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Papers by Shweta Srivastava
Monthly Notices of the Royal Astronomical Society, 2020
We present low frequency, GMRT observations at 240, 610, and 1300 MHz of IC 711, a narrow angle t... more We present low frequency, GMRT observations at 240, 610, and 1300 MHz of IC 711, a narrow angle tail (NAT) radio galaxy. The total angular extent of the radio emission, ∼22 arcmin, corresponds to a projected linear size of ∼900 kpc, making it the longest among the known head-tail radio galaxies. The objectives of the GMRT observations were to investigate the radio morphology, especially of the long tail structure, at low frequencies. The radio structure, especially initial ∼10 arcmin of tail being a long straight feature, does not seem to be consistent with a simple circular motion around the cluster centre, as previously suggested in the literature. Two sharp bends after the straight section of the tail cast doubt on the prevailing idea in the literature that the long narrow tails represent trails left behind by the fast moving parent optical galaxy with respect to the cluster medium, as the optical galaxy could not have undergone such sharp bends in its path, under any conceivable...
Monthly Notices of the Royal Astronomical Society
We report low-frequency observations of Wolf-Rayet galaxies, NGC 4214 and NGC 4449 at 610, 325 an... more We report low-frequency observations of Wolf-Rayet galaxies, NGC 4214 and NGC 4449 at 610, 325 and 150 MHz, using the Giant Meterwave Radio Telescope (GMRT). We detect diffuse extended emission from NGC 4214 at and NGC 4449. NGC 4449 is observed to be five times more radio luminous than NGC 4214, indicating vigorous star formation. We estimate synchrotron spectral index after separating the thermal free-free emission and obtain α nt = −0.63 ± 0.04 (S∝ ν α nt) for NGC 4214 and −0.49 ± 0.02 for NGC 4449. About 22 per cent of the total radio emission from NGC 4214 and ∼9 per cent from NGC 4449 at 610 MHz is thermal in origin. We also study the spectra of two compact star-forming regions in NGC 4214 from 325 MHz to 15 GHz and obtain α nt = −0.32 ± 0.02 for NGC 4214-I and α nt = −0.94 ± 0.12 for NGC 4214-II. The luminosities of these star-forming regions (∼10 19 WHz −1) appear to be similar to those in circumnuclear rings in normal disc galaxies observed with similar linear resolution. We detect the supernova remnant SNR J1228+441 in NGC 4449 and estimate the spectral index of the emission between 325 and 610 MHz to be −1.8 in the epoch 2008-2009. The galaxies follow the radio-FIR correlation slopes suggesting that star formation in Wolf-Rayet galaxies, which are low-metallicity systems, are similar to that of normal disc galaxies.
2015 1st URSI Atlantic Radio Science Conference (URSI AT-RASC), 2015
Wolf-Rayet galaxies have a large population of massive WR stars which dominate the optical spectr... more Wolf-Rayet galaxies have a large population of massive WR stars which dominate the optical spectra of these galaxies. Some of these galaxies have been observed in the radio continuum at frequencies >1 GHz which reveal a variety of spectra ranging from power law to flat spectra. A small subset has also been observed at lower frequencies which suggest that there is no single combination of physical mechanisms that can explain all the observed spectra. However a larger sample at the low GMRT frequencies may help discriminate between the various physical models. In this ongoing study, we have observed & detected emission at 610, 240 and 150 MHz from two WR galaxies: Mrk 008 and Mrk 1089. In this poster paper we present the integrated spectra of these galaxies and the preliminary results from our study.
Journal of Astrophysics and Astronomy, 2011
In this paper, we present the first low frequency (< 1.4 GHz) radio continuum study of a Wolf Ray... more In this paper, we present the first low frequency (< 1.4 GHz) radio continuum study of a Wolf Rayet galaxy NGC 4214 using the Giant Meterwave Radio Telescope (GMRT). We detect diffuse extended emission from the galaxy disk at 325 MHz and find that the radio emission closely follows the ultraviolet emission mapped by GALEX. The galaxy is undergoing continuous star formation which can explain the diffuse emission. We suggest that the diffuse radio continuum emission and X-ray emission detected in the northern part of NGC 4214 is associated with a background galaxy, 2MASX J12153795+3622218.
The Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7) is a targeted survey probin... more The Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7) is a targeted survey probing the narrow-line regions (NLRs) of a representative sample of ∼ 140 nearby (z < 0.02) Seyfert galaxies by means of optical integral field spectroscopy. The survey is based on a homogeneous data set observed using the Wide Field Spectrograph WiFeS. The data provide a 25 × 38 arcsec 2 field-of-view around the galaxy centre at typically ∼ 1.5 arcsec spatial resolution and cover a wavelength range between ∼ 3400 − 7100 A at spectral resolutions of ∼ 100 km s −1 and ∼ 50 km s −1 in the blue and red parts, respectively. The survey is primarily designed to study gas excitation and star formation around AGN, with a special focus
The Astrophysical Journal, 2015
We present Wide Field Spectrograph (WiFeS) integral field spectroscopy and HST FOS spectroscopy f... more We present Wide Field Spectrograph (WiFeS) integral field spectroscopy and HST FOS spectroscopy for the LINER galaxy NGC 1052. We infer the presence of a turbulent accretion flow forming a smallscale accretion disk. We find a large-scale outflow and ionisation cone along the minor axis of the galaxy. Part of this outflow region is photoionised by the AGN, and shares properties with the ENLR of Seyfert galaxies, but the inner (R 1.0 arcsec) accretion disk and the region around the radio jet appear shock excited. The emission line properties can be modelled by a "double shock" model in which the accretion flow first passes through an accretion shock in the presence of a hard X-ray radiation, and the accretion disk is then processed through a cocoon shock driven by the overpressure of the radio jets. This model explains the observation of two distinct densities (∼ 10 4 and ∼ 10 6 cm −3 ), and provides a good fit to the observed emission line spectrum. We derive estimates for the velocities of the two shock components and their mixing fractions, the black hole mass, the accretion rate needed to sustain the LINER emission and derive an estimate for the jet power. Our emission line model is remarkably robust against variation of input parameters, and so offers a generic explanation for the excitation of LINER galaxies, including those of spiral type such as NGC 3031 (M81).
Monthly Notices of the Royal Astronomical Society, 2020
We present low frequency, GMRT observations at 240, 610, and 1300 MHz of IC 711, a narrow angle t... more We present low frequency, GMRT observations at 240, 610, and 1300 MHz of IC 711, a narrow angle tail (NAT) radio galaxy. The total angular extent of the radio emission, ∼22 arcmin, corresponds to a projected linear size of ∼900 kpc, making it the longest among the known head-tail radio galaxies. The objectives of the GMRT observations were to investigate the radio morphology, especially of the long tail structure, at low frequencies. The radio structure, especially initial ∼10 arcmin of tail being a long straight feature, does not seem to be consistent with a simple circular motion around the cluster centre, as previously suggested in the literature. Two sharp bends after the straight section of the tail cast doubt on the prevailing idea in the literature that the long narrow tails represent trails left behind by the fast moving parent optical galaxy with respect to the cluster medium, as the optical galaxy could not have undergone such sharp bends in its path, under any conceivable...
Monthly Notices of the Royal Astronomical Society
We report low-frequency observations of Wolf-Rayet galaxies, NGC 4214 and NGC 4449 at 610, 325 an... more We report low-frequency observations of Wolf-Rayet galaxies, NGC 4214 and NGC 4449 at 610, 325 and 150 MHz, using the Giant Meterwave Radio Telescope (GMRT). We detect diffuse extended emission from NGC 4214 at and NGC 4449. NGC 4449 is observed to be five times more radio luminous than NGC 4214, indicating vigorous star formation. We estimate synchrotron spectral index after separating the thermal free-free emission and obtain α nt = −0.63 ± 0.04 (S∝ ν α nt) for NGC 4214 and −0.49 ± 0.02 for NGC 4449. About 22 per cent of the total radio emission from NGC 4214 and ∼9 per cent from NGC 4449 at 610 MHz is thermal in origin. We also study the spectra of two compact star-forming regions in NGC 4214 from 325 MHz to 15 GHz and obtain α nt = −0.32 ± 0.02 for NGC 4214-I and α nt = −0.94 ± 0.12 for NGC 4214-II. The luminosities of these star-forming regions (∼10 19 WHz −1) appear to be similar to those in circumnuclear rings in normal disc galaxies observed with similar linear resolution. We detect the supernova remnant SNR J1228+441 in NGC 4449 and estimate the spectral index of the emission between 325 and 610 MHz to be −1.8 in the epoch 2008-2009. The galaxies follow the radio-FIR correlation slopes suggesting that star formation in Wolf-Rayet galaxies, which are low-metallicity systems, are similar to that of normal disc galaxies.
2015 1st URSI Atlantic Radio Science Conference (URSI AT-RASC), 2015
Wolf-Rayet galaxies have a large population of massive WR stars which dominate the optical spectr... more Wolf-Rayet galaxies have a large population of massive WR stars which dominate the optical spectra of these galaxies. Some of these galaxies have been observed in the radio continuum at frequencies >1 GHz which reveal a variety of spectra ranging from power law to flat spectra. A small subset has also been observed at lower frequencies which suggest that there is no single combination of physical mechanisms that can explain all the observed spectra. However a larger sample at the low GMRT frequencies may help discriminate between the various physical models. In this ongoing study, we have observed & detected emission at 610, 240 and 150 MHz from two WR galaxies: Mrk 008 and Mrk 1089. In this poster paper we present the integrated spectra of these galaxies and the preliminary results from our study.
Journal of Astrophysics and Astronomy, 2011
In this paper, we present the first low frequency (< 1.4 GHz) radio continuum study of a Wolf Ray... more In this paper, we present the first low frequency (< 1.4 GHz) radio continuum study of a Wolf Rayet galaxy NGC 4214 using the Giant Meterwave Radio Telescope (GMRT). We detect diffuse extended emission from the galaxy disk at 325 MHz and find that the radio emission closely follows the ultraviolet emission mapped by GALEX. The galaxy is undergoing continuous star formation which can explain the diffuse emission. We suggest that the diffuse radio continuum emission and X-ray emission detected in the northern part of NGC 4214 is associated with a background galaxy, 2MASX J12153795+3622218.
The Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7) is a targeted survey probin... more The Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7) is a targeted survey probing the narrow-line regions (NLRs) of a representative sample of ∼ 140 nearby (z < 0.02) Seyfert galaxies by means of optical integral field spectroscopy. The survey is based on a homogeneous data set observed using the Wide Field Spectrograph WiFeS. The data provide a 25 × 38 arcsec 2 field-of-view around the galaxy centre at typically ∼ 1.5 arcsec spatial resolution and cover a wavelength range between ∼ 3400 − 7100 A at spectral resolutions of ∼ 100 km s −1 and ∼ 50 km s −1 in the blue and red parts, respectively. The survey is primarily designed to study gas excitation and star formation around AGN, with a special focus
The Astrophysical Journal, 2015
We present Wide Field Spectrograph (WiFeS) integral field spectroscopy and HST FOS spectroscopy f... more We present Wide Field Spectrograph (WiFeS) integral field spectroscopy and HST FOS spectroscopy for the LINER galaxy NGC 1052. We infer the presence of a turbulent accretion flow forming a smallscale accretion disk. We find a large-scale outflow and ionisation cone along the minor axis of the galaxy. Part of this outflow region is photoionised by the AGN, and shares properties with the ENLR of Seyfert galaxies, but the inner (R 1.0 arcsec) accretion disk and the region around the radio jet appear shock excited. The emission line properties can be modelled by a "double shock" model in which the accretion flow first passes through an accretion shock in the presence of a hard X-ray radiation, and the accretion disk is then processed through a cocoon shock driven by the overpressure of the radio jets. This model explains the observation of two distinct densities (∼ 10 4 and ∼ 10 6 cm −3 ), and provides a good fit to the observed emission line spectrum. We derive estimates for the velocities of the two shock components and their mixing fractions, the black hole mass, the accretion rate needed to sustain the LINER emission and derive an estimate for the jet power. Our emission line model is remarkably robust against variation of input parameters, and so offers a generic explanation for the excitation of LINER galaxies, including those of spiral type such as NGC 3031 (M81).