arXiv subjects

J. N. Chengalur

Publications and source records attributed to J. N. Chengalur.

At least 19 recordsLinked to original sources

arXiv2022

A Green Pea starburst arising from a galaxy-galaxy merger

Green Pea galaxies are low-redshift starburst dwarf galaxies, with properties similar to those of the high-redshift galaxies that reionized the Universe. We report the first mapping of the spatial distribution of atomic hydrogen (HI) in and around a Green Pea, GP J0213+0056 at z=0.0399, using the Giant Metrewave Radio Telescope (GMRT). Like many Green Peas, GP J0213+0056 shows strong HI 21 cm emission in single-dish spectroscopy, strong Ly-alpha emission, and a high [OIII]$λ$5007/[OIII]$λ$3727 luminosity ratio, O32 $\approx$ 8.8, consistent with a high leakage of Lyman-continuum radiation. Our GMRT HI 21 cm images show that the HI 21 cm emission in the field of GP J0213+0056 arises from an extended broken-ring structure around the Green Pea, with the strongest emission coming from a region between GP J0213+0056 and a companion galaxy lying $\approx$ 4.7 kpc away, and little HI 21cm emission coming from the Green Pea itself. We find that the merger between GP J0213+0056 and its companion is likely to have triggered the starburst, and led to a disturbed HI spatial and velocity distribution, which in turn allowed Ly-alpha (and, possibly, Lyman-continuum) emission to escape the Green Pea. Our results suggest that such mergers, and the resulting holes in the HI distribution, are a natural way to explain the tension between the requirements of cold gas to fuel the starburst and the observed leakage of Ly-alpha and Lyman-continuum emission in Green Pea galaxies and their high-redshift counterparts.

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arXiv2021

The Atomic Gas Mass of Green Pea Galaxies

We have used the Arecibo Telescope and the Green Bank Telescope to carry out a deep search for H{\sc i}~21\,cm emission from a large sample of Green Pea galaxies, yielding 19 detections, and 21 upper limits on the H{\sc i} mass. We obtain H{\sc i} masses of $\rm M_{HI} \approx (4-300) \times 10^8 \, \rm M_\odot$ for the detections, with a median H{\sc i} mass of $\approx 2.6 \times 10^9 \, \rm M_\odot$; for the non-detections, the median $3σ$ upper limit on the H{\sc i} mass is $\approx 5.5 \times 10^8 \, \rm M_\odot$. These are the first estimates of the atomic gas content of Green Pea galaxies. We find that the H{\sc i}-to-stellar mass ratio in Green Peas is consistent with trends identified in star-forming galaxies in the local Universe. However, the median H{\sc i} depletion timescale in Green Peas is $\approx 0.6$~Gyr, an order of magnitude lower than that obtained in local star-forming galaxies. This implies that Green Peas consume their atomic gas on very short timescales. A significant fraction of the Green Peas of our sample lie $\gtrsim 0.6$~dex ($2σ$) above the local $\rm M_{HI} - M_B$ relation, suggesting recent gas accretion. Further, $\approx 30$\% of the Green Peas are more than $\pm 2σ$ deviant from this relation, suggesting possible bimodality in the Green Pea population. We obtain a low H{\sc i}~21\,cm detection rate in the Green Peas with the highest O32~$\equiv$~[O{\sc iii}]$λ$5007/[O{\sc ii}]$λ$3727 luminosity ratios, O32~$> 10$, consistent with the high expected Lyman-continuum leakage from these galaxies.

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arXiv2019

The Expanded Giant Metrewave Radio Telescope

With 30 antennas and a maximum baseline length of 25 km, the Giant Metrewave Radio Telescope (GMRT) is the premier low-frequency radio interferometer today. We have carried out a study of possible expansions of the GMRT, via adding new antennas and installing focal plane arrays (FPAs), to improve its point-source sensitivity, surface brightness sensitivity, angular resolution, field of view, and U-V coverage. We have carried out array configuration studies, aimed at minimizing the number of new GMRT antennas required to obtain a well-behaved synthesized beam over a wide range of angular resolutions for full-synthesis observations. This was done via two approaches, tomographic projection and random sampling, to identify the optimal locations for the new antennas. We report results for the optimal locations of the antennas of an expanded array (the "EGMRT"), consisting of the existing 30 GMRT antennas, 30 new antennas at short distances, $\leq 2.5$ km from the array centre, and 26 new antennas at long distances, $\approx 5-25$ km from the array centre. The collecting area and the field of view of the proposed EGMRT array would be larger by factors of, respectively, $\approx 3$ and $\approx 30$, than those of the GMRT. Indeed, the EGMRT continuum sensitivity and survey speed with 550-850 MHz FPAs installed on the 45 antennas within a distance of $\approx 2.5$ km of the array centre would be far better than those of any existing interferometer, and comparable to the sensitivity and survey speed of Phase-1 of the Square Kilometre Array.

astro-ph.IM↗

arXiv2017

Space Weather and Solar Wind Studies with OWFA

In this paper, we review the results of interplanetary scintillation (IPS) observations made with the legacy system of the Ooty Radio Telescope (ORT) and compare them with the possibilities opened by the upgraded ORT, the Ooty Wide Field Array (OWFA). The stability and the sensitivity of the legacy system of ORT allowed the regular monitoring of IPS on a grid of large number of radio sources and the results of these studies have been useful to understand the physical processes in the heliosphere and space weather events, such as coronal mass ejections, interaction regions and their propagation effects. In the case of OWFA, its wide bandwidth of 38 MHz, the large field of view of $\sim$27$^\circ$ and increased sensitivity provide a unique capability for the heliospheric science at 326.5 MHz. IPS observations with the OWFA would allow one to monitor more than 5000 sources per day. This, in turn, will lead to much improved studies of space weather events and solar wind plasma, overcoming the limitations faced with the legacy system. We also highlight some of the specific aspects of the OWFA, potentially relevant for the studies of coronal plasma and its turbulence characteristics.

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arXiv2015

Study of the Lynx-Cancer void galaxies-V. The extremely isolated galaxy UGC4722

We present a detailed study of the extremely isolated Sdm galaxy UGC4722 (M_B = -17.4) located in the nearby Lynx-Cancer void. UGC4722 is a member of the catalogue of isolated galaxies, and has also been identified as one of the most isolated galaxies in the Local Supercluster. Optical images of the galaxy however show that it has a peculiar morphology with an elongated ~ 14 kpc long plume. New observations with the Russian 6-m telescope (BTA) and the Giant Metrewave Radio Telescope (GMRT) of the ionised and neutral gas in UGC4722 reveal the second component responsible for the disturbed morphology of the system. This is a small, almost completely destroyed, very gas-rich dwarf (M_B = -15.2, M_HI/L_B ~4.3). We estimate the oxygen abundance for both galaxies to be 12+log(O/H) ~ 7.5-7.6, which is 2-3 times lower than what is expected from the luminosity-metallicity relation for similar galaxies in denser environments. The ugr colours of the plume derived from Sloan Digital Sky Survey (SDSS) images are consistent with a simple stellar population with a post starburst age of 0.45-0.5 Gyr. This system hence appears to be the first known case of a minor merger with a prominent tidal feature consisting of a young stellar population.

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arXiv2013

The spin temperature of high-redshift damped Lyman-$α$ systems

We report results from a programme aimed at investigating the temperature of neutral gas in high-redshift damped Lyman-$α$ absorbers (DLAs). This involved (1) HI 21cm absorption studies of a large DLA sample, (2) VLBI studies to measure the low-frequency quasar core fractions, and (3) optical/ultraviolet spectroscopy to determine DLA metallicities and velocity widths. Including literature data, our sample consists of 37 DLAs with estimates of the spin temperature $T_s$ and the covering factor. We find a strong $4σ$) difference between the $T_s$ distributions in high-z (z>2.4) and low-z (z<2.4) DLA samples. The high-z sample contains more systems with high $T_s$ values, $\gtrsim 1000$ K. The $T_s$ distributions in DLAs and the Galaxy are also clearly (~$6σ$) different, with more high-$T_s$ sightlines in DLAs than in the Milky Way. The high $T_s$ values in the high-z DLAs of our sample arise due to low fractions of the cold neutral medium. For 29 DLAs with metallicity [Z/H] estimates, we confirm the presence of an anti-correlation between $T_s$ and [Z/H], at $3.5σ$ significance via a non-parametric Kendall-tau test. This result was obtained with the assumption that the DLA covering factor is equal to the core fraction. Monte Carlo simulations show that the significance of the result is only marginally decreased if the covering factor and the core fraction are uncorrelated, or if there is a random error in the inferred covering factor. We also find evidence for redshift evolution in DLA $T_s$ values even for the z>1 sub-sample. Since z>1 DLAs have angular diameter distances comparable to or larger than those of the background quasars, they have similar efficiency in covering the quasars. Low covering factors in high-z DLAs thus cannot account for the observed redshift evolution in spin temperatures. (Abstract abridged.)

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arXiv2013

Detection of fast transients with radio interferometric arrays

Next-generation radio arrays, including the Square Kilometre Array (SKA) and its pathfinders, will open up new avenues for exciting transient science at radio wavelengths. Their innovative designs, comprising a large number of small elements, pose several challenges in digital processing and optimal observing strategies. The Giant Metrewave Radio Telescope (GMRT) presents an excellent test-bed for developing and validating suitable observing modes and strategies for transient experiments with future arrays. Here we describe the first phase of the ongoing development of a transient detection system for GMRT that is planned to eventually function in a commensal mode with other observing programs. It capitalizes on the GMRT's interferometric and sub-array capabilities, and the versatility of a new software backend. We outline considerations in the plan and design of transient exploration programs with interferometric arrays, and describe a pilot survey that was undertaken to aid in the development of algorithms and associated analysis software. This survey was conducted at 325 and 610 MHz, and covered 360 deg$^2$ of the sky with short dwell times. It provides large volumes of real data that can be used to test the efficacies of various algorithms and observing strategies applicable for transient detection. We present examples that illustrate the methodologies of detecting short-duration transients, including the use of sub-arrays for higher resilience to spurious events of terrestrial origin, localization of candidate events via imaging and the use of a phased array for improved signal detection and confirmation. In addition to demonstrating applications of interferometric arrays for fast transient exploration, our efforts mark important steps in the roadmap toward SKA-era science.

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arXiv2013

A coupling pair of dwarfs in Lynx

We report on discovery of unique binary system of dIr galaxies which looks like a low surface brightness tidal stream in the halo of another dwarf galaxy. Both the galaxies are detected in the HI line with the Giant Metrewave Radio Telescope (GMRT). The pair components have absolute blue magnitudes -16.0 and -13.2, heliocentric radial velocities 1799 and 1842 km/s, and hydrogen masses-to-luminosity ratios 0.4 and 1.6 in solar units, respectively. The binary system is characterized by a high orbital mass-to-luminosity ratio of 49 in solar units and a short crossing time of 0.22 Gyr.

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arXiv2011

The central point source in G76.9+1.0

We describe the serendipitous discovery of a radio point source in a 618 MHz image of the supernova remnant(SNR) G76.9+1.0. The SNR has a bipolar structure and the point source is located near a faint bridge of emission joining the two lobes of emission. The point source was also detected in follow-up higher frequency(1170 MHz) observations. The spectral index for the point source obtained from the GMRT observations is alpha = -2.1. The steep spectrum, as well as the location of the point source near the centre of the SNR establish the fact that it is indeed the pulsar J2022+3842 associated with this SNR. Consistent with this, subsequent analysis of archival Chandra X-ray data shows a point source coincident with the radio point source, as well as diffuse extended X-ray emission surrounding the unresolved source. However, no pulsed emission was detected despite deep searches at both 610 MHz and 1160 MHz although pulsed emission has been seen at 2 GHz with the GBT. It appears that the most likely reason for not detecting the pulsed signal at the GMRT is temporal broadening: for the estimated DM towards this SNR, the pulse broadening time could be as large as tens of milliseconds. The diffuse X-ray emission is elongated along the same direction as the bipolar structure seen in the radio. We interpret the radio lobes as having been formed from an equatorial wind. Although direct detection of pulsed signal has not been possible, we show convincingly that sensitive, high-resolution, radio imaging at multiple frequencies is a useful method to search for pulsar candidates.

astro-ph.HE↗

arXiv2010

Probing fundamental constant evolution with neutral atomic gas lines

We have detected narrow HI 21cm and CI absorption at $z \sim 1.4 - 1.6$ towards Q0458$-$020 and Q2337$-$011, and use these lines to test for possible changes in the fine structure constant $α$, the proton-electron mass ratio $μ$, and the proton gyromagnetic ratio $g_p$. A comparison between the HI 21cm and CI line redshifts yields $ΔX/X = [+6.8 \pm 1.0] \times 10^{-6}$ over $0 < \le 1.46$, where $X = g_p α^2/μ$, and the errors are purely statistical, from the gaussian fits. The simple line profiles and the high sensitivity of the spectra imply that statistical errors in this comparison are an order of magnitude lower than in previous studies. Further, the CI lines arise in cold neutral gas that also gives rise to HI 21cm absorption, and both background quasars are core-dominated, reducing the likelihood of systematic errors due to local velocity offsets between the hyperfine and resonance lines. The dominant source of systematic error lies in the absolute wavelength calibration of the optical spectra, which appears uncertain to $\sim 2$ km/s, yielding a maximum error in $ΔX/X$ of $\sim 6.7 \times 10^{-6}$. Including this, we obtain $ΔX/X = [+6.8 \pm 1.0 (statistical) \pm 6.7 (max. systematic)] \times 10^{-6}$ over $0 < \le 1.46$. Using literature constraints on $Δμ/μ$, this is inconsistent with claims of a smaller value of $α$ from the many-multiplet method, unless fractional changes in $g_p$ are larger than those in $α$ and $μ$.

astro-ph.CO↗

arXiv2009

A search for HI 21cm absorption in strong MgII absorbers in the redshift desert

We report results from a deep search for redshifted HI 21cm absorption in 55 strong MgII$λ$2796 absorbers (having $W (MgII) > 0.5 Å$) at intermediate redshifts, $0.58 < z_{\rm abs} < 1.70$, with the Green Bank Telescope (GBT) and the Giant Metrewave Radio Telescope (GMRT). Nine detections of HI 21cm absorption were obtained, all at $1.17 < z_{\rm abs} < 1.68$, including three systems reported earlier by Gupta et al. (2007). Absorption was not detected at $> 3σ$ significance in 32 other MgII absorbers, with 26 of these providing strong upper limits to the HI 21cm optical depth, $τ_{3σ} < 0.013$ per $\sim 10$ km/s. For the latter 26 systems, the spin temperature $T_s$ of the absorber must be $> [800 \times f]$ K (where $f$ is the covering factor), if the HI column density is $\ge 2 \times 10^{20}$ cm$^{-2}$, i.e. if the absorber is a damped Lyman-$α$ system (DLA). Data on the remaining 13 systems of the sample were affected by radio frequency interference and were hence not useful. Excluding "associated" systems (within 3000 km/s of the quasar redshift), the detection rate of HI 21cm absorption in strong MgII absorbers is $x_{\rm 21,MgII} ({\bar z} = 1.1) = 25^{+11}_{-8}$%, at a $3σ$ optical depth sensitivity of $\sim 0.013$ per 10 km/s. Comparing the detection rates of HI 21cm and damped Lyman-$α$ absorption in strong MgII absorber samples yields a detection rate of HI 21cm absorption in DLAs of $x_{\rm 21,DLA} ({\bar z} = 1.1) = (73 \pm 27)$%, consistent with the detection rate of HI 21cm absorption in low-$z$ DLAs. Since HI 21cm absorption arises in cold neutral gas, this indicates that most gas-rich galaxies contain significant fractions of cold HI by $z \sim 1$. (abridged)

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arXiv2009

The covering factor of high redshift damped Lyman-$α$ systems

We have used the Very Long Baseline Array to image 18 quasars with foreground damped Lyman-$α$ systems (DLAs) at 327, 610 or 1420 MHz, to measure the covering factor $f$ of each DLA at or near its redshifted HI 21cm line frequency. Including six systems from the literature, we find that none of 24 DLAs at $0.09 < z < 3.45$ has an exceptionally low covering factor, with $f \sim 0.45 - 1$ for the 14 DLAs at $z > 1.5$, $f \sim 0.41 - 1$ for the 10 systems at $z < 1$, and consistent covering factor distributions in the two sub-samples. The observed paucity of detections of HI 21cm absorption in high-$z$ DLAs thus cannot be explained by low covering factors and is instead likely to arise due to a larger fraction of warm HI in these absorbers.

astro-ph.CO↗

arXiv2007

H 21cm emission as a tracer of gas during the evolution from protoplanetary to debris disks

We present models for the HI 21 cm emission from circumstellar disks and use them to convert observed upper limits on the HI 21 cm flux to limits on the total disk gas mass. The upper limits we use come from earlier Australia Telescope Compact Array observations of the debris disk around beta Pictoris as well as fresh Giant Meterwave Radio Telescope observations of HD135344, LkCa 15 and HD163296. Our observations and models span a range of disk types, from young proto-planetary disks to old debris disks. The models self-consistently calculate the gas chemistry (H/H2 balance) and the thermal structure of UV irradiated disks. Atomic hydrogen production is dominated by UV irradiation in transition phase objects as well as debris disks, but for very young disks, HI production by stellar X-rays (which we do not account for) is important. We use a simple radiative transfer approach to convert the model disk parameters into predicted HI 21 cm line maps and spectral profiles. This allows a direct comparison of the observations to the model. We find that the HI traces the disk surface layers, and that the Hi emission, if detected, could be used to study the e ects of irradiation and evaporation, in addition to the kinematics of the disk. Our models cover massive protoplanetary disks, transition phase disks and dusty debris disks. In massive protoplanetary disks, UV produced HI constitutes less than 0.5% of the total disk mass, while X-rays clearly dominate the chemistry and thus the HI production. For the two such disks that we have observed, viz. those around LkCa 15 and HD163296, the predicted 21 cm flux is below the current detection limit. On the other hand, transition phase disks at distances of 100 pc have predicted 21 cm fluxes that are close to the detection limit.... (abstract abbreviated)

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arXiv2007

HI 21cm absorption at $z \sim 3.39$ towards PKS 0201+113

We report the GMRT detection of HI 21cm absorption from the $z \sim 3.39$ damped Lyman-$α$ absorber (DLA) towards PKS 0201+113, the highest redshift at which 21cm absorption has been detected in a DLA. The absorption is spread over $\sim 115$ km s$^{-1}$ and has two components, at $z = 3.387144 (17)$ and $z = 3.386141 (45)$. The stronger component has a redshift and velocity width in agreement with the tentative detection of Briggs et al. (1997), but a significantly lower optical depth. The core size and DLA covering factor are estimated to be $\lesssim 100$ pc and $f \sim 0.69$, respectively, from a VLBA 328 MHz image. If one makes the conventional assumption that the HI column densities towards the optical and radio cores are the same, this optical depth corresponds to a spin temperature of $\ts \sim [(955 \pm 160) \times (f/0.69)] $ K. However, this assumption may not be correct, given that no metal-line absorption is seen at the redshift of the stronger 21cm component, indicating that this component does not arise along the line of sight to the optical QSO, and that there is structure in the 21cm absorbing gas on scales smaller than the size of the radio core. We model the 21cm absorbing gas as having a two-phase structure with cold dense gas randomly distributed within a diffuse envelope of warm gas. For such a model, our radio data indicate that, even if the optical QSO lies along a line-of-sight with a fortuitously high ($\sim 50$%) cold gas fraction, the average cold gas fraction is low, ($\lesssim 17%$), when averaged over the the spatial extent of the radio core. Finally, the large mismatch between peak 21cm and optical redshifts and the complexity of both profiles makes it unlikely that the $z \sim 3.39$ DLA will be useful in tests of fundamental constant evolution.

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arXiv2006

HI 21cm Absorption at $z \sim 2.347$ towards PKS B0438-436

We report the detection of redshifted HI~21cm absorption in the $z \sim 2.347$ damped Lyman-$α$ absorber (DLA) towards PKS B0438-436, with the Green Bank Telescope. This is the second-highest redshift at which 21cm absorption has been detected in a DLA. The absorption extends over $\sim 60$ km/s and has two distinct components, at $z = 2.347477 (12)$ and $z = 2.347869 (20)$. A similar velocity structure is seen in optical metal lines, although the peak absorption here is offset by $\sim 11$ km/s from the peak in the 21cm line. We obtain a high spin temperature $T_s \sim (886 \pm 248) \times (f/0.58)$ K, using a covering factor estimated from 2.3 GHz VLBI data. However, the current data cannot rule out a low spin temperature. The non-detection of CO 3-2 absorption places the upper limit $N_{CO} < 3.8 \times 10^{15} \times (T_x/10)$ cm$^{-2}$ on the CO column density.

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arXiv2003

Detection of OH and wide HI absorption toward B0218+357

We present deep GMRT OH and WSRT HI absorption spectra of the $z = 0.6846$ gravitational lens toward B0218+357. Both the 1665 MHz and 1667 MHz OH lines are clearly detected for the first time while a new wide absorption component was detected (at low significance) in the HI spectrum; the OH spectra yielded an OH column density of $N_{OH} = 2.3 \times 10^{15}$ cm^{-2}. The ratio of 1667 and 1665 MHz equivalent widths is $\sim 1.8$ while the redshift of peak OH absorption is $z = 0.68468 \pm 0.000008$ for both lines; this redshift agrees with that obtained from the HI line. The velocity spread (between nulls) of the HI absorption is $\sim 140$ km/s, while that of both OH lines is $\sim 100$ km/s; the HI and OH spectra are broadly similar in that they each have two principal narrow components and a wide absorption trough. We argue that the wide absorption is likely to arise from source components in the Einstein ring and derive a rotation velocity of $\sim 150$ km/s at a distance of 1.5 kpc from the centre of the $z \sim 0.6846$ galaxy.

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arXiv2002

Multi-frequency GMRT Observations of the HII regions S 201, S 206, and S 209 : Galactic Temperature Gradient

We present radio continuum images of three Galactic HII regions, S 201, S 206, and S 209 near 232, 327, and 610 MHz using the Giant Meterwave Radio Telescope (GMRT). The GMRT has a mix of short and long baselines, therefore, even though the data have high spatial resolution, the maps are still sensitive to diffuse extended emission. We find that all three HII regions have bright cores surrounded by diffuse envelopes. We use the high resolution afforded by the data to estimate the electron temperatures and emission measures of the compact cores of these HII regions. Our estimates of electron temperatures are consistent with a linear increase of electron temperature with Galacto-centric distance for distances up to 18 kpc (the distance to the most distant HII region in our sample).

astro-ph↗

arXiv2002

The electron temperatures of HII regions S 201, S 206, and S 209 : Multi-frequency GMRT observations

Three Galactic HII regions, viz., S 201, S 206, and S 209 have been imaged at three frequencies, viz., 232, 327, and 610 MHz using the GMRT. The resolutions of these images are typically 15" at 232, 10" at 327, and 6" at 610 MHz. These are the highest resolution low frequency images of these HII regions. We found that all three HII regions have core--envelope morphologies. We use the high resolution afforded by the data to estimate the electron temperatures of the compact cores of these HII regions. These estimates of the electron temperatures are consistent with an increase in the temperature with Galacto-centric distance; an effect attributed to a decrease in the heavy elements abundances at large Galacto-centric distances.

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