arXiv subjects

Veeresh Singh

Publications and source records attributed to Veeresh Singh.

29 records · Page 2Linked to original sources

arXiv2015

Discovery of rare double-lobe radio galaxies hosted in spiral galaxies

Double-lobe radio galaxies in the local Universe have traditionally been found to be hosted in elliptical or lenticular galaxies. We report the discovery of four spiral-host double-lobe radio galaxies (J0836+0532, J1159+5820, J1352+3126 and J1649+2635) that are discovered by cross-matching a large sample of 187005 spiral galaxies from SDSS DR7 to the full catalogues of FIRST and NVSS. J0836+0532 is reported for the first time. The host galaxies are forming stars at an average rate of 1.7 $-$ 10 M$_{\odot}$ yr$^{-1}$ and possess Super Massive Black Holes (SMBHs) with masses of a few times 10$^{8}$ M$_{\odot}$. Their radio morphologies are similar to FR-II radio galaxies with total projected linear sizes ranging from 86 kpc to 420 kpc, but their total 1.4 GHz radio luminosities are only in the range 10$^{24}$ $-$ 10$^{25}$ W Hz$^{-1}$. We propose that the formation of spiral-host double-lobe radio galaxies can be attributed to more than one factor, such as the occurrence of strong interactions, mergers, and the presence of unusually massive SMBHs, such that the spiral structures are not destroyed. Only one of our sources (J1649+2635) is found in a cluster environment, indicating that processes other than accretion through cooling flows e.g., galaxy-galaxy mergers or interactions could be plausible scenarios for triggering radio-loud AGN activity in spiral galaxies.

astro-ph.GA↗

arXiv2015

J021659-044920: a relic giant radio galaxy at z ~ 1.3

We report the discovery of a relic Giant Radio Galaxy (GRG) J021659-044920 at redshift $z \sim 1.3$ that exhibits large-scale extended, nearly co-spatial, radio and X-ray emission from radio lobes, but no detection of Active Galactic Nuclei core, jets and hotspots. The total angular extent of the GRG at the observed frame 0.325 GHz, using Giant Metrewave Radio Telescope observations is found to be ${\sim}$ 2.4 arcmin, that corresponds to a total projected linear size of $\sim$ 1.2 Mpc. The integrated radio spectrum between 0.240 and 1.4 GHz shows high spectral curvature ($α_{\rm 0.610~GHz}^{\rm 1.4~GHz} - α_{\rm 0.240~GHz}^{\rm 0.325~GHz}$ $>$ 1.19) with sharp steepening above 0.325 GHz, consistent with relic radio emission that is $\sim$ 8 $\times$ 10$^{6}$ yr old. The radio spectral index map between observed frame 0.325 and 1.4~GHz for the two lobes varies from 1.4 to 2.5 with the steepening trend from outer-end to inner-end, indicating backflow of plasma in the lobes. The extended X-ray emission characterized by an absorbed power-law with photon index $\sim$ 1.86 favours inverse-Compton scattering of the Cosmic Microwave Background (ICCMB) photons as the plausible origin. Using both X-ray and radio fluxes under the assumption of ICCMB we estimate the magnetic field in the lobes to be 3.3 $μ$G. The magnetic field estimate based on energy equipartition is $\sim$ 3.5 $μ$G. Our work presents a case study of a rare example of a GRG caught in dying phase in the distant Universe.

astro-ph.GA↗

arXiv2015

Radio--Far infrared correlation in "blue cloud" galaxies with 0<z<1.2

We study the radio--far infrared (FIR) correlation in "blue cloud" galaxies chosen from the PRism MUltiobject Survey (PRIMUS) up to redshift ($z$) of 1.2 in the XMM-LSS field. We use rest-frame emission at 1.4 GHz in the radio and both monochromatic (at 70$μ$m) and bolometric (between $8-1000~μ$m) emission in the FIR. To probe the nature of the correlation up to $z\sim1.2$, where direct detection of blue star-forming galaxies is impossible with current technology, we employ the technique of image stacking at 0.325 and 1.4 GHz in the radio and in six infrared bands, viz. 24, 70, 160, 250, 350 and $500~μ$m. For comparison, we also study the correlation for more luminous galaxies that are directly detected. The stacking analysis allows us to probe the radio--FIR correlation for galaxies that are up to 2 orders of magnitude fainter than the ones detected directly. The $k-$correction in the infrared wavebands is obtained by fitting the observed spectral energy distribution (SED) with a composite mid-IR power law and a single temperature greybody model. We find that the radio luminosity at 1.4 GHz ($L_{\rm 1.4GHz}$) is strongly correlated with monochromatic FIR luminosity at 70 $μ$m ($L_{\rm 70μm}$) having slope $1.09\pm0.05$ and with bolometric luminosity ($L_{\rm TIR}$) having slope $1.11\pm0.04$. The quantity $q_{\rm TIR} (=\log_{10}[L_{\rm TIR}/(3.75\times 10^{12} L_{\rm 1.4 GHz})])$ is observed to decrease with redshift as $q_{\rm TIR} \propto (1+z)^{-0.16\pm0.03}$ probably caused due to the non-linear slope of the radio--FIR correlation. Within the uncertainties of our measurement and the limitations of our flux-limited and color-selected sample, we do not find any evolution of the radio--FIR correlation with redshift.

astro-ph.GA↗

arXiv2014

Kiloparsec-scale radio emission in Seyfert and LINER galaxies

Seyfert and LINER galaxies are known to exhibit compact radio emission on $\sim$ 10 to 100 parsec scales, but larger Kiloparsec-Scale Radio structures (KSRs) often remain undetected in sub-arcsec high resolution observations. We investigate the prevalence and nature of KSRs in Seyfert and LINER galaxies using the 1.4 GHz VLA FIRST and NVSS observations. Our sample consists of 2651 sources detected in FIRST and of these 1737 sources also have NVSS counterparts. Considering the ratio of total to peak flux density ($θ$ $=$ ${\rm (S_{\rm int}/S_{\rm peak})^{1/2}}$) as a parameter to infer the presence of extended radio emission we show that $\geq$ 30$\%$ of FIRST detected sources possess extended radio structures on scales larger than 1.0 kpc. The use of low-resolution NVSS observations help us to recover faint extended KSRs that are resolved out in FIRST observations and results in $\geq$ 42.5$\%$ KSR sources in FIRST-NVSS subsample. This fraction is only a lower limit owing to the combination of projection, resolution and sensitivity effects. Our study demonstrates that KSRs may be more common than previously thought and are found across all redshifts, luminosities and radio-loudness. The extranuclear radio luminosity of KSR sources is found to be positively correlated with the core radio luminosity as well as the [O~III] $λ$5007Å line luminosity and this can be interpreted as KSRs being powered by AGN rather than star-formation. The distributions of the FIR-to-radio ratios and mid-IR colors of KSR sources are also consistent with their AGN origin. However, contribution from star-formation cannot be ruled out particularly in sources with low radio luminosities.

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arXiv2014

Multiwavelength characterization of faint Ultra Steep Spectrum radio sources : A search for high-redshift radio galaxies

Ultra Steep Spectrum (USS) radio sources are one of the efficient tracers of powerful High-z Radio Galaxies (HzRGs). In contrast to searches for powerful HzRGs from radio surveys of moderate depths, fainter USS samples derived from deeper radio surveys can be useful in finding HzRGs at even higher redshifts and in unveiling a population of obscured weaker radio-loud AGN at moderate redshifts. Using our 325 MHz GMRT observations (5-sigma ~ 800 microJy) and 1.4 GHz VLA observations (5-sigma ~ 80 - 100 microJy) available in two subfields (viz., VLA-VIMOS VLT Deep Survey (VLA-VVDS) and Subaru X-ray Deep Field (SXDF)) of the XMM-LSS field, we derive a large sample of 160 faint USS radio sources and characterize their nature. The optical, IR counterparts of our USS sample sources are searched using existing deep surveys, at respective wavelengths. We attempt to unveil the nature of our faint USS sources using diagnostic techniques based on mid-IR colors, flux ratios of radio to mid-IR, and radio luminosities. Redshift estimates are available for 86/116 (~ 74%) USS sources in the VLA-VVDS field and for 39/44 (~ 87%) USS sources in the SXDF fields with median values (z_median) ~ 1.18 and ~ 1.57, which are higher than that for non-USS radio sources (z_median non-USS ~ 0.99 and ~ 0.96), in the two subfields, respectively. The flux ratio of radio to mid-IR (S_1.4 GHz/S_3.6 micron) versus redshift diagnostic plot suggests that more than half of our USS sample sources distributed over z ~ 0.5 to 3.8 are likely to be hosted in obscured environments. A significant fraction (~ 26% in the VLA-VVDS and ~ 13% in the SXDF) of our USS sources without redshift estimates mostly remain unidentified in the existing optical, IR surveys, and exhibit high radio to mid-IR flux ratio limits similar to HzRGs, and thus, can be considered as potential HzRG candidates.

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arXiv2014

Probing obscured, high redshift galaxies using deep P-band continuum imaging with GMRT

We have carried out a deep (150 micro Jy rms) P-band, continuum imaging survey of about 40 square degrees of sky in the XMM-LSS, Lockman Hole and ELAIS-N1 fields with the GMRT. Our deep radio data, combined with deep archival observations in the X-ray (XMM/Chandra), optical (SDSS, CFHTLS), near-infrared (UKIDSS, VISTA/VIDEO), mid-infrared (Spitzer/SWIRE, Spitzer/SERVS) and far-infrared (Spitzer/SWIRE, Herschel/HerMES) will enable us to obtain an accurate census of star-forming and active galaxies out to z~2. This panchromatic coverage enables accurate determination of photometric redshifts and accurate modeling of the spectral energy distribution. We are using our large, merged photometric catalog of over 10000 galaxies to pursue a number of science goals.

astro-ph.GA↗

arXiv2014

Unveiling the population of high-redshift radio galaxies using centimeter GMRT survey

Ultra Steep Spectrum (USS) radio sources are one of the efficient tracers of High Redshift Radio Galaxies (HzRGs). To search for HzRGs candidates, we investigate properties of a large sample of faint USS sources derived from our deep 325 MHz GMRT observations combined with 1.4 GHz VLA data on the two subfields (i.e., VLA-VIMOS VLT Deep Survey (VVDS) and Subaru X-ray Deep Field (SXDF)) in the XMM-LSS field. The available redshift estimates show that majority of our USS sample sources are at higher redshifts with the median redshifts ~ 1.18 and ~ 1.57 in the VLA-VVDS and SXDF fields. In the VLA-VVDS field, ~ 20% of USS sources lack the redshift estimates as well as the detection in the deep optical, IR surveys, and thus these sources may be considered as potential high-z candidates. The radio luminosity distributions suggest that a substantial fraction (~ 40%) of our USS sample sources are radio-loud sources, distributed over redshifts ~ 0.5 to 4.

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arXiv2013

Low-frequency radio observations of Seyfert galaxies: A test to the unification scheme

Aims. We present low-frequency radio imaging and spectral properties of a well defined sample of Seyfert galaxies using GMRT 240/610 MHz dual frequency observations. Radio spectra of Seyfert galaxies over 240 MHz to 5.0 GHz are investigated using 240 MHz, 610 MHz flux densities derived from GMRT, and 1.4 GHz and 5.0 GHz flux densities mainly from published VLA data. We test the predictions of Seyfert unification scheme by comparing the radio properties of Seyfert type 1s and type 2s. Methods. We choose a sample such that the two Seyfert subtypes have matched distributions in parameters that are independent to the orientation of AGN, obscuring torus and the host galaxy. Our sample selection criteria allow us to assume that the two Seyfert subtypes are intrinsically similar within the framework of the unification scheme. Results. The new observations at 240/610 MHz, together with archival observations at 1.4 GHz, 5.0 GHz show that type 1s and type 2s have statistically similar radio luminosity distributions at 240 MHz, 610 MHz, 1.4 GHz and 5.0 GHz. The spectral indices at selected frequency intervals as well as index measured over 240 MHz to 5.0 GHz for the two Seyfert subtypes have similar distributions with median spectral index $/sim$ -0.7, consistent with the synchrotron emission from optically thin plasma. In our snap-shot 240/610 MHz GMRT observations, most of the Seyfert galaxies show primarily an unresolved central radio component, except a few sources in which faint kpc-scale extended emission is apparent at 610 MHz. Our results on the statistical comparison of the multifrequency radio properties of our sample Seyfert galaxies are in agreement with the predictions of the Seyfert unification scheme.

astro-ph.CO↗

arXiv2011

Suzaku X-ray spectral study of the Compton-thick Seyfert galaxy NGC 5135

We present the 0.5 - 50 keV Suzaku broad-band X-ray spectral study of the Compton-thick AGN in NGC 5135. The Suzaku observation provides the first detection of NGC 5135 above 10 keV that allowed us, for the first time, to estimate the absorbing column density, the intrinsic X-ray luminosity, the strength of the reflection component and the viewing angle of the torus for this AGN. The 0.5 - 10 keV spectrum of NGC 5135 is characterized by the standard components for a Compton-thick source: a scattered continuum, a prominent Fe Kα emission line (EW ~ 2.1 keV) and a soft excess. At higher energies (E > 10 keV) the intrinsic AGN continuum shows up, implying an absorbing column density of the order of ~ 2.5 \times 10^24 cm^-2 and the intrinsic 2.0 - 10 keV X-ray luminosity of ~ 1.8 \times 10^43 erg s^-1. Assuming a toroidal geometry of the reprocessing material we show that an edge-on view of the obscuring torus is preferred in this source.

astro-ph.HE↗

arXiv2011

Measuring the level of nuclear activity in Seyfert galaxies and the unification scheme

Context: The unification scheme of Seyfert galaxies hypothesizes that Seyfert type 1s and type 2s are intrinsically similar and the observed differences between the two subtypes are solely due to the differing orientations of toroidal-shaped obscuring material around the AGN. In the framework of the unification scheme, both the Seyfert subtypes are expected to show similar intrinsic nuclear properties, such as the absorption-corrected AGN X-ray luminosity, bolometric luminosity, accretion rate and the mass of the supermassive black hole. Aims: To test the predictions of the Seyfert unification scheme, we make statistical comparison of the distributions of: (i) the absorption-corrected 2.0 - 10 keV X-ray luminosities, (ii) the bolometric luminosities, (iii) the black hole masses and, (iv) the Eddington ratios, of Seyfert type 1s and type 2s. Methods: We use an optically selected Seyfert sample in which type 1s and type 2s are matched in properties that are independent to the orientation of the obscuring torus, the AGN axis and the host galaxy. Results: The distributions of the absorption-corrected 2.0 - 10 keV X-ray luminosities, the bolometric luminosities, the black masses and, the Eddington ratios are statistically similar for the two Seyfert subtypes, consistent with the orientation and obscuration based Seyfert unification scheme. The Eddington ratio distributions suggest that both the Seyfert subtypes are accreting at sub-Eddington level with wide span of Eddington ratios i.e., 10^-4 - 10^-1.

astro-ph.CO↗

arXiv2011

X-ray spectral properties of Seyfert galaxies and the unification scheme

Aims: The unification scheme of Seyfert galaxies predicts that the observed differences between type 1 and type 2 Seyfert galaxies are solely due to the differing orientations of the toroidal-shaped obscuring material around AGN. The observed X-ray spectra of Seyfert type 2s compared to type 1s are expected to be affected by higher absorbing column density due to the edge-on view of the obscuring torus. We study the 0.5 - 10 keV X-ray spectral properties of Seyfert type 1s and type 2s with the aim to test the predictions of Seyfert unification scheme in the X-ray regime. Methods: We use an optically selected Seyfert sample in which type 1s and type 2s have matched distributions in the orientation independent parameters of AGN and host galaxy. Results: The 0.5 - 10 keV XMM-Newton pn X-ray spectra of Seyfert galaxies are in general best fitted with a model consists of an absorbed power-law, a narrow Gaussian fitted to the Fe Kα emission line and an often seen soft excess component characterized by either a thermal plasma model with temperature kT \sim 0.1 - 1.0 keV and/or a steep power-law. The 2.0 - 10 keV hard X-ray continuum emission in several Seyfert type 2s is reflection dominated and suggests the Compton-thick obscuration. Results on the statistical comparison of the distributions of the observed X-ray luminosities in the soft (0.5 - 2.0 keV) and hard (2.0 - 10.0 keV) bands, the X-ray absorbing column densities, the equivalent widths of Fe Kα line and the flux ratios of hard X-ray to [OIII] λ5007Å for the two Seyfert subtypes are consistent with the obscuration and orientation based unification scheme.

astro-ph.HE↗