GMRT Science Highlights

Angular momentum of dwarf galaxies
Mass and specific angular momentum are two fundamental physical parameters of galaxies. Kurapati et al. (2018) use high-resolution HI 21cm observations and broad band photometry to measure the baryonic mass (M) and baryonic specific angular momentum (j) of 11 dwarf galaxies that lie in the Lynx-Cancer void. They find that the specific angular momentum of void dwarf galaxies is similar to that of other dwarf galaxies in average density environments. However, all dwarf galaxies (regardless of environment) have significantly higher specific angular momentum than expected from an extrapolation of the relation between specific angular momentum and baryonic mass for large spiral galaxies. The figure shows the difference between the observed specific angular momentum of dwarf galaxies and the specific angular momentum expected from the bulge-less spiral relation, as a function of baryonic mass. The elevation in specific angular momentum occurs for dwarf galaxies with masses lower than roughly a billion solar masses. Galaxies above this mass threshold have relatively low baryonic specific angular momenta, following the relation obtained for massive galaxies with zero bulge fraction. Interestingly, the above mass threshold is very similar to the mass below which galaxy discs start to become systematically thick. Kurapati et al. examine the possibility that both these effects, viz. the thickening of disks and the increase in specific angular momentum, are results of feedback from star formation. Such feedback would preferentially remove the low angular momentum gas from the central parts of dwarfs (thus increasing the specific angular momentum of the system) and also inject mechanical energy into the system, leading to thicker discs. They find however, that the observed amount of observed star formation in their sample galaxies is insufficient to produce the observed increase in the specific angular momentum. It hence appears that some other, as yet unknown mechanism, plays a role in producing the observed enhancement in specific angular momentum.
Detection of the Galactic warm neutral medium in HI 21cm absorption
Patra et al. report a deep GMRT search for Galactic HI 21-cm absorption towards the quasar B0438-436, yielding the detection of wide, weak HI 21-cm absorption, with a velocity-integrated HI 21-cm optical depth of 0.0188 +/- 0.0036 km/s. Comparing this with the HI column density measured in the Parkes Galactic All-Sky Survey gives a column density-weighted harmonic mean spin temperature of 3760 +/- 365 K, one of the highest measured in the Galaxy. This is consistent with most of the HI along the sightline arising in the stable warm neutral medium. The low-peak HI 21-cm optical depth towards B0438-436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the HI 21-cm absorption and emission spectra. This yields a gas kinetic temperature T_k <= (4910 +/- 1900) K, and a spin temperature T_s = (1000 +/- 345) K for the gas that gives rise to the HI 21-cm absorption. The GMRT data are consistent with the HI 21-cm absorption arising from either the stable WNM, with T_s << T_k, T_k~5000 K, and little penetration of the background Lyman-alpha radiation field into the neutral hydrogen, or the unstable neutral medium, with T_sT_k~1000 K. The figure shows results of the multi-Gaussian joint decomposition of the (A) HI 21-cm emission and (B) HI 21-cm absorption spectra. The top panels show the best-fit model (solid curve) overlaid on the two spectra, while the bottom two panels show the residuals from the fit.
Probing Star Formation in Galaxies at z~1
Bera et al. carried out deep GMRT 610 MHz imaging of four fields of the DEEP2 Galaxy Redshift Survey, and stacked the radio emission from a sample of nearly 4000 blue star-forming galaxies at 0.7<z< 1.45 to detect the median rest-frame 1.4 GHz radio continuum emission of the galaxies. The authors used the local relation between total star formation rate (SFR) and radio 1.4 GHz luminosity to infer a median total SFR of (24.4 +/- 1.4) solar masses per year for blue star-forming galaxies at these redshifts. They detect the main-sequence relation between SFR and stellar mass, and find that the power-law index of the main sequence shows no change over z~0.7-1.45. They also find that the nebular line emission suffers less extinction than the stellar continuum, contrary to the situation in the local Universe; further, the ratio of nebular extinction to stellar extinction increases with decreasing redshift. They combined their results with earlier GMRT HI 21cm emission studies of the DEEP2 fields to obtain an upper limit of 0.87 Gyr to the atomic gas depletion time of star-forming galaxies at z~1.3. Neutral atomic gas thus appears to be a transient phase in high-z star-forming galaxies. The left panel of the figure shows the stacked rest-frame 1.4 GHz radio emission of the galaxies of the sample, detected at high statistical significance; the right panel shows a similar stack at neighbouring locations ("off-source") which shows no signal, indicating that the detected signal of the left panel is very unlikely to arise from systematic effects.
A Post-correlation Beamformer for Time-domain Studies of Pulsars and Transients
Roy, Chengalur & Pen have demonstrated that a new way of beam-forming called post-correlation beam-forming (i.e. beam-forming which involves only phased sums of the correlation of the voltages of different antennas in an array) significantly improves the capabilities and sensitivity of the upgraded GMRT for discovering new pulsars and fast radio bursts (FRBs). Compared with the traditionally used incoherent (IA) and phased (PA) beam-forming techniques in radio telescopes for time-domain astronomy, this new technique dramatically reduces the effect of red-noise and radio frequency interference, yielding more than factor of 2 improvements in the  GMRT time-domain survey sensitivity. The eye-catching improvements in the signal-to-noise of the pulses from PSR J2144-3933 can be seen in the single-pulse time-series from the post-correlation beam-former. The extremely well-cleaned post-correlation beam also has an order of magnitude reduction in red-noise, as is clear in the power spectra plot. The post-correlation beam formation beautifully brings out the hitherto unexplored capability of interferometric arrays (the future of radio astronomy) over single dish telescopes.  We describe a time-domain survey with the GMRT using this post-correlation beam formation,  which will be one of the most sensitive surveys for pulsars and FRBs at low and mid-range radio frequencies.
Curvature in the spectrum of a remnant radio galaxy with the uGMRT
The origin of cosmic rays in the intra-cluster medium (ICM) has been attributed to re-acceleration of charged particles in shocks and turbulence. For these re-acceleration mechanisms to work, it is expected that there will be reservoirs of seed cosmic ray electrons in the ICM. Radio galaxies with jets and lobes are strong candidates for providing these seeds. Dr. Kale and collaborators have used the unique broad band observing capabilities of the recently operational upgraded Giant Metrewave Radio Telescope (uGMRT) to study an enigmatic "dead radio galaxy" or a "remnant radio relic" in the galaxy cluster Abell 4038. They have shown that the spectrum of the source varies considerably across its extent and undergoes extreme changes from high to low frequencies that are quantified in a parameter called the "spectral curvature". The authors fine that the assumption that the spectra of seed particles are simple power-laws may not be correct, given the extreme spectral curvature measured using the uGMRT images. Their study has recently been accepted for publication in the Monthly Notices of the Royal Astronomical Society.
Electron Cyclotron Maser Emission from a radio star
HD133880 is a B-type rapidly-rotating star, with a period less than 1 day, on the main sequence. It is characterised by the presence of an asymmetric dipolar magnetic field of kiloGauss strength. Gyro-synchrotron radio emission has earlier been detected from this star. In 2015, Chandra et al. reported strong enhancement in the star's radio flux (at 610 MHz and 1420 MHz) at certain rotational phases, but the phase coverage was too limited for a detailed study. In the present work, Das, Chandra & Wade aimed to understand the origin of the radio pulses, by using the Giant Metrewave Radio Telescope (GMRT) 610 and 1420 MHz receivers to observe the star over a complete rotation. The GMRT 610 MHz data revealed a dramatic increase (by an order of magnitude) in the star's radio emission at a narrow epoch (phase 0.73) during its rotation, and in the right circular polarization; this can be seen in the upper panel of the attached figure. The observed enhancement is confined to a narrow range of phases and is approximately 100% polarised. Further, the enhancement occurs when the line of sight magnetic field is nearly zero, as can be seen in the lower panel of the figure. Das et al. find that the GMRT data single out Electron Cyclotron Maser Emission as the likely cause of the observed enhanced radiation. This maser process arises, under suitable conditions, due to the interaction of electromagnetic waves with a population of mildly relativistic electrons in a magnetised plasma. Previously, only one magnetic star (CU Vir) was known to host this mechanism, and it was unclear if this is a specific property of CU Vir or a common property of magnetic stars. The discovery of the maser mechanism in a second star rules out the first possibility and, since the maser process is more favourable at low frequencies, emphasizes the importance of more low frequency studies of magnetic stars to further understand the physical conditions that give rise to the maser.
The angular momentum content of gas-rich dwarf galaxies
A galaxy's spin is intricately connected to its morphology --- spiral galaxies spin faster and hence are thinner whereas elliptical galaxies have lower specific angular momentum and are puffier. The mass and the angular momentum of a galaxy are related via their evolutionary history. Various researchers in the past have reported a power-law scaling relation between the mass and the specific angular momentum of large spiral galaxies. Chowdhury and Chengalur used archival GMRT, VLA and WSRT HI 21cm data of five gas-rich dwarf galaxies and found that the specific angular momentum in these smaller, less massive, dwarf galaxies is significantly higher than that expected from the earlier studies of spiral disks. The figure shows the location of these dwarf galaxies in the specific angular momentum - mass plane, and compares them with the distribution of spiral galaxies. All the five gas-rich dwarf galaxies lie outside the 95% probability band of the relation for spiral galaxies. The chance probability that the dwarf galaxies belong to the same angular momentum - mass distribution as the spirals is less than one part in a million. The authors suggest two mechanisms through which the dwarfs may acquire their higher specific angular momentum: (i) preferential outflow of low angular momentum gas due to stellar feedback, and (ii) cosmic cold mode accretion, which is known to dominate in less massive galaxies.
A fourth radio arc in Abell 2626
The supermassive black holes at the centres of active galaxies can lead to the formation of spectacular jets that are detectable in deep radio imaging studies. When such black holes are situated close to the centres of galaxy clusters, they experience a dense environment. The radio jets can be affected by the black hole itself and by the environment, leading to complex morphologies. A system of three concave arcs was earlier known towards the galaxy cluster Abell 2626. Kale & Gitti used the 610 MHz receivers of the GMRT to discover a fourth arc in the sytem, that completes an intriguing symmetric structure of four arcs around the central massive galaxy that itself has two active nuclei. The origin of the exotic source is as yet unknown, but may be a rare event of precessing jets from the double nuclei of the central galaxy or a similarly rare configuration of a gravitational lens. The image shows the GMRT radio image in blue, overlaid on X-ray (red) and optical (green) images.
A Giant Radio Galaxy at z ~ 0.57
Giant radio galaxies (GRGs) are radio galaxies whose linear extent is more than 1 Mpc. Most of the known GRGs are less than a billion light years away from us. The sharp decline in the number of GRGs at larger distances, i.e. higher redshifts, is a mystery because the number of normal radio sources is actually higher at high redshifts. We recently used the GMRT to carry out a deep 150 MHz study of a small region of the sky in the Lynx constellation, and discovered a large GRG, of size 7 million light years, at a distance of about 5 billion light years, i.e. a redshift of 0.57. We used the GMRT to carry out detailed imaging studies of the GRG, at 325 MHz, 610 MHz and 1420 MHz; the new data suggest that the object is probably a double-double radio galaxy. Further, the radio core of the galaxy shows an unusually steep spectrum, which may imply that there is yet another unresolved pair of lobes within the core, making this GRG a candidate triple-double radio galaxy. Further investigations of the central region of the GRG, to test if it is a re-started radio source, are now under way using the European Very Long Baseline Interferometry Network (EVN), which has the resolution to probe the central region very close to the supermassive black hole. The figure shows the GMRT 610 MHz image of the new GRG, overlaid on the optical SDSS gri-composite image. The optical host galaxy is shown separately in the rectangular box. The double-lobe structure on either side of the central core is clearly visible.
GMRT monitoring of the X-ray binary V404 Cygni during its June 2015 outburst
Chandra & Kanekar used the GMRT at 1280, 610, 325 and 235 MHz to monitor the black hole X-ray binary V404 Cygni during its 2015 June outburst, extending for a period of 2.5 weeks, and beginning on June 26.9 UT, a day after the strongest radio/X-ray outburst. They find the low-frequency radio emission of V404 Cygni to be extremely bright and fast-decaying in the outburst phase, with an inverted spectrum below 1.5 GHz and an intermediate X-ray state. The radio emission settles to a weak, quiescent state roughly 11 days after the outburst, with a flat radio spectrum and a soft X-ray state. Combining the GMRT measurements with flux density estimates from the literature, the authors identify a spectral turnover in the radio spectrum at ~1.5 GHz on June 26.9 UT (see the attached image), indicating the presence of a synchrotron self-absorbed emitting region. They use the measured flux density at the turnover frequency with the assumption of equipartition of energy between the particles and the magnetic field to infer the jet radius, magnetic field, minimum total energy, and transient jet power. The relatively low value of the jet power, despite V404 Cygni’s high black hole spin parameter, suggests that the radio jet power does not correlate with the spin parameter.
Discovery of a radio relic in the low mass galaxy cluster PLCK G200.9-28.2
Kale et al. used the Giant Metrewave Radio Telescope (GMRT), the XMM-Newton X-ray Observatory, and the Jansky Very Large Array to discover a new radio relic in the galaxy cluster PLCKG200.9-28.2 at z~0.22. Such arc-like radio relics are usually found at the periphery of massive colliding clusters, and are extremely rare, arising in fewer than 5% of merging clusters. Despite their rarity, radio relics are an excellent tracer of the shocks that are expected to be driven in the diffuse intra-cluster medium by violent cluster collisions. Indeed, it is very difficult to even detect these shocks at other wavelengths. So far, radio relics have been found only in the vicinity of merging massive clusters. The new radio relic detected by Kale et al. is very interesting because it arises in a cluster of low mass, the lowest mass at which such a relic has ever been seen! This demonstrates that violent mergers in low-mass clusters are capable of producing strong shock waves in their diffuse media. In the adjoining figure, the 235 MHz emission imaged with the GMRT is shown in red and the X-ray emission imaged with the XMM-Newton satellite observatory is shown in blue. The elongated source seen in red is the new radio relic.
GMRT imaging of a high-energy supernova remnant
Nayana et al. used the Giant Metrewave Radio Telescope (GMRT) to detect 325 and 610 MHz radio emission from HESS J1731-347, one of only five known very-high-energy (VHE; > 0.1 TeV) shell-type supernova remnants (SNRs). Multiple filaments of the SNR are clearly seen in the GMRT 610 and 325 MHz images, shown, respectively, in the left and right panels of the adjacent figure. However, the faintest feature in the GMRT bands corresponds to the peak in the VHE emission. This anti-correlation can be explained if the observed VHE gamma-ray emission has a leptonic origin. The individual filaments of the SNR (indicated by "1", "2", "3", and "4") have steep radio spectra, consistent with a non-thermal origin.
An AGN's rendezvous with a radio relic
van Weeren et al. used data from the Chandra X-ray Observatory, the Giant Metrewave Radio Telescope, the Jansky Very Large Array, and other telescopes to discover a cosmic event never seen before. Galaxy clusters contain multiple sources of radio emission, including active galactic nuclei (AGNs), radio halos and radio relics. A long-standing problem in studies of clusters is how low-Mach-number shocks can accelerate electrons efficiently to produce the observed radio relics. van Weeren et al. discovered, for the first time, a direct connection between a radio relic and an AGN (a radio galaxy) in the merging galaxy cluster Abell 3411-3412 by combining radio, X-ray and optical data. This discovery indicates that fossil relativistic electrons from AGNs are re-accelerated at cluster shocks. It also implies that radio galaxies play an important role in governing the non-thermal component of the intra-cluster medium in merging clusters. For the first time, two of the most powerful phenomena in the Universe have been clearly linked together in the same system. Image credits: X-ray: NASA/CXC/SAO/R. van Weeren et al; Optical: NAOJ/Subaru;