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News Article | November 7, 2015
Site: www.techtimes.com

A team of astronomers from the National Center for Radio Astrophysics in India discovered a Giant Radio Galaxy (GRG) nine billion light years away through the Giant Metrewave Radio Telescope (GMRT). The galaxy is said to be in its dying phase, thereby providing experts with information regarding the structure of galaxies that are about to fade away. Radio galaxies discharge strong radio waves that obscure the optical signals of similar galaxies. Those that are enormous in size are very rare and the newly discovered one, named J021659-044920, emits radio signals equivalent to an extent of four million light years. Despite its mighty ability to produce such massive radio waves, the J021659-044920 is only a few hundred thousand light years in optical size. This then raises questions of how such an optically-sized galaxy generates multi-million light years worth of radio waves. Experts said that a huge black hole at the center of the galaxy pushes big scale jets of hot plasma to move in opposite directions, resulting in large radio lobes that are emitted into space. The black hole may eventually stop generating radio jets under special conditions. When this happens, the illuminating radio lobes fade away within millions of years because of inadequate resumption. The recent discovery of the J021659-044920 is something extraordinary because the astronomers were able to catch it during its dying stage. The radio jet looks as if it has been turned off and the radio lobes have began dwindling. Fading radio lobes are said to be caused by energy loss through two possible ways. The first mechanism is via the so-called inverse Compton scattering, characterized by the transfer of energy from the radio lobes to the photons of cosmic microwave backdrop. Such process leads to the creation of a dim X-ray emission. Another possible way by which energy may be lost is by releasing radio signals, which are presented as the gigantic radio lobes. "Our work presents a case study of a rare example of a GRG caught in dying phase in the distant Universe," the authors wrote. The researchers also studied their findings alongside observations made via other ground-based telescopes. Through this, they were able to analyze the various spectral features of the radio galaxy, adding in-depth understanding about its one-of-a-kind structure.

Paranjape A.,ETH Zurich | Choudhury T.R.,National Center for Radio Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2014

The size distribution of ionized regions during the epoch of reionization - a key ingredient in understanding the H I power spectrum observable by 21 cm experiments - can be modelled analytically using the excursion set formalism of random walks in the smoothed initial density field. To date, such calculations have been based on simplifying assumptions carried forward from the earliest excursion set models of two decades ago. In particular, these models assume that the random walks have uncorrelated steps and that haloes can form at arbitrary locations in the initial density field. We extend these calculations by incorporating recent technical developments that allow us to (a) include the effect of correlations in the steps of the walks induced by a realistic smoothing filter and (b) more importantly, account for the fact that dark matter haloes preferentially form near peaks in the initial density. A comparison with previous calculations shows that including these features, particularly the peaks constraint on halo locations, has large effects on the size distribution of the HII bubbles surrounding these haloes. For example, when comparing models at the same value of the globally averaged ionized volume fraction, the typical bubble sizes predicted by our model are more than a factor of 2 larger than earlier calculations. Our results can potentially have a significant impact on estimates of the observable HI power spectrum. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Chengalur J.N.,National Center for Radio Astrophysics | Pustilnik S.A.,Circassia
Monthly Notices of the Royal Astronomical Society | Year: 2013

The Giant Metrewave Radio Telescope (GMRT) HI observations, done as part of an ongoing study of dwarf galaxies in the Lynx-Cancer void, resulted in the discovery of a triplet of extremely gas rich galaxies located near the centre of the void. The triplet members SDSS J0723+3621, SDSS J0723+3622 and SDSS J0723+3624 have absolute magnitudes MB of -14.2, -11.9 and -9.7 and M(H I)/LB of ~2.9, ~10 and ~25, respectively. The gas mass fractions, as derived from the Sloan Digital Sky Survey (SDSS) photometry and the GMRT data, are 0.93, 0.997 and 0.997, respectively. The faintest member of this triplet, SDSS J0723+3624, is one of the most gas rich galaxies known. We find that all three galaxies deviate significantly from the Tully-Fisher relation, but follow the baryonic Tully-Fisher relation. All three galaxies also have a baryon fraction that is significantly smaller than the cosmic baryon fraction. For the largest galaxy in the triplet, this is in contradiction to numerical simulations. The discovery of this very unique dwarf triplet lends further support to the idea that the void environment is conducive to the formation of galaxies with unusual properties. These observations provide further motivation to do deep searches of voids for a 'hidden' very gas rich galaxy population with MB ≥ -11. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Kanekar N.,National Center for Radio Astrophysics
Astrophysical Journal Letters | Year: 2014

I report the detection of H I 21 cm absorption in two high column density damped Lyα absorbers (DLAs) at z ≈ 2 using new wide-band 250-500 MHz receivers on board the Giant Metrewave Radio Telescope. The integrated H I 21 cm optical depths are 0.85 ± 0.16 km s-1 (TXS1755+578) and 2.95 ± 0.15 km s-1 (TXS1850+402). For the z = 1.9698 DLA toward TXS1755+578, the difference in H I 21 cm and C I profiles and the weakness of the radio core suggest that the H I 21cm absorption arises toward radio components in the jet, and that the optical and radio sightlines are not the same. This precludes an estimate of the DLA spin temperature. For the z = 1.9888 DLA toward TXS1850+402, the absorber covering factor is likely to be close to unity, as the background source is extremely compact, with the entire 5 GHz emission arising from a region of ≤ 1.4 mas in size. This yields a DLA spin temperature of Ts = (372 ± 18) × (f/1.0) K, lower than typical Ts values in high-z DLAs. This low spin temperature and the relatively high metallicity of the z = 1.9888 DLA ([Zn/H] =(- 0.68 ± 0.04)) are consistent with the anti-correlation between metallicity and spin temperature that has been found earlier in damped Lyα systems. © 2014. The American Astronomical Society. All rights reserved.

Wagg J.,European Southern Observatory | Kanekar N.,National Center for Radio Astrophysics
Astrophysical Journal Letters | Year: 2012

We have used the Green Bank Telescope to carry out a deep search for redshifted CO J = 2-1 line emission from an extended (>17kpc) Lyα blob (LAB), "Himiko," at z 6.595. Our non-detection of CO J = 2-1 emission places the strong 3σ upper limit of L′CO < 1.8 × 1010 × (ΔV/250)1/2 Kkms-1pc 2 on the CO line luminosity. This is comparable to the best current limits on the CO line luminosity in LABs at z 3 and lower-luminosity Lyα emitters at z ≳ 6.5. High-z LABs appear to have lower CO line luminosities than the host galaxies of luminous quasars and submillimeter galaxies at similar redshifts, despite their high stellar mass. Although the CO-to-H2 conversion factor is uncertain for galaxies in the early universe, we assume X CO = 0.8 M (Kkms-1pc2)-1 to obtain the limit M(H2) <1.4 × 1010 M on Himiko's molecular gas mass; this is a factor of ≳ 2.5 lower than the stellar mass in the z 6.595 LAB. © 2012. The American Astronomical Society. All rights reserved..

Ekta B.,National Center for Radio Astrophysics | Chengalur J.N.,National Center for Radio Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2010

We present Giant Metrewave Radio Telescope H I observations of two extremely metal- deficient (XMD) galaxies, UM 133 and SDSS J011914.27-093546.4, with no known nearby companions. Earlier H I observations of XMD galaxies have shown that disturbed H I mor- phologies and kinematics, generally related to tidal interaction with a companion, are common in them. However, some of these galaxies were already known to be in pairs before the H I observations. The two galaxies studied here were specifically selected because they do not have any known nearby companions. None the less, we find that both the galaxies have highly disturbed H I morphologies and velocity fields. These could have arisen from interactions with more distant companions than considered in our isolation criteria or merger with extremely gas-rich dwarf companions. It is also possible that distorted H I distributions of these XMD galaxies are the result of cold gas accretion from the intergalactic medium. Our observations provide further support to the idea that inflow of metal-poor gas from the outskirts of a galaxy to the central star-forming regions (in the case of interaction), or cold gas accretion is the probable cause for the observed low emission-line metallicities of XMD galaxies. © 2010 The Authors. Journal compilation © 2010 RAS.

Ekta B.,National Center for Radio Astrophysics | Chengalur J.N.,National Center for Radio Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2010

Extremely metal-deficient (XMD) galaxies, by definition, have oxygen abundances ≤1/10 solar, and form a very small fraction of the local gas-rich, star-forming dwarf galaxy population. We examine their positions in the luminosity-metallicity (L-. Z) and mass-metallicity (M-. Z) planes, with respect to the L-. Z and M-. Z relations of other gas-rich, star-forming dwarf galaxies, viz., blue compact galaxies (BCGs) and dwarf irregular (dI) galaxies. We find that while the metallicities of some low-luminosity XMD galaxies are consistent with those expected from the L-. Z relation, other XMD galaxies are deviant, and more so as the luminosity and/or metal-deficiency increases. We determine the 95 per cent confidence interval around the L-. Z relation for BCGs, and find that its lower boundary is given by 12 + log (O/H) = - 0.177 MB + 4.87. We suggest that a galaxy should be regarded as XMD, in a statistically significant manner, only if it lies below this boundary in the L-. Z plane. Of our sample of XMD galaxies, we find that more than half are XMD by this criterion, and in fact, nine of the galaxies lie below the 99.5 per cent confidence interval about the L-. Z relation.We also determine the gas mass fractions and chemical yields of galaxies in all three samples. We find that the effective chemical yield increases with increasing baryonic mass, consistent with what is expected if outflows of metal-enriched gas are important in determining the effective yield. XMD galaxies have lower effective yield than BCG/dI galaxies of similar baryonic mass. This suggests that some process, peculiar to XMD galaxies, has resulted in their low measured metallicities. Motivated by the fact that interactions are common in XMD galaxies, we suggest that improved (tidally driven) mixing of the interstellar media (ISM) in XMD galaxies leads to a lowering of both, the measured metallicity and the calculated effective yield. In isolated dwarf galaxies, the outer parts of the stellar envelope probably do not participate in the star formation, but are still generally included in the calculation of effective yield. This results in an overestimate of the effective yield. We suggest that XMD galaxies are deviant from the L-. Z relation because of a combination of being gas rich (i.e. having processed less gas into stars) and having more uniform mixing of metals in their ISM. © 2010 The Authors. Journal compilation © 2010 RAS.

Joshi B.C.,National Center for Radio Astrophysics
International Journal of Modern Physics D | Year: 2013

In the last decade, the use of an ensemble of radio pulsars to constrain the characteristic strain caused by a stochastic gravitational wave background has advanced the cause of detection of very low frequency gravitational waves (GWs) significantly. This electromagnetic means of GW detection, called Pulsar Timing Array (PTA), is reviewed in this paper. The principle of operation of PTA, the current operating PTAs and their status are presented along with a discussion of the main challenges in the detection of GWs using PTA. © 2013 World Scientific Publishing Company.

Mitra S.,Harish Chandra Research Institute | Ferrara A.,Normal School of Pisa | Choudhury T.R.,National Center for Radio Astrophysics
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2013

The escape fraction, fesc, of ionizing photons from high-redshift galaxies is a key parameter to understand cosmic reionization and star formation history. Yet, in spite of many efforts, it remains largely uncertain. We propose a novel, semi-empirical approach based on a simultaneous match of the most recently determined luminosity functions of galaxies in the redshift range 6 ≤ z ≤ 10 with reionization models constrained by a large variety of experimental data. From this procedure, we obtain the evolution of the best-fitting values of fesc along with their 2s limits. We find that, averaged over the galaxy population, (i) the escape fraction increases from fesc = 0.068+0.054 -0.047 at z = 6 to fesc = 0.179+0.331 -0.132 at z = 8 and (ii) at z = 10 we can only put a lower limit of fesc > 0.146. Thus, although errors are large, there is an indication of a 2.6 times increase of the average escape fraction from z = 6 to 8, which might partially release the 'starving reionization' problem. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Kanekar N.,National Center for Radio Astrophysics
Astrophysical Journal Letters | Year: 2011

We report deep Green Bank Telescope spectroscopy in the redshifted NH 3 (1,1), CS 1-0, and H2CO 000-101 lines from the z ∼ 0.685 absorber toward B0218+357. The inversion (NH3) and rotational (CS, H2CO) line frequencies have different dependences on the proton-electron mass ratio μ, implying that a comparison between the line redshifts is sensitive to changes in μ. A joint three-component fit to the NH3, CS, and H2CO lines yields [Δμ/μ] = (-3.5 ± 1.2) × 10-7, from z ∼ 0.685 to today, where the error includes systematic effects from comparing lines from different species and possible frequency-dependent source morphology. Two additional sources of systematic error remain, due to time variability in the source morphology and velocity offsets between nitrogen-bearing and carbon-bearing species. We find no statistically significant (≥3σ) evidence for changes in μ and obtain the stringent 3σ constraint, [Aμ/μ] < 3.6 × 10 -7, over 6.2 Gyr; this is the best present limit on temporal changes in μ from any technique, and for any look-back time, by a factor >5. © 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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