Faint Images Of The Radio Sky At Twenty Centimeters Research Articles

A GREEN BANK TELESCOPE SURVEY FOR H I 21 cm ABSORPTION IN THE DISKS AND HALOS OF LOW-REDSHIFT GALAXIES

We present an H i 21 cm absorption survey with the Green Bank Telescope (GBT) of galaxy–quasar pairs selected by combining galaxy data from the Sloan Digital Sky Survey (SDSS) and radio sources from the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey. Our sample consists of 23 sight lines through 15 low-redshift foreground galaxy–background quasar pairs with impact parameters ranging from 1.7 kpc up to 86.7 kpc. We detected one absorber in the GBT survey from the foreground dwarf galaxy, GQ1042+0747, at an impact parameter of 1.7 kpc and another possible absorber in our follow-up Very Large Array (VLA) imaging of the nearby foreground galaxy UGC 7408. The line widths of both absorbers are narrow (FWHM of 3.6 and 4.8km s−1). The absorbers have sub-damped Lyα column densities, and most likely originate in the disk gas of the foreground galaxies. We also detected H i emission from three foreground galaxies including UGC 7408. Although our sample contains both blue and red galaxies, the two H i absorbers as well as the H i emissions are associated with blue galaxies. We discuss the physical conditions in the 21 cm absorbers and some drawbacks of the large GBT beam for this type of survey.

Herschel-ATLAS: the far-infrared-radio correlation at z < 0.5★

We use data from the Herschel-ATLAS to investigate the evolution of the far-infrared--radio correlation over the redshift range 0<z<0.5. Using the total far-infrared luminosity of all >5sigma sources in the Herschel-ATLAS Science Demonstration Field and cross-matching these data with radio data from the Faint Images of the Radio Sky at Twenty-Centimetres (FIRST) survey and the NRAO VLA Northern Sky Survey (NVSS), we obtain 104 radio counterparts to the Herschel sources. With these data we find no evidence for evolution in the far-infrared--radio correlation over the redshift range 0<z<0.5, where the median value for the ratio between far-infrared and radio luminosity, $q_{\rm IR}$, over this range is $q_{\rm IR} = 2.40\pm 0.12$ (and a mean of $q_{\rm IR}=2.52 \pm 0.03$ accounting for the lower limits), consistent with both the local value determined from {\em IRAS} and values derived from surveys targeting the high-redshift Universe. By comparing the radio fluxes of our sample measured from both FIRST and NVSS we show that previous results suggesting an increase in the value of $q_{\rm IR}$ from high to low redshift may be the result of resolving out extended emission of the low-redshift sources with relatively high-resolution interferometric data, although AGN contamination could still play a significant role. We also find tentative evidence that the longer wavelength, cooler dust is heated by an evolved stellar population which does not trace the star-formation rate as closely as the shorter wavelength $\ltsim 250~\mu$m emission or the radio emission, supporting suggestions based on detailed models of individual galaxies.

Flat-spectrum radio quasars from the SDSS DR3 quasar catalogue

We constructed a sample of 185 flat-spectrum radio quasars (FSRQs) by cross-correlating Shen et al.'s Sloan Digital Sky Survey Data Release 3 (SDSS DR3) X-ray quasar sample with the Faint Images of the Radio Sky at Twenty Centimeters (FIRST) and Green Bank 6-cm survey (GB6) radio catalogues. From the spectral energy distribution (SED) constructed using multi-band (radio, UV, optical, infrared and X-ray) data, we derived the synchrotron peak frequency and peak luminosity. The black hole mass MBH and the broad-line region (BLR) luminosity (then the bolometric luminosity Lbol) were obtained by measuring the linewidth and strength of broad emission lines from SDSS spectra. We define a subsample of 118 FSRQs for which non-thermal jet emission is thought to dominate over thermal emission from the accretion disc and host galaxy. For this subsample, we found 25 FSRQs having synchrotron peak frequency νpeak > 1015 Hz, which is higher than the typical value for FSRQs. These sources with high νpeak could be targets for the Fermi gamma-ray telescope. Only a weak anti-correlation is found between the synchrotron peak frequency and peak luminosity, while no strong correlation is present between either the synchrotron peak frequency and black hole mass or the synchrotron peak frequency and the Eddington ratio Lbol/LEdd. When combining the FSRQ subsample with Wu et al.'s sample of 170 BL Lac objects, a strong anti-correlation between the synchrotron peak frequency and luminosity is apparently present, covering about seven orders of magnitude in νpeak. However, the anti-correlation differs from the blazar sequence in having large scatter. At a similar peak frequency, the peak luminosity of FSRQs with νpeak > 1015 Hz is systematically higher than that of BL Lac objects, with some FSRQs outside the range covered by BL Lac objects. Although high νpeak values are found in some FSRQs, they do not reach the extreme value of BL Lacs. For the subsample of 118 FSRQs, we found significant correlations between the peak luminosity and black hole mass, the Eddington ratio and the BLR luminosity, indicating that the jet physics may be strongly related to the accretion process.

The Combined NVSS-FIRST Galaxies (CoNFIG) sample - I. Sample definition, classification and evolution

The CoNFIG (Combined NVSS–FIRST Galaxies) sample is a new sample of 274 bright radio sources at 1.4 GHz. It was defined by selecting all sources with S1.4 GHz≥ 1.3 Jy from the NRAO Very Large Array (VLA) Sky Survey (NVSS) in the north field of the Faint Images of the Radio Sky at Twenty centimetres (FIRST) survey. New radio observations obtained with the VLA for 31 of the sources are presented. The sample has complete Fanaroff–Riley (FRI)/FRII morphology identification; optical identifications and redshifts are available for 80 and 89 per cent of the sample, respectively, yielding a mean redshift of ∼0.71. One of the goals of this survey is to get better definitions of luminosity distributions and source counts of FRI/FRII sources separately, in order to determine the evolution of the luminosity function for each type of source. We present a preliminary analysis, showing that these data are an important step towards examining various evolutionary schemes for these objects and to confirm or correct the dual population unified scheme for radio active galactic nuclei (AGN). Improving our understanding of radio galaxy evolution will give better insight into the role of AGN feedback in galaxy formation.

Radio Star Candidates from FIRST and 2MASS Databases

AbstractWe positionally cross-identified FIRST (the Faint Images of the Radio Sky at Twenty centimeters) catalogue with 2MASS (the Two Micron All Sky Survey) pointed-source database and collected the data from radio band and near infrared band. Then the data were cross-matched with the Véron-Cetty &amp; Véron 2006 catalog and the Tycho-2 catalog, respectively. Therefore the known samples of quasars and stars are obtained. We applied principal component analysis (PCA) on the known sample. The overall sample may be projected in the principal component space. From the space, we can easily locate the area that radio stars occupy, and select out radio star candidates. With the follow-up observation of these candidates, the properties of radio stars may be studied.

Comparing Pattern Recognition Feature Sets for Sorting Triples in the FIRST Database

Pattern recognition techniques have been used with increasing success for coping with the tremendous amounts of data being generated by automated surveys. Usually this process involves construction of training sets, the typical examples of data with known classifications. Given a feature set, along with the training set, statistical methods can be employed to generate a classifier. The classifier is then applied to process the remaining data. Feature set selection, however, is still an issue. This paper presents techniques developed for accommodating data for which a substantive portion of the training set cannot be classified unambiguously, a typical case for low-resolution data. Significance tests on the sort-ordered, sample-size-normalized vote distribution of an ensemble of decision trees is introduced as a method of evaluating relative quality of feature sets. The technique is applied to comparing feature sets for sorting a particular radio galaxy morphology, bent-doubles, from the Faint Images of the Radio Sky at Twenty Centimeters (FIRST) database. Also examined are alternative functional forms for feature sets. Associated standard deviations provide the means to evaluate the effect of the number of folds, the number of classifiers per fold, and the sample size on the resulting classifications. The technique also may be applied to situations for which, although accurate classifications are available, the feature set is clearly inadequate, but is desired nonetheless to make the best of available information.

A sample of radio-loud active galactic nuclei in the Sloan Digital Sky Survey

A sample of 2712 radio-luminous galaxies is defined from the second data release of the Sloan Digital Sky Survey (SDSS) by cross-comparing the main spectroscopic galaxy sample with two radio surveys: the National Radio Astronomy Observatories (NRAO) Very Large Array (VLA) Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty centimeters (FIRST) survey. The comparison is carried out in a multistage process and makes optimal use of both radio surveys by exploiting the sensitivity of the NVSS to extended and multicomponent radio sources in addition to the high angular resolution of the FIRST images. A radio source sample with 95 per cent completeness and 98.9 per cent reliability is achieved, far better than would be possible for this sample if only one of the surveys was used. The radio source sample is then divided into two classes: radio-loud active galactic nuclei (AGN) and galaxies in which the radio emission is dominated by star formation. The division is based on the location of a galaxy in the plane of 4000-A break strength versus radio luminosity per unit stellar mass and provides a sample of 2215 radio-loud AGN and 497 star-forming galaxies brighter than 5 mJy at 1.4 GHz. A full catalogue of positions and radio properties is provided for these sources. The local radio luminosity function is then derived both for radio-loud AGN and for star-forming galaxies and is found to be in agreement with previous studies. By using the radio to far-infrared (FIR) correlation, the radio luminosity function of star-forming galaxies is also compared to the luminosity function derived in the FIR. It is found to agree well at high luminosities but less so at lower luminosities, confirming that the linearity of the radio to FIR correlation breaks down below about 10 22 W Hz -1 at 1.4GHz.

The cosmic evolution of low-luminosity radio sources from the Sloan Digital Sky Survey Data Release 1

In this paper we constrain the evolution in the comoving space density for low-luminosity [typically Fanaroff–Riley type I (FRI)] radio sources as a function of cosmic epoch by matching deep radio surveys [Westerbork Northern Sky Survey (WENSS), Faint Images of the Radio Sky at Twenty-centimetres (FIRST) and NRAO VLA Sky Survey (NVSS)] with the Sloan Digital Sky Survey (SDSS). This results in 1014 matched radio sources covering an effective area of 0.217 sr, 686 with L325 MHz < 1025 W Hz−1 sr−1, which is an order of magnitude larger than any previous study at these luminosities. Using the non-parametric V/Vmax test we show that low-luminosity radio sources evolve differently from their more powerful, predominantly Fanaroff–Riley type II (FRII), counterparts. Indeed, we find that the lower luminosity population is consistent with a constant comoving space density with redshift, as opposed to the strong positive evolution for the more powerful sources.

Radio Continuum Emission at 1.4 GHz from KISS Emission-Line Galaxies

We have searched the Faint Images of the Radio Sky at Twenty centimeters (FIRST) and the NRAO VLA Sky Survey (NVSS) 1.4 GHz radio surveys for sources that are coincident with emission-line galaxy (ELG) candidates from the KPNO International Spectroscopic Survey (KISS). A total of 207 of the 2157 KISS ELGs (∼10%) in the first two Hα-selected survey lists were found to possess radio detections in FIRST, NVSS, or both. Follow-up spectra exist for all of the radio detections, allowing us to determine the activity type (star-forming vs. active nucleus) for the entire sample. We explore the properties of the radio-detected KISS galaxies in order to gain a better insight into the nature of radio-emitting galaxies in the local universe (z < 0.1). No dwarf galaxies were detected, despite the large numbers of low-luminosity galaxies present in KISS, suggesting that lower mass, lower luminosity objects do not possess strong galaxian-scale magnetic fields. Because of the selection technique used for KISS, our radio ELGs represent a quasi–volume-limited sample, which allows us to develop a clearer picture of the radio galaxy population at low redshift. Nearly two-thirds of the KISS radio galaxies are starburst or star-forming galaxies, which is in stark contrast to the results of flux-limited radio surveys, which are dominated by active galactic nuclei (AGNs) and elliptical galaxies (i.e., classical radio galaxies). While there are many AGNs among the KISS radio galaxies, there are no objects with large radio powers in our local volume. We derive a radio luminosity function (RLF) for the KISS ELGs that agrees very well with previous RLFs that adequately sample the lower luminosity radio population.

Radio structure of the most distant radio-detected quasar at the 10-mas scale

We present a high-resolution radio image of SDSS 0836+0054, identified recently as the most distant radio-detected quasar at a redshift of z= 5.82. The observation was carried out with 10 antennas of the European Very Long Baseline Interferometry (VLBI) Network, spread from Europe to China and South Africa, at 1.6-GHz frequency on 2002 June 8. The source was detected with a total flux density of 1.1 mJy, equal to its flux density measured in the Very Large Array (VLA) Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey. We found no indication of multiple images produced by gravitational lensing. The radio structure of the quasar at ∼10-mas angular resolution appears somewhat resolved. It resembles the radio structure typical for lower-redshift radio-loud active galactic nuclei. We obtained so far the best astrometric position of the source with an accuracy better than 8 mas, limited mainly by the structural effects in the phase-reference calibrator source.

Classifying bent-double galaxies

Astronomy data sets have led to interesting problems in mining scientific data. These problems will likely become more challenging as the astronomy community brings several surveys online as part of the National Virtual Observatory, giving rise to the possibility of mining data across many different surveys. In this article, we discuss the work we performed while using the catalog from the FIRST (Faint Images of the Radio Sky at Twenty centimetres) survey to classify galaxies with a bent-double morphology, meaning those galaxies that appear to be bent in shape. We describe the approach we took to mine this data, the issues we addressed in working with a real data set, and the lessons we learned in the process.

Optical identifications of 4000 radio sources at the 1mJy level

We present an analysis of the properties of optical counterparts of radio sources down to 1 mJy. Optical identifications have been obtained by matching together objects from the APM (Automatic Plate Measuring) and FIRST (Faint Images of the Radio Sky at Twenty centimetres) surveys over the region 14813≲RA(2000)≲21813,−277≲Dec.≲225. Selecting radio sources down to 1 mJy, and adopting a uniform optical limit of bJ=21.5, we find 3176 have a counterpart in the APM catalogue, corresponding to 13 per cent of the radio sample. For bJ≤20.5 we can divide radio sources into resolved radio galaxies and stellar-like objects (principally quasi-stellar objects – QSO). We find the population of radio galaxies to be mainly composed of early-type galaxies with very red colours (bJ−R up to ∼4) and a radio-to-optical ratio 102≲r≲104. The contribution of starbursting objects is negligible. In general QSO show r≳104 and 0≲bJ−R≲1. On the basis of the R magnitudes, we estimate the sample of radio galaxies to be complete up to z≃0.3. We can therefore divide the whole sample of radio sources into a low-z and high-z population. The low-z one includes the objects identified as galaxies in the APM survey and the high-z one includes sources either identified as QSO or with no optical counterpart for bJ≤20.5. We find that radio galaxies are strongly clustered and are highly biased tracers of the underlying mass distribution. Models for the angular correlation function w(θ) show good agreement with the observations if we assume a bias factor b≃2 at z≃0.3.

Intrinsic Absorption in Radio‐selected Quasars

Moderate resolution spectra have been recorded for a sample of radio-selected QSOs (z ~ 2.5) discovered in the Faint Images of the Radio Sky at Twenty centimeters (FIRST) VLA survey. This work is motivated by the study of a heterogeneous set of QSO absorption line spectra that showed that as many as 36% of the absorbers normally thought to be intergalactic are correlated with physical properties of the background QSO, including the radio source spectral index. Spectra were taken for 24 newly discovered quasars in order to test this finding on a more homogeneous data set. The spectra have been searched for the most frequently observed absorption lines. The absorption line properties are compared to the radio properties of the quasars. The primary results are summarized as follows: (1) an excess of C IV absorbers in flat-spectrum quasars as compared to steep-spectrum quasars is confirmed (based on a total sample of 165 systems), (2) a small excess of high-velocity C IV absorbers is seen in radio-quiet QSOs as compared to radio-loud quasars, and (3) the Sloan Digital Sky Survey QSO spectra will allow for the detection of a high-velocity, narrow, intrinsic population of absorbers that is as small as 7% of the C IV absorber population in QSOs at z ~ 2.5.

A Search for Submillisecond Pulsations in Unidentified FIRST and NVSS Radio Sources

We have searched 92 unidentified sources from the Faint Images of the Radio Sky at Twenty Centimeters (FIRST) and NRAO VLA Sky Survey (NVSS) 1400 MHz radio survey catalogs for radio pulsations at 610 MHz. The selected radio sources are bright, have no identification with extragalactic objects, are pointlike, and are more than 5% linearly polarized. Our search was sensitive to submillisecond pulsations from pulsars with dispersion measures less than ~500 pc cm-3 in the absence of scattering. We have detected no pulsations from these sources and consider possible effects that might prevent detection. We conclude that, as a population, these sources are unlikely to be pulsars.

Determining the Fraction of Intrinsic CivAbsorption in Quasi‐stellar Object Absorption‐Line Systems

We present the results of an exhaustive study of QSO absorption line systems (QSOALSs) with respect to intrinsic QSO properties using an updated catalog of data in the literature. We have searched the literature for 6 and 20 cm radio flux densities and have studied 20 cm contour plots from the Faint Images of the Radio Sky at Twenty centimeters (FIRST) VLA Survey in order to compare the absorption properties with radio luminosity, radio spectral index, and radio morphology. Although the data in our catalog are decidedly heterogeneous, great care has been taken to account for potential biases during the course of our research. Analysis of relatively unbiased subcatalogs allows us to investigate the properties of QSOALSs with better statistics than with any single homogeneous catalog. This work focuses particularly on the nature of C iv QSOALSs and their distribution in velocity space in light of intrinsic QSO properties. We find that the distribution of narrow, C iv absorption systems with relative velocities exceeding 5000 km s{sup {minus}1} is dependent not only on the optical luminosity of the QSOs but also on the radio luminosity, the radio spectral index, and the radio morphology of the QSOs. These observations are apparentlymore » inconsistent with the hypothesis that these systems are predominantly due to intervening galaxies, and it would seem that the contamination of the intervening systems (from 5000 to 75,000 km s{sup {minus}1}) by those that are intrinsic to the environment of the QSO is significantly larger than expected. We stress the need for truly homogeneous and unbiased surveys of QSOALSs to confirm these results from our inhomogeneous data set. {copyright} {ital {copyright} 1999.} {ital The American Astronomical Society}« less

The Angular Two‐Point Correlation Function for the FIRST Radio Survey

The FIRST (Faint Images of the Radio Sky at Twenty centimeters) survey now covers 1550 deg2 of sky, where 07h16 α 17h40 and 28°.3 δ 42°. This yields a catalog of 138,665 sources above the survey threshold of 1 mJy, about one-third of which are in double-lobed and multicomponent sources. We have used these data to obtain the first high-significance measurement of the two-point angular correlation for a deep radio sample. We find that the correlation function between 002 and 2° is well fitted by a power law of the form Aθγ, where A ≈ 3 × 10–3 and γ ≈ -1.1. On small scales (θ < 02), double and multicomponent sources are shown to have a larger clustering amplitude than that of the whole sample. Sources with flux densities below 2 mJy are found to have a shallower slope than that obtained for the whole sample, consistent with there being a significant contribution from starbursting galaxies at these faint fluxes. The cross-correlation of radio sources and Abell clusters is determined. A preliminary approach to inferring spatial information is outlined.