frequencies, the star-forming galaxy population exhibits an optically thin synchrotron spectral index indicative of energy injection from supernovae. At lower frequencies, the spectra flatten significantly with decreasing flux density, and the fraction of sources with peaked spectra increases. This is the first superMIGHTEE uGMRT data release. Subsequent releases will include spectropolarimetric and spectral line image cubes, as well as images at lower frequencies. The goal of the superMIGHTEE ultra-wideband dataset is to enhance our understanding of the evolution of active galactic nuclei and star-forming galaxies over cosmic time, (abridged).

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arXiv2025

MIGHTEE: exploring the relationship between spectral index, redshift and radio luminosity

It has been known for many years that there is an apparent trend for the spectral index (α) of radio sources to steepen with redshift z, which has led to attempts to select high-redshift objects by searching for radio sources with steep spectra. In this study we use data from the MeerKAT, LOFAR, GMRT, and uGMRT telescopes, particularly using the MIGHTEE and superMIGHTEE surveys, to select compact sources over a wide range of redshifts and luminosities. We investigate the relationship between spectral index, luminosity and redshift and compare our results to those of previous studies. Although there is a correlation between α and z in our sample for some combinations of frequency where good data are available, there is a clear offset between the α-z relations in our sample and those derived previously from samples of more luminous objects; in other words, the α-z relation is different for low and high luminosity sources. The relationships between α and luminosity are also weak in our sample but in general the most luminous sources are steeper-spectrum and this trend is extended by samples from previous studies. In detail, we argue that both a α-luminosity relation and an α-z relation can be found in the data, but it is the former that drives the apparent α-z relation observed in earlier work, which only appears because of the strong redshift-luminosity relation in bright, flux density-limited samples. Steep-spectrum selection should be applied with caution in searching for high-z sources in future deep surveys.

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arXiv2024

A multi-wavelength study of Galactic H II regions with extended emission

H II regions are the signposts of massive ($M\geq\,8\,M_\odot$) star-forming sites in our Galaxy. It has been observed that the ionizing photon rate inferred from the radio continuum emission of H II regions is significantly lower ($\sim$ 90%) than that inferred from far-infrared fluxes measured by IRAS. This discrepancy in the ionizing photon rates may arise due to there being significant amounts of dust within the H II regions or the presence of extended emission that is undetected by high-resolution radio interferometric observations. Here, we study a sample of eight compact and ultracompact H II regions with extended emission to explore its role in resolving the discrepancy. We have used observations at the uGMRT (1.25-1.45 GHz) and data from the GLOSTAR survey (4-8 GHz) to estimate the ionizing photon rate from the radio continuum emission. We have also estimated the ionizing photon rate from the infrared luminosity by fitting a spectral energy distribution function to the infrared data from the GLIMPSE, MIPSGAL, and Hi-GAL surveys. The excellent sensitivity of the radio observations to extended emission allows us to investigate the actual fraction of ionizing photons that are absorbed by dust in compact and ultracompact H II regions. Barring one source, we find a direct association between the radio continuum emission from the compact and diffuse components of the H II region. Our study shows that the ionizing photon rates estimated using the radio and infrared data are within reasonable agreement (5-28%) if we include the extended emission. We also find multiple candidate ionizing stars in all our sources, and the ionizing photon rates from the radio observations and candidate stars are in reasonable agreement.

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arXiv2023

Spectral age distribution for radio-loud active galaxies in the XMM-LSS field

Jets of energetic particles, as seen in FR type-I and FR type-II sources, ejected from the center of Radio-Loud AGN affect the sources surrounding intracluster medium/intergalactic medium. Placing constraints on the age of such sources is important in order to measure the jet powers and determine the effects on feedback. To evaluate the age of these sources using spectral age models, we require high-resolution multi-wavelength data. The new sensitive and high-resolution MIGHTEE survey of the XMM-LSS field along with data from the Low Frequency Array (LOFAR) and the Giant Metrewave Radio Telescope (GMRT) provide data taken at different frequencies with similar resolution, which enables us to determine the spectral age distribution for radio loud AGN in the survey field. In this study we present a sample of 28 radio galaxies with their best fitting spectral age distribution analyzed using the Jaffe-Perola (JP) model on a pixel-by-pixel basis. Fits are generally good and objects in our sample show maximum ages within the range of 2.8 Myr to 115 Myr with a median of 8.71 Myr. High-resolution maps over a range of frequencies are required to observe detailed age distributions for small sources and high-sensitivity maps will be needed in order to observe fainter extended emission. We do not observe any correlation between the total physical size of the sources and their age and we speculate both dynamical models and the approach to spectral age analysis may need some modification to account for our observations.

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arXiv2022

A multiwavelength study of the W33 Main ultracompact HII region

The dynamics of ionized gas around the W33 Main ultracompact HII region is studied using observations of hydrogen radio recombination lines and a detailed multiwavelength characterization of the massive star-forming region W33 Main is performed. We used the Giant Meterwave Radio Telescope (GMRT) to observe the H167$α$ recombination line at 1.4 GHz at an angular resolution of 10 arcsec, and Karl. G. Jansky Very Large Array (VLA) data acquired in the GLOSTAR survey to study the dynamics of ionized gas. We also observed the radio continuum at 1.4 GHz and 610 MHz with the GMRT and used GLOSTAR 4$-$8 GHz continuum data to characterize the nature of the radio emission. In addition, archival data from submillimeter to near-infrared wavelengths were used to study the dust emission and identify YSOs in the W33 Main star-forming region. The radio recombination lines were detected at good signal to noise in the GLOSTAR data, while the H167$α$ radio recombination line was marginally detected with the GMRT. The spectral index of radio emission in the region determined from GMRT and GLOSTAR shows the emission to be thermal in the entire region. Along with W33 Main, an arc-shaped diffuse continuum source, G12.81$-$0.22, was detected with the GMRT data. The GLOSTAR recombination line data reveal a velocity gradient across W33 Main and G12.81$-$0.22. The electron temperature is found to be 6343 K and 4843 K in W33 Main and G12.81$-$0.22, respectively. The physical properties of the W33 Main molecular clump were derived by modeling the dust emission using data from the ATLASGAL and Hi-GAL surveys and they are consistent with the region being a relatively evolved site of massive star formation. The gas dynamics and physical properties of G12.81$-$0.22 are consistent with the HII region being in an evolved phase and its expansion on account of the pressure difference is slowing down.

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arXiv2022

Chandra view of Abell 407: the central compact group of galaxies and the interaction between the radio AGN and the ICM

Abell 407 (A407) is a unique galaxy cluster hosting a central compact group of nine galaxies (named as 'Zwicky's Nonet'; G1 - G9 in this work) within a 30 kpc radius region. The cluster core also hosts a luminous radio active galactic nucleus (AGN), 4C 35.06 with helically twisted jets extending over 200 kpc. With a 44 ks Chandra observation of A407, we characterize the X-ray properties of its intracluster medium (ICM) and central galaxies. The mean X-ray temperature of A407 is 2.7 keV and the $M_{200}$ is $1.9 \times 10^{14} {M_{\odot}}$. We suggest that A407 has a weak cool core at $r < 60$ kpc scales and at its very center, $< 1$-2 kpc radius, a small galaxy corona associated with the strong radio AGN. We also conclude that the AGN 4C 35.06 host galaxy is most likely G3. We suggest that the central group of galaxies is undergoing a `slow merge' procedure. The range of the merging time-scale is $0.3\sim2.3$ Gyr and the stellar mass of the future brightest cluster galaxy (BCG) will be $7.4\times10^{11} M_{\odot}$. We find that the regions which overlap with the radio jets have higher temperature and metallicity. This is consistent with AGN feedback activity. The central entropy is higher than that for other clusters, which may be due to the AGN feedback and/or merging activity. With all these facts, we suggest that A407 is a unique and rare system in the local universe that could help us to understand the formation of a massive BCG.

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arXiv2021

The Discovery of a Remnant Radio Galaxy in A2065 Using GMRT

The upgraded Giant Metrewave Radio Telescope (GMRT) has been used to map the cluster A2065 at z = 0.0726. We report the discovery of a remnant radio galaxy at the peripheral cluster region. The spatially resolved radio emission from the remnant radio galaxy shows an elongated, bar-shaped structure, whose size is $\approx$ 52$^{\prime\prime}$ $\times$ 110$^{\prime\prime}$ ($\simeq$ 72 $\times$ 152 kpc$^2$). Our study with the multiwavelength GMRT data and \textit{Chandra} data shows that across the remnant radio galaxy there is a hint of a surface-brightness edge in the hot X-ray gas. We detect tentative flattening of the radio spectral index as the old plasma at the near end of the surface-brightness edge is reinvigorated by the passage of possible shock front and shows the expected change in radio emission characteristics. We suggest that the remnant radio galaxy has been seeded by the lobes of the active galactic nucleus (AGN), hosted by the WISEA J152228.01$+$274141.3 source, demonstrating the connection between AGNs and remnant radio sources. Although the number of known remnant radio sources is beginning to increase, we emphasize the need for better data to understand the physics and nature of poorly understood remnant radio sources.

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arXiv2020

NGC 4869 in the Coma cluster: twist, wrap, overlap and bend

The upgraded Giant Metrewave Radio Telescope (GMRT) has been used to image the head-tail radio galaxy NGC 4869 in the Coma cluster with an angular resolution of 6.26 arcsec at 250-500 MHz and 2.18 arcsec at the 1050-1450 MHz bands. The archival legacy GMRT data have also been used to image the source with angular resolutions from 4.9 to 21.8 arcsec at 610 MHz, 325 MHz, 240 MHz, and 150 MHz. We find that the ~200 kpc scale radio morphology consists of five distinct regions with the clear presence of a pinch at ~1.4 arcmin (= 38.8 kpc) and a ridge at ~3.4 arcmin (= 94.2 kpc) from the head. The sharp bend by ~70 deg at ~3.5 arcmin (= 97 kpc) from the head is possibly due to projection effects. The radio spectra show progressive spectral steepening as a function of distance from the head and there is possibly re-acceleration of the synchrotron electrons and perhaps also magnetic field re-generation in the 6-208 arcsec (= 2.8-96.1 kpc) region of the jet. We report a steep spectrum sheath layer enveloping a flat spectrum spine, hinting at a transverse velocity structure with a fast-moving spine surrounded by a slow-moving sheath layer. We also derive the lifetimes of the radiating electrons and equipartition parameters. A plausible explanation for the characteristic feature, a ridge of emission perpendicular to the direction of tail is the flaring of a straight, collimated radio jet as it crosses a surface brightness edge due to Kelvin-Helmholtz instabilities.

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arXiv2020

Upgraded GMRT observations of the Coma cluster of galaxies: The observations

We have used the upgraded Giant Metrewave Radio Telescope to map the Coma cluster of galaxies at 250-500 MHz and 1050-1450 MHz bands. These 6.26 arcsec and 2.18 arcsec resolutions observations allow detailed radio structures to be determined of all detected radio sources that show both discrete pointlike and extended morphologies. We present images of a subset of 32 brightest (flux density >= 30 mJy) and dominant sources, and several sources show discrete pointlike radio morphologies. We find the steepening of the spectra consistent with synchrotron cooling in the majority of sources and the median for spectral indices is -0.78, suggesting that ~59% sources have steep spectra. The nature and the statistical properties of the radio sources in the Coma cluster will be discussed in subsequent papers.

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arXiv2019

GMRT Low-frequency Imaging of an Extended Sample of X-shaped Radio Galaxies

We present a low-frequency imaging study of an extended sample of X-shaped radio sources using the Giant Metrewave radio telescope (GMRT) at two frequencies (610 and 240 MHz). The sources were drawn from a Very Large Array FIRST-selected sample and extends an initial GMRT study at the same frequencies, of 12 X-shaped radio galaxies predominantly from the 3CR catalog (Lal & Rao 2007). Both the intensity maps and spectral index maps of the 16 newly observed sources are presented. With the combined sample of 28 X-shaped radio sources, we found no systematic differences in the spectral properties of the higher surface brightness, active lobes versus the lower surface brightness, off-axis emission. The properties of the combined sample are discussed, including the possible role of a twin active galactic nuclei model in the formation of such objects.

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arXiv2017

Giant Metrewave Radio Telescope Observations of Head-Tail Radio Galaxies

We present results from a study of seven large known head-tail radio galaxies based on observations using the Giant Metrewave Radio Telescope at 240 and 610 MHz. These observations are used to study the radio morphologies and distribution of the spectral indices across the sources. The overall morphology of the radio tails of these sources is suggestive of random motions of the optical host around the cluster potential. The presence of the multiple bends an d wiggles in several head-tail sources is possibly due to the precessing radio jets. We find steepening of the spectral index along the radio tails. The prevailing equipartition magnetic field also decreases a long the radio tails of these sources. These steepening trends are attributed to the synchrotron aging of plasma toward the ends of the tails. The dynamical ages of these sample sources have been estimated to be ~100 Myr, which is a factor of six more than the age estimates from the radiative losses due to synchrotron cooling.

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arXiv2017

The Case for Electron Re-Acceleration at Galaxy Cluster Shocks

On the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Clusters grow through accretion from these large-scale filaments and by mergers with other clusters and groups. In a growing number of galaxy clusters, elongated Mpc-size radio sources have been found, so-called radio relics. These relics are thought to trace relativistic electrons in the intracluster plasma accelerated by low-Mach number collisionless shocks generated by cluster-cluster merger events. A long-standing problem is how low-Mach number shocks can accelerate electrons so efficiently to explain the observed radio relics. Here we report on the discovery of a direct connection between a radio relic and a radio galaxy in the merging galaxy cluster Abell 3411-3412. This discovery indicates that fossil relativistic electrons from active galactic nuclei are re-accelerated at cluster shocks. It also implies that radio galaxies play an important role in governing the non-thermal component of the intracluster medium in merging clusters.

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