Showing papers on "NRAO VLA Sky Survey published in 2019"

Large disparity in cosmic reference frames determined from the sky distributions of radio sources and the microwave background radiation

Abstract: The angular distribution of the cosmic microwave background radiation (CMBR) in the sky shows a dipole asymmetry, ascribed to the peculiar velocity of the observer (or rather of the Solar system), where peculiar velocity is defined as motion relative to the local comoving coordinates. The peculiar velocity thus determined turns out to be $370\text{ }\text{ }\mathrm{km}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ in the direction $\mathrm{RA}=168\ifmmode^\circ\else\textdegree\fi{}$, $\mathrm{Dec}=\ensuremath{-}7\ifmmode^\circ\else\textdegree\fi{}$. On the other hand, a dipole asymmetry in the sky distribution of radio sources in the NRAO VLA Sky Survey (NVSS), carried out by the Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO), comprising a catalog of 1.8 million sources, yielded a value for the observer's velocity approximately four times larger than the CMBR value, though the direction turned out to be in agreement with that of the CMBR dipole. This large difference in the observer's speeds with respect to the reference frames of NVSS radio sources and of CMBR, confirmed since by many independent groups, is rather disconcerting, as the observer's motion with respect to local comoving coordinates should be independent of the technique used to determine it. A genuine difference in relative speeds of two cosmic reference frames could jeopardize the cosmological principle; thence, it is crucial to confirm such discrepancies using independent samples of radio sources. We investigate here the dipole in the sky distribution of radio sources in the recent TIFR GMRT Sky Survey (TGSS), carried out by the Giant Metrewave Radio Telescope (GMRT) of the Tata Institute of Fundamental Research (TIFR), comprising a dataset of 0.62 million sources, to determine the observer's motion. We find a significant disparity in the observer's speeds relative to all three reference frames, determined from the radio source datasets and the CMBR, which does not fit with the cosmological principle, a starting point for the standard modern cosmology.

Clustering properties of TGSS radio sources

Abstract: We investigate the clustering properties of radio sources in the Alternative Data Release 1 of the TIFR GMRT Sky Survey (TGSS), focusing on large angular scales, where previous analyses have detected a large clustering signal. After appropriate data selection, the TGSS sample we use contains ~110,000 sources selected at 150 MHz over ~70% of the sky. The survey footprint is largely superimposed on that of the NRAO VLA Sky Survey (NVSS) with the majority of TGSS sources having a counterpart in the NVSS sample. These characteristics make TGSS suitable for large-scale clustering analyses and facilitate the comparison with the results of previous studies. In this analysis we focus on the angular power spectrum, although the angular correlation function is also computed to quantify the contribution of multiple-component radio sources. We find that on large angular scales, corresponding to multipoles $2 \leq \ell \leq 30$, the amplitude of the TGSS angular power spectrum is significantly larger than that of the NVSS. We do not identify any observational systematic effects that may explain this mismatch. We have produced a number of physically motivated models for the TGSS angular power spectrum and found that all of them fail to match observations, even when taking into account observational and theoretical uncertainties. The same models provide a good fit to the angular spectrum of the NVSS sources. These results confirm the anomalous nature of the TGSS large-scale power, which has no obvious physical origin and seems to indicate that unknown systematic errors are present in the TGSS dataset.

Radio sources associated with Optical Galaxies and having Unresolved or Extended morphologies (ROGUE). I. A catalog of SDSS galaxies with FIRST core identifications.

Abstract: We present the catalog of Radio sources associated with Optical Galaxies and having Unresolved or Extended morphologies I (ROGUE~I), consisting of 32,616 spectroscopically selected galaxies. It is the largest handmade catalog of this kind, obtained by cross-matching galaxies from the Sloan Digital Sky Survey (SDSS) Data Release 7 and radio sources from both the First Images of Radio Sky at Twenty Centimetre (FIRST) survey and the NRAO VLA Sky Survey \textit{without imposing a limit to the radio flux densities}. The catalog provides a \textit{visual} classification of radio and optical morphologies of galaxies presenting a FIRST core within 3\arcsec\ of the optical position. The radio morphological classification is performed by examining the radio-optical overlays of linear sizes equal to 1 Mpc at the source distance, while the 120\arcsec\ image snapshots from the SDSS database are used for optical classification. The results of our search are: (i) single-component unresolved and elongated, radio sources constitute the major group in the ROGUE I catalog ($\sim$90%), and $\sim$8% exhibiting {\it extended} morphologies, (ii) samples of 269, 730, and 115 Fanaroff-Riley (FR) type I, II, and hybrid galaxies, respectively, are presented (iii) we report 55 newly discovered giant/possible giant, 16 double-double, 9 X-shaped, and 25 Z-shaped radio sources, (iv) on the optical front, most galaxies have elliptical morphologies ($\sim$62%) while spirals form the second major category ($\sim$17%) followed by distorted ($\sim$ 12%) and lenticular ($\sim$7%) morphologies, (v) division between the FR I and the FR~II sources in the radio-optical luminosity plane is blurred, in tune with recent studies.

A broad-band spectro-polarimetric view of the NVSS rotation measure catalogue - I. Breaking the nπ-ambiguity

Abstract: The NRAO VLA Sky Survey (NVSS) Rotation Measure (RM) catalogue is invaluable for the study of cosmic magnetism. However, the RM values reported in it can be affected by n$\pi$-ambiguity, resulting in deviations of the reported RM from the true values by multiples of +-652.9 rad m-2. We therefore set off to observationally constrain the fraction of sources in the RM catalogue affected by this ambiguity. New broadband spectro-polarimetric observations were performed with the Karl G. Jansky Very Large Array (VLA) at 1--2 GHz, with 23 n$\pi$-ambiguity candidates selected by their peculiarly high |RM| values. We identified nine sources with erroneous RM values due to n$\pi$-ambiguity and 11 with reliable RM values. In addition, we found two sources to be unpolarised and one source to be inconsistent with neither n$\pi$-ambiguity nor reliable RM cases. By comparing the statistical distributions of the above two main classes, we devised a measure of how much a source's RM deviates from that of its neighbours: $\Delta/\sigma$, which we found to be a good diagnostic of n$\pi$-ambiguity. With this, we estimate that there are at least 50 sources affected by n$\pi$-ambiguity among the 37,543 sources in the catalogue. Finally, we explored the Faraday complexities of our sources revealed by our broadband observations.

A high-resolution self-consistent whole sky foreground model

Abstract: The neutral hydrogen 21 cm line is potentially a very powerful probe of the observable universe, and a number of on-goingexperiments are trying to detect it at cosmological distances. However, the presence of strong foreground radiationssuch as the galactic synchrotron radiation, galactic free-free emission and extragalactic radio sourcesmake it a very challenging task. For the design of 21 cm experiments and analysis of their data, simulation is an essential tool,and good sky foreground model is needed. With existing data the whole sky maps are available only in low angular resolutionsor for limited patches of sky, which is inadequate in the simulation of these new 21 cm experiments.In this paper, we present the method of constructing a high resolution self-consistent sky model at low frequencies,which incorporates both diffuse foreground and point sources.Our diffuse map is constructed by generating physical foreground components includingthe galactic synchrotron emission and galactic free-free emission. The point source sample is generated using theactual data from the NRAO VLA Sky Survey (NVSS) and the Sydney University Molonglo SkySurvey (SUMSS) where they are available and complete in flux limit, and mock point sourcesaccording to statistical distributions. The entire model is made self-consistent by removing the integrated flux ofthe point sources from the diffuse map so that this part of radiation is not double counted.We show that with the point sources added, a significant angular power is introduced in the mock sky map,which may be important for foreground subtraction simulations.Our sky maps and point source catalogues are available to download.

A broad-band spectro-polarimetric view of the NVSS rotation measure catalogue - II. Effects of off-axis instrumental polarization

Abstract: The NRAO VLA Sky Survey (NVSS) Rotation Measure (RM) catalogue has enabled numerous studies in cosmic magnetism, and will continue being a unique dataset complementing future polarisation surveys. Robust comparisons with these new surveys will however require further understandings in the systematic effects present in the NVSS RM catalogue. In this paper, we make careful comparisons between our new on-axis broadband observations with the Karl G. Jansky Very Large Array and the NVSS RM results for 23 sources. We found that two unpolarised sources were reported as polarised at about 0.5% level in the RM catalogue, and noted significant differences between our newly derived RM values and the catalogue values for the remaining 21 sources. These discrepancies are attributed to off-axis instrumental polarisation in the NVSS RM catalogue. By adopting the 0.5% above as the typical off-axis instrumental polarisation amplitude, we quantified its effect on the reported RMs with a simulation, and found that on average the RM uncertainties in the catalogue have to be increased by $\approx$ 10% to account for the off-axis instrumental polarisation effect. This effect is more substantial for sources with lower fractional polarisation, and is a function of the source's true RM. Moreover, the distribution of the resulting RM uncertainty is highly non-Gaussian. With the extra RM uncertainty incorporated, we found that the RM values from the two observations for most (18 out of 21) of our polarised targets can be reconciled. The remaining three are interpreted as showing hints of time variabilities in RM.

RadioGAN – Translations between different radio surveys with generative adversarial networks

Abstract: Radio surveys are widely used to study active galactic nuclei. Radio interferometric observations typically trade-off surface brightness sensitivity for angular resolution. Hence, observations using a wide range of baseline lengths are required to recover both bright small-scale structures and diffuse extended emission. We investigate if generative adversarial networks (GANs) can extract additional information from radio data and might ultimately recover extended flux from a survey with a high angular resolution and vice versa. We use a GAN for the image-to-image translation between two different data sets, namely the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) and the NRAO VLA Sky Survey (NVSS) radio surveys. The GAN is trained to generate the corresponding image cutout from the other survey for a given input. The results are analyzed with a variety of metrics, including structural similarity as well as flux and size comparison of the extracted sources. RadioGAN is able to recover extended flux density within a $20\%$ margin for almost half of the sources and learns more complex relations between sources in the two surveys than simply convolving them with a different synthesized beam. RadioGAN is also able to achieve subbeam resolution by recognizing complicated underlying structures from unresolved sources. RadioGAN generates over a third of the sources within a $20\%$ deviation from both original size and flux for the FIRST to NVSS translation, while for the NVSS to FIRST mapping it achieves almost $30\%$ within this range.

Radio spectral index from NVSS and TGSS

Abstract: I extract the radio spectral index, $\alpha$, from 541,195 common sources observed in the 150 MHz TIFR GMRT Sky Survey (TGSS) and the 1.4 GHz NRAO VLA Sky Survey (NVSS). This large common source catalogue covers about $80\%$ of the sky. The flux density limits in these surveys are such that the observed galaxies are presumably hosts of active galactic nuclei (AGNs). I confirm the steepening of $\alpha$ with increasing flux density for this large sample and provide a parametric fit between $\alpha$ and flux density. Next, I divide the data into a low flux (LF) and a high flux (HF) density sample of roughly equal number of galaxies. The LF sample contains all galaxies below 100 mJy TGSS and 20 mJy NVSS flux density and the HF sample is all galaxies above 100 mJy TGSS and 20 mJy NVSS. I observe an increase in $\alpha$ with source size (TGSS measured), saturating for large sizes to $0.89\pm0.22$ and $0.76\pm 0.21$ for the LF and HF sources, respectively. I discuss the observed results and possible physical mechanisms to explain observed $\alpha$ dependence with source size for LF and HF samples.

Clustering properties of TGSS radio sources

Abstract: We investigate the clustering properties of radio sources in the Alternative Data Release 1 of the TIFR GMRT Sky Survey (TGSS), focusing on large angular scales, where previous analyses have detected a large clustering signal. After appropriate data selection, the TGSS sample we use contains ~110,000 sources selected at 150 MHz over ~70% of the sky. The survey footprint is largely superimposed on that of the NRAO VLA Sky Survey (NVSS) with the majority of TGSS sources having a counterpart in the NVSS sample. These characteristics make TGSS suitable for large-scale clustering analyses and facilitate the comparison with the results of previous studies. In this analysis we focus on the angular power spectrum, although the angular correlation function is also computed to quantify the contribution of multiple-component radio sources. We find that on large angular scales, corresponding to multipoles $2 \leq \ell \leq 30$, the amplitude of the TGSS angular power spectrum is significantly larger than that of the NVSS. We do not identify any observational systematic effects that may explain this mismatch. We have produced a number of physically motivated models for the TGSS angular power spectrum and found that all of them fail to match observations, even when taking into account observational and theoretical uncertainties. The same models provide a good fit to the angular spectrum of the NVSS sources. These results confirm the anomalous nature of the TGSS large-scale power, which has no obvious physical origin and seems to indicate that unknown systematic errors are present in the TGSS dataset.

Studying the Flux Density of Bright Active Galaxies at Different Spectral Bands

Abstract: Statistical studies are reported in this article for an active galactic nuclei sample of different type of active galaxies Seyferts 1, Seyferts 2, and Quasars. These sources have been selected from a Catalogue for bright X-ray galaxies. The name of this index is ROSAT Bright Source Catalogue (RBSC) and the NRAO VLA Sky Survey (NVSS). In this research, multi-wavelength observational bands Radio at 1.4 GHz, Optical at 4400 A0, and X-ray at energy 0.1-2.4 KeV have been adopted in this study. The behavior of flux density ratios has been studied , with respect to the absolute magnitude . Furthermore, the Seyfert1 and Seyfert 2 objects are combined in one group and the QSOs are collectest in another group. Also, it has been found that the ratios , are increasing towards fainter optical absolute magnitude especially in Quasars.

Identification of clusters of galaxies and physical properties of clusters and their member galaxies

Abstract: Clusters of galaxies are the largest gravitationally bound systems in the Universe and are located at the knots of cosmic web. They provide powerful constraints on galaxy evolution, the formation of large scale structure of the Universe and many astrophysical processes. This paper summarizes the results we obtained in the last decade on identification of galaxy clusters, studies of the properties of clusters and cluster member galaxies. We developed a method to identify galaxy clusters by using photometric redshifts of galaxies. The method was applied to the survey data of Sloan Digital Sky Survey (SDSS), and we identified 39668 clusters from the sixth data release of SDSS, and later identified 158103 clusters from the twelfth data release of SDSS, which doubles the number of known clusters and extends the redshift range of clusters from 0.3 to 0.8. Recently, we identified 47600 clusters from all sky data of Two Micron All-Sky Survey (2MASS), Wide-field Infrared Survey Explorer (WISE) and SuperCOSMOS, which significantly enlarge the number of clusters outside of the SDSS sky region. More high-redshift clusters were also recently identified from SDSS×WISE and the deep survey data, which significantly enlarge the number of cluster at high redshift. By inspecting the SDSS color images, we found many clusters showing giant arcs, acting as the lensed system for background galaxies. Many following-up studies have been done to understand the properties of galaxy clusters, including their spatial distribution in the Universe, the dynamical state and diffuse radio emission. The two point correlation function of 79091 clusters was calculated to detect the Baryon Acoustic Oscillations (BAO) signal with 3.7σ confidence, which is the first significant detection of the BAO signal by using galaxy clusters as tracers. Based on optical photometric data, we developed a method to quantify the dynamical state of clusters and calculated the relaxation parameter for 2092 rich clusters. We found that the cluster dynamical state is significantly correlated with the absolute magnitude of Brightest Cluster Galaxy (BCG) and also depends on the magnitude difference between the first BCG and the second BCG. The diffuse radio emission of galaxy clsuters detected in merging clusters is related to dynamical state, so that the radio power of diffuse radio emission, cluster mass proxy and dynamical parameters form a fundamental plane. The properties of cluster member galaxies have been investigated in several aspects. By using 2092 clusters with dynamical states, we calculated the composite luminosity function of member galaxies. We found that more relaxed clusters have fewer bright member galaxies, but host a brighter BCG. By cross-matching the largest optical cluster sample with radio data obtained by the Very Large Array (VLA) of National Radio Astronomy Observatory (NRAO) through the NRAO VLA Sky Survey (NVSS) and Faint Images of the Radio Sky at Twenty-centimeters (FIRST), we got the largest complete sample of radio loud BCG. We obtained the radio luminosity function of BCGs, and found that it is related to the optical luminosity of BCG and dynamical state of clusters. BCGs with larger optical luminosity have higher fraction of radio loud. BCGs in more relaxed clusters have higher radio power. It is gareat era now as more deeper sky surveys on many bands are going on. We expect many breakthroughs in the research areas related to galaxy clusters.

On the clustering properties of TGSS radio sources

Abstract: We investigate the clustering properties of radio sources in the Alternative Data Release 1 of the TIFR GMRT Sky Survey (TGSS), focusing on large angular scales, where previous analyses have detected a large clustering signal. After appropriate data selection, the TGSS sample we use contains ~110,000 sources selected at 150 MHz over ~70% of the sky. The survey footprint is largely superimposed on that of the NRAO VLA Sky Survey (NVSS) with the majority of TGSS sources having a counterpart in the NVSS sample. These characteristics make TGSS suitable for large-scale clustering analyses and facilitate the comparison with the results of previous studies. In this analysis we focus on the angular power spectrum, although the angular correlation function is also computed to quantify the contribution of multiple-component radio sources. We find that on large angular scales, corresponding to multipoles $2 \leq \ell \leq 30$, the amplitude of the TGSS angular power spectrum is significantly larger than that of the NVSS. We do not identify any observational systematic effects that may explain this mismatch. We have produced a number of physically motivated models for the TGSS angular power spectrum and found that all of them fail to match observations, even when taking into account observational and theoretical uncertainties. The same models provide a good fit to the angular spectrum of the NVSS sources. These results confirm the anomalous nature of the TGSS large-scale power, which has no obvious physical origin and seems to indicate that unknown systematic errors are present in the TGSS dataset.