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Patra N.N.,NCRA TIFR | Chengalur J.N.,NCRA TIFR | Begum A.,IISER Bhopal
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present the HI column density distribution function, f (NHI), as measured from dwarf galaxies observed as part of the Faint Irregular Galaxy Giant Metrewave Radio Telescope (GMRT) Survey (FIGGS). We find that the shape of the dwarf galaxy f (NHI) is significantly different from the f (NHI) for high-redshift damped Lyman a absorbers (DLAs) or the f (NHI) for a representative sample of z = 0 gas-rich galaxies. The dwarf f (NHI) falls much more steeply at high HI column densities as compared to the other determinations. While ~10 per cent of the cross-section above NHI = 1020.3 atoms cm-2 at z = 0 is provided by dwarf galaxies, the fraction falls to ≲1 per cent by NHI ~ 1021.5 atoms cm-2. In the local universe, the contribution to the high NHI end of the f (NHI) distribution comes predominantly from the inclined discs of large galaxies. Dwarf galaxies, both because of their smaller scalelengths and their larger intrinsic axial ratios, do not produce large HI column densities even when viewed edge-on. If high-column-density DLAs/Gamma Ray Burst (GRB) hosts correspond to galaxies like the local dwarfs, this would require that either (i) the absorption arises from merging and not isolated systems or (ii) the observed lines of sight are strongly biased towards high-column-density regions. © 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Roychowdhury S.,NCRA TIFR | Chengalur J.N.,NCRA TIFR
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2012

We study the radio-far-infrared (FIR) correlation in a sample of faint dwarf irregular galaxies usingNRAOVLASky Survey (NVSS) data for 1.4-GHz radio flux, Spitzer Multiband Imaging Photometer (MIPS) 70-μm data for FIR flux and GALEX far-ultraviolet data to estimate the star formation rates (SFRs). Since our target galaxies are extremely faint, we stack images of many galaxies together to estimate the average radio and FIR fluxes. We find that for a given SFR both 70-μm and 1.4-GHz fluxes are low compared to the calibration for large spirals. None the less, the ratio of 70μm to 1.4GHz flux agrees within error bars with that seen for large galaxies. The radio-FIR correlation thus appears to be the result of a 'conspiracy'. We use the SFR to estimate the non-thermal fraction of the 1.4-GHz radio emission and find it to be around 50 per cent, much smaller than the 90 per cent typical for spirals. We also estimate the equipartition magnetic field and find it to be ~2μG, about five times smaller than that typical for spirals. © 2012 The Authors. Monthly Notices of the Royal Astronomical Society. © 2012 RAS.


Roychowdhury S.,NCRA TIFR | Chengalur J.N.,NCRA TIFR | Kaisin S.S.,Russian Academy of Sciences | Begum A.,University of Wisconsin - Madison | Karachentsev I.D.,Russian Academy of Sciences
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2011

We study the relationship between the gas column density (σHI) and the star formation rate surface density (σSFRI) for a sample of extremely small (MB ~ -13, Δ V50 ~ 30 km s-1)dwarf irregular galaxies. We find a clear stochasticity in the relation between the gas column density and star formation. All gas with σHI≳ 10 M⊙ pc-2 has some ongoing star formation, but the fraction of the gas with ongoing star formation decreases as the gas column density decreases and falls to about 50 per cent at σHI ~ 3M⊙ pc-2. Further, even for the most dense gas, the MTSY.-1.circledot star formation efficiency is at least a factor of ~2 smaller than typical of star-forming regions in spirals. We also find that the ratio of Hα emission to far-ultraviolet emission increases with the increasing gas column density. This is unlikely to be due to increasing dust extinction because the required dust-to-gas ratios are too high. We suggest instead that this correlation arises because massive (i.e. Hα-producing) stars are formed preferentially in regions with high gas density. © 2011 The Authors Monthly Notices of the Royal Astronomical Society. © 2011 RAS.


Roy S.,NCRA TIFR | Hyman S.D.,Sweet Briar College | Pal S.,University of Western Australia | Lazio T.J.W.,U.S. Navy | And 2 more authors.
Astrophysical Journal Letters | Year: 2010

We report on the detection of strong circularly polarized emission from the transient bursting source GCRT J1745 - 3009 based on new analysis of 325 MHz Giant Metrewave Radio Telescope observations conducted on 2003 September 28. We place 8 R· as the upper limit on the size of the emission region. The implied high brightness temperature required for an object beyond 1 pc and the high fraction of circular polarization firmly establish the emission as coherent. Electron cyclotron or plasma emission from a highly subsolar magnetically dominated dwarf located ≤4 kpc away could have given rise to the GCRT radio emission. © 2010. The American Astronomical Society. All rights reserved..


We have observed the Galactic center (GC) region at 0.154 and 0.255 GHz with the Giant Metrewave Radio Telescope. A total of 62 compact likely extragalactic (EG) sources are detected. Their scattering sizes decrease linearly with increasing angular distance from the GC up to about 1°. The apparent scattering sizes of the sources are more than an order of magnitude less than predicted earlier by the NE2001 model of Galactic electron distribution within 359.°5 < l < 0.°5 and -0.°5 < b < 0.°5 (Hyperstrong Scattering Region) of the Galaxy. High free-free optical depths (τ) are observed toward most of the extended non-thermal sources within 0.°6 from the GC. Significant variation of τ indicates that the absorbing medium is patchy at an angular scale of ∼10′ and n e is ∼10 cm-3, which matches the NE2001 model. This model predicts the EG sources to be resolved out from 1.4 GHz interferometric surveys. However, out of 10 EG sources expected in the region, 8 likely EG are present in the 1.4 GHz catalog. Ionized interfaces of dense molecular clouds to the ambient medium are most likely responsible for strong scattering and low radio frequency absorption. However, dense GC clouds traced by CS J = 1-0 emission are found to have a narrow distribution of ∼0.°2 across the Galactic plane. Angular distribution of most EG sources seen through the so-called Hyperstrong Scattering Region are random in b, and typically ∼7 out of 10 sources will not be seen through the dense molecular clouds, which explains why most of them are not scatter broadened at 1.4 GHz. © 2013. The American Astronomical Society. All rights reserved.


Roy S.,NCRA TIFR | Pal S.,Indian Center for Space Physics | Pal S.,Ionospheric and Earthquake Research Center
Astrophysical Journal | Year: 2013

We report the discovery of a shell-like structure G354.4+0.0 of size 1.′6 that shows the morphology of a shell supernova remnant (SNR). Part of the structure shows polarized emission in a NRAO VLA sky survey map. Based on 330 MHz and 1.4 GHz Giant Metrewave Radio Telescope observations and existing observations at higher frequencies, we conclude that the partial shell structure showing synchrotron emission is embedded in an extended H II region of size ∼4′. The spectrum of the diffuse H II region turns over between 1.4 GHz and 330 MHz. The H I absorption spectrum shows this objected to be located more than 5 kpc from Sun. Based on its morphology, non-thermal polarized emission, and size, this object is one of the youngest SNRs discovered in the Galaxy with an estimated age of ∼100-500 yr. © 2013. The American Astronomical Society. All rights reserved.


Roychowdhury S.,NCRA TIFR | Chengalur J.N.,NCRA TIFR | Karachentsev I.D.,Russian Academy of Sciences | Kaisina E.I.,Russian Academy of Sciences
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2013

We use the measured B-band axial ratios of galaxies from an updated catalogue of Local Volume galaxies to determine the intrinsic shape of dwarf irregular galaxies (de Vaucouleurs' morphological types 8-10). We find that the shapes change systematically with luminosity, with fainter galaxies being thicker. In particular, we divide our sample into sub-samples and find that the most luminous dwarfs (-19.6 < MB < -14.8) have thin discs (thickness ~0.2), ~with the disc being slightly elliptical (axial ratio ~0.8). At intermediate luminosity, viz. -14.8 < MB < -12.6, the galaxies are still characterized by elliptical discs (axial ratio ~0.7), but the discs are somewhat thicker (thickness ~0.4). The faintest dwarfs, viz. those with -12.6 < MB < -6.7 are well described as being oblate spheroids with an axial ratio ~0.5. The increasing thickness of the stellar discs of dwarf irregulars with decreasing luminosity is compatible with the increasing ratio of velocity dispersion to rotational velocity with decreasing galaxy size. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Srianand R.,Inter-University Center for Astronomy and Astrophysics | Gupta N.,Netherlands Institute for Radio Astronomy | Petitjean P.,CNRS Paris Institute of Astrophysics | Noterdaeme P.,CNRS Paris Institute of Astrophysics | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present the results of a systematic Green Bank Telescope and Giant Metrewave Radio Telescope survey for 21-cm absorption in a sample of 10 damped Lyman α systems (DLAs) at 2 ≤z abs≤ 3.4. Analysis of L-band very long baseline array images of the background QSOs are also presented. We detect 21-cm absorption in only one DLA (at z abs= 3.1745 towards J1337+3152). Thus the detection rate of 21-cm absorption is ~10per cent when no limit on the integrated optical depth (∫τ(v)dv) is imposed and ~13per cent for a 3σ limit of 0.4 km s -1. Combining our data with the data from the literature (a sample of 28 DLAs) and assuming the measured core fraction at milliarcsecond scale to represent the gas covering factor, we find that the Hi gas in DLAs at z≥ 2 is predominantly constituted by a warm neutral medium. The detection rate of 21-cm absorption seems to be higher for systems with higher N(Hi) or metallicity. However, no clear correlation is found between the integrated 21-cm optical depth (or the spin temperature, T S) and either N(Hi), metallicity or velocity spread of the low-ionization species. There are 13 DLAs in our sample for which high-resolution optical spectra covering the expected wavelength range of H 2 absorption are available. We report the detection of H 2 molecules in the z abs= 3.3871 21-cm absorber towards J0203+1134 (PKS 0201+113). In eight cases, neither H 2 (with molecular fraction f(H 2) ≤ 10 -6) nor 21-cm absorption (with T S/f c≥ 700K) is detected. The lack of 21-cm and H 2 absorption in these systems can be explained if most of the Hi in these DLAs originates from low-density high-temperature gas. In one case we have a DLA with 21-cm absorption not showing H 2 absorption. In two cases, both species are detected but do not originate from the same velocity component. In the remaining two cases 21-cm absorption is not detected despite the presence of H 2 with evidence for the presence of cold gas. All this is consistent with the idea that the H 2 components seen in DLAs are compact (with sizes of ≤15pc) and contain only a small fraction (i.e. typically ≤10per cent) of the total N(Hi) measured in the DLAs. This implies that the molecular fractions f(H 2) reported from the H 2 surveys should be considered as conservative lower limits for the H 2 components. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Roychowdhury S.,NCRA TIFR | Chengalur J.N.,NCRA TIFR | Chiboucas K.,Gemini Observatory | Chiboucas K.,University of Hawaii at Manoa | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present the results from a search for Hi emission from a sample of newly discovered dwarf galaxies in the M81 group. Hi is detected in three galaxies, all of which are classified as blue compact dwarf galaxies (BCDs). The Hi masses of these galaxies are ∼10 6 M ⊙, making them some of the lowest mass BCDs known. For these three galaxies far-ultraviolet (FUV) images (from GALEX) and Hα images (from the Russian 6m BTA telescope) are available. The Hα emission is very faint and, in principle, could be produced by a single O star. Further, in all cases we find offsets between the peak of the FUV emission and that of the Hα emission. Offsets between the most recent sites of star formation (i.e. those traced by Hα) and the older sites (i.e. those traced by FUV) would be natural if the star formation is stochastic. In spite of the expectation that the effects of mechanical feedback from star formation would most directly be seen in the smallest galaxies with low gravitational potentials, we only see tentative evidence of outflowing Hi gas associated with the star-forming region in one of the galaxies. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Patra N.N.,NCRA TIFR | Chengalur J.N.,NCRA TIFR | Karachentsev I.D.,Special Astrophysical Observatory | Kaisin S.S.,IISER Bhopal | Begum A.,IISER Bhopal
Monthly Notices of the Royal Astronomical Society | Year: 2016

We present the results of a study of the amount and distribution of cold atomic gas, as well its correlation with recent star formation in a sample of extremely faint dwarf irregular galaxies. Our sample is drawn from the Faint Irregular Galaxy GMRT Survey (FIGGS) and its extension, FIGGS2. We use two different methods to identify cold atomic gas. In the first method, line-of-sight HI spectra were decomposed into multiple Gaussian components and narrow Gaussian components were identified as cold HI. In the second method, the brightness temperature (TB) is used as a tracer of cold HI. We find that the amount of cold gas identified using the TB method is significantly larger than the amount of gas identified using Gaussian decomposition. We also find that a large fraction of the cold gas identified using the TB method is spatially coincident with regions of recent star formation, although the converse is not true. That is only a small fraction of the regions with recent star formation are also covered by cold gas. For regions where the star formation and the cold gas overlap, we study the relationship between the star formation rate density and the cold HI column density. We find that the star formation rate density has a power-law dependence on the HI column density, but that the slope of this power law is significantly flatter than that of the canonical Kennicutt-Schmidt relation. © 2015 The Authors.

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