Identification of newly discovered radio-emitting X-ray sources: results from spectroscopy

We present optical identifications for a sample of 20 previously unknown X-ray/radio sources that are present both in the source catalogue of ROSAT PSPC pointed observations ( ROSAT SRC) and in the NRAO VLA Sky Survey (NVSS). The optical spectroscopy was carried out with the 2.1-m telescope at San Pedro Martir (Mexico) during 1995 April and September. We have identified 15 active galactic nuclei [including 12 broad-emission-line (FWHM >1000 km s⁻¹) objects, one bona fide BL Lac, one BL Lac candidate and one narrow-line (FWHM < 1000 km s⁻¹) radio galaxy] and five radio galaxies. We derive the X-ray fluxes and luminosities by analysing the PSPC exposures, and show the radio morphology from the NVSS maps.  We find that the correlation between the monochromatic X-ray luminosity at 2 keV and the core radio luminosity at 5 GHz for the radio galaxies in our sample follows that found for the 3CR radio galaxies, suggesting a possible nuclear origin for the X-ray emission in these sources. This correlation is weaker in the case of broad-line objects, indicating the presence of another (unbeamed) mechanism for the X-ray emission only weakly related to the radio emission.     

High-redshift QSOs in the FIRST survey

In a pilot search for high-redshift radio quasi-stellar objects (QSOs), we have obtained spectra of 55 FIRST sources     with very red     starlike optical identifications. 10 of the candidates are QSOs with redshifts     (four were previously known), six with     . The remaining 45 candidates comprise: one     broad-absorption-line (BAL) QSO; three low-redshift galaxies with narrow emission lines; 18 probable radio galaxies; and 23 M stars (mainly misidentifications). The success rate (high-redshift QSOs / spectroscopically-observed candidates) for this search is 1/2 for     , and 1/9 for     . With an effective search area of 4030 deg², the surface density of high-redshift     QSOs discovered with this technique is 0.0015 deg⁻².     

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-Å 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 10²² W Hz⁻¹ at 1.4 GHz.     

Broad-line Balmer decrements in blue active galactic nuclei

We have investigated the broad-line Balmer decrements (Hα/Hβ) for a large, homogeneous sample of Seyfert 1 galaxies and quasi-stellar objects (QSOs) using spectroscopic data obtained in the Sloan Digital Sky Survey. The sample, drawn from the Fourth Data Release, comprises 446 low-redshift ( z ≲ 0.35) active galactic nuclei (AGN) that have blue optical continua as indicated by the spectral slopes in order to minimize the effect of dust extinction. We find that (i) the distribution of the intrinsic broad-line Hα/Hβ ratio can be well described by log-Gaussian, with a peak at  Hα/Hβ= 3.06  and a standard deviation of about 0.03 dex only; (ii) the Balmer decrement does not correlate with AGN properties such as luminosity, accretion rate and continuum slope, etc.; (iii) on average, the Balmer decrements are found to be only slightly larger in radio-loud sources (3.37) and sources having double-peaked emission-line profiles (3.27) compared to the rest of the sample. We therefore suggest that the broad-line Hα/Hβ ratio can be used as a good indicator for dust extinction in the AGN broad-line region; this is especially true for radio-quiet AGN with regular emission-line profiles, which constitute the vast majority of the AGN population.     

Deep low-frequency observations with the Giant Metrewave Radio Telescope: a search for relic radio emission

We present deep multifrequency observations using the Giant Metrewave Radio Telescope (GMRT) at 153, 244, 610 and 1260 MHz of a field centred on J0916+6348, to search for evidence of fossil radio lobes which could be due to an earlier cycle of episodic activity of the parent galaxy, as well as haloes and relics in clusters of galaxies. We do not find any unambiguous evidence of episodic activity in a list of 374 sources, suggesting that such activity is rare even in relatively deep low-frequency observations. We examine the spectra of all the sources by combining our observations with those from the Westerbork Northern Sky Survey (WENSS), NRAO (National Radio Astronomy Observatories) VLA (Very Large Array) Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty-Centimeters Survey (FIRST). Considering only those which have measurements at a minimum of three different frequencies, we find that almost all sources are consistent with a straight spectrum with a median spectral index,  α∼ 0.8 [S(ν) ∝ν^−α  ], which appears steeper than theoretical expectations of the injection spectral index. We identify 14 very steep-spectrum sources with  α≥ 1.3  . We examine their optical fields and discuss the nature of some of these sources.     

Radio sources in the 2dF Galaxy Redshift Survey – II. Local radio luminosity functions for AGN and star-forming galaxies at 1.4 GHz

We have cross-matched the 1.4-GHz NRAO VLA Sky Survey (NVSS) with the first 210 fields observed in the 2dF Galaxy Redshift Survey (2dFGRS), covering an effective area of 325 deg² (about 20 per cent of the final 2dFGRS area). This yields a set of optical spectra of 912 candidate NVSS counterparts, of which we identify 757 as genuine radio identifications – the largest and most homogeneous set of radio source spectra ever obtained. The 2dFGRS radio sources span the redshift range     to 0.438, and are a mixture of active galaxies (60 per cent) and star-forming galaxies (40 per cent). About 25 per cent of the 2dFGRS radio sources are spatially resolved by NVSS, and the sample includes three giant radio galaxies with projected linear size greater than 1 Mpc. The high quality of the 2dF spectra means we can usually distinguish unambiguously between AGN and star-forming galaxies. We make a new determination of the local radio luminosity function at 1.4 GHz for both active and star-forming galaxies, and derive a local star formation density of         .     

CENSORS: A Combined EIS–NVSS Survey Of Radio Sources – I. Sample definition, radio data and optical identifications

A new sample of radio sources, with the designated name CENSORS (A Combined EIS–NVSS Survey Of Radio Sources), has been defined by combining the National Radio Astronomy Observatory Very Large Array Sky Survey (NVSS) at 1.4 GHz with the ESO Imaging Survey (EIS) Patch D, a 3° by 2° region of sky centred at RA     , Dec. −21°00'00' (J2000). New radio observations of 199 NVSS radio sources with NVSS flux densities   S _{1.4 GHz} > 7.8 mJy  are presented, and are compared with the EIS I -band imaging observations which reach a depth of   I ∼ 23  ; optical identifications are obtained for over two-thirds of the ∼150 confirmed radio sources within the EIS field. The radio sources have a median linear size of 6 arcsec, consistent with the trend for lower flux density radio sources to be less extended. Other radio source properties, such as the lobe flux density ratios, are consistent with those of brighter radio source samples. From the optical information, 30–40 per cent of the sources are expected to lie at redshifts   z ≳ 1.5  .
One of the key goals of this survey is to accurately determine the high-redshift evolution of the radio luminosity function. These radio sources are at the ideal flux density level to achieve this goal; at redshifts   z ∼ 2  they have luminosities which are around the break of the luminosity function and so provide a much more accurate census of the radio source population at those redshifts than the existing studies of extreme, high radio power sources. Other survey goals include investigating the dual-population unification schemes for radio sources, studying the radio luminosity dependence of the evolution of radio source environments, and understanding the radio power dependence of the K – z relation for radio galaxies.     

On the streaming motions of haloes and galaxies

Little is known about the statistics of gravitationally lensed quasars at large (7–30 arcsec) image separations, which probe masses on the scale of galaxy clusters. We have carried out a survey for gravitationally lensed objects, among sources in the FIRST 20-cm radio survey that have unresolved optical counterparts in the digitizations of the Palomar Observatory Sky Survey. From the statistics of ongoing surveys that search for quasars among FIRST sources, we estimate that there are about 9100 quasars in this source sample, making this one of the largest lensing surveys to date. Using broad-band imaging, we have isolated all objects with double radio components separated by 5–30 arcsec that have unresolved optical counterparts with similar BVI colours. Our criteria for similar colours conservatively allow for observational error and for colour variations due to time delays between lensed images. Spectroscopy of these candidates shows that none of the pairs are lensed quasars. This sets an upper limit (95 per cent confidence) on the lensing fraction in this survey of 3.3×10⁻⁴, assuming 9100 quasars. Although the source redshift distribution is poorly known, a rough calculation of the expected lensing frequency and the detection efficiencies and biases suggests that simple theoretical expectations are of the same order of magnitude as our observational upper limit. Our procedure is novel in that our exhaustive search for lensed objects does not require prior identification of the quasars in the sample as such. Characterization of the FIRST-selected quasar population will enable use of our result to constrain quantitatively the mass properties of clusters.     

Distant FR I radio galaxies in the Hubble Deep Field: implications for the cosmological evolution of radio-loud AGN

Deep and high-resolution radio observations of the Hubble Deep Field and flanking fields have shown the presence of two distant edge-darkened FR I radio galaxies, allowing for the first time an estimate of their high-redshift space density. If it is assumed that the space density of FR I radio galaxies at     is similar to that found in the local Universe, then the chance of finding two FR I radio galaxies at these high radio powers in such a small area of sky is < 1 per cent. This suggests that these objects were significantly more abundant at     than at present, effectively ruling out the possibility that FR I radio sources undergo no cosmological evolution. We suggest that FR I and FR II radio galaxies should not be treated as intrinsically distinct classes of objects, but that the cosmological evolution is simply a function of radio power with FR I and FR II radio galaxies of similar radio powers undergoing similar cosmological evolutions. Since low-power radio galaxies have mainly FR I morphologies and high-power radio galaxies have mainly FR II morphologies, this results in a generally stronger cosmological evolution for the FR IIs than the FR Is. We believe that additional support from the V / V _max test for evolving and non-evolving population of FR IIs and FR Is respectively is irrelevant, since this test is sensitive over very different redshift ranges for the two classes.     

Radio imaging of the Subaru/XMM–Newton Deep Field – II. The 37 brightest radio sources

We study the 37 brightest radio sources in the Subaru/ XMM–Newton Deep Field. We have spectroscopic redshifts for 24 of 37 objects and photometric redshifts for the remainder, yielding a median redshift z _med for the whole sample of   z _med≃ 1.1  and a median radio luminosity close to the 'Fanaroff–Riley type I/type II (FR I/FR II)' luminosity divide. Using mid-infrared (mid-IR) ( Spitzer MIPS 24 μm) data we expect to trace nuclear accretion activity, even if it is obscured at optical wavelengths, unless the obscuring column is extreme. Our results suggest that above the FR I/FR II radio luminosity break most of the radio sources are associated with objects that have excess mid-IR emission, only some of which are broad-line objects, although there is one clear low-accretion-rate object with an FR I radio structure. For extended steep-spectrum radio sources, the fraction of objects with mid-IR excess drops dramatically below the FR I/FR II luminosity break, although there exists at least one high-accretion-rate 'radio-quiet' QSO. We have therefore shown that the strong link between radio luminosity (or radio structure) and accretion properties, well known at z ∼ 0.1, persists to z ∼ 1. Investigation of mid-IR and blue excesses shows that they are correlated as predicted by a model in which, when significant accretion exists, a torus of dust absorbs ∼30 per cent of the light, and the dust above and below the torus scatters ≳1 per cent of the light.     

Evolutionary unification in composite active galactic nuclei

In this paper, we present an evolutionary unification scenario, involving supermassive black holes (SMBHs) and starbursts (SBs) with outflow (OF), that seems capable of explaining most of the observational properties (of at least part) of active galactic nuclei (AGN).
The scenario includes a nuclear/circumnuclear SB closely associated with the AGN where the narrow-line region (NLR), broad-line region (BLR) and broad absorption line (BAL) region are produced in part by the OF process with shells and in compact supernova remnants (cSNRs).
The OF process in BAL quasi-stellar objects (QSOs) with extreme infrared (IR) and Fe  ii emission is studied. In addition, the Fe  ii problem regarding the BLR of AGN is analysed. The correlations between the BAL, IR emission, Fe  ii intensity and the intrinsic properties of the AGN are not clearly understood. We suggest here that the behaviour of the BAL, IR and Fe  ii emission in AGN can be understood within an evolutionary and composite model for AGN.
In our model, strong BAL systems and Fe  ii emission are present (and intense) in young IR objects. Parameters like the BALs, IR emission, Fe  ii /Hβ intensity ratio, Fe  ii equivalent width (EW), broad-line width, [O  iii ]λ5007-Å intensity and width, NLR size, X-ray spectral slope in radio quiet (RQ) AGN plus lobe separation, and lobe to core intensity ratio in radio loud (RL) AGN are proposed to be fundamentally time-dependent variables inside time-scales of the order of 10⁸ yr. Orientation/obscuration effects take the role of a second parameter providing the segregation between Seyfert 1/Seyfert 2 galaxies (Sy1/Sy2) and broad-/narrow-line radio galaxies (BLRG/NLRG).     

An excess of damped Lyman α galaxies near quasi-stellar objects

We present a sample of 33 damped Lyman α systems (DLAs) discovered in the Sloan Digital Sky Survey (SDSS) whose absorption redshifts ( z _abs) are within 6000 km s⁻¹ of the quasi-stellar object's (QSO) systemic redshift ( z _sys). Our sample is based on  731 2.5 < z _sys < 4.5  non-broad absorption line (non-BAL) QSOs from Data Release 3 (DR3) of the SDSS. We estimate that our search is ≈100 per cent complete for absorbers with N (H  i )  ≥ 2 × 10²⁰ cm⁻²  . The derived number density of DLAs per unit redshift, n ( z ), within  Δ v < 6000 km s⁻¹  is higher (3.5σ significance) by almost a factor of 2 than that of intervening absorbers observed in the SDSS DR3, i.e. there is evidence for an overdensity of galaxies near the QSOs. This provides a physical motivation for excluding DLAs at small velocity separations in surveys of intervening 'field' DLAs. In addition, we find that the overdensity of proximate DLAs is independent of the radio-loudness of the QSO, consistent with the environments of radio-loud and radio-quiet QSOs being similar.     

A search for distant radio-loud quasars in the CLASS survey: three new radio-selected quasars at z > 4

We report on the search for distant radio-loud quasars in the Cosmic Lens All Sky Survey (CLASS) of flat spectrum radio sources with S _5GHz>30 mJy . Unresolved optical counterparts were selected from APM scans of POSS-I plates, with e <19.0 and red o − e >2.0 colours, in an effective area of ∼6400 deg². Four sources were found to be quasars with z >4 , of which one was previously known. This sample bridges the gap between the strong radio surveys with S _5GHz>200 mJy and the samples of radio-weak quasars that can be generated via radio observations of optically selected quasars. In addition, four new quasars at z >3 have been found. The selection criteria result in a success-rate of ∼1:7 for radio-loud quasars at z >4 , which is a significant improvement over previous studies. This search yields a surface density of 1 per 1600 deg², which is about a factor of ∼15 lower than that found in a similar search for radio-quiet quasars at z >4 . The study presented here is strongly biased against quasars beyond z >4.5 , since the e -passband of the POSS-I only samples the spectra shortward of 1200 Å at these redshifts.     

M BH–σ relation in Sloan Digital Sky Survey flat-spectrum radio quasars

The relationship between the black hole mass and velocity dispersion indicated with [O  iii ] linewidth is investigated for a sample of 87 flat-spectrum radio quasars selected from the Sloan Digital Sky Survey Data Release 3 quasar catalogue. We found that the   M _BH−σ_[O III]  relation is different from the Tremaine et al. relation for nearby inactive galaxies, with a larger black hole mass at given velocity dispersion. There is no strong evidence of cosmology evolution in the   M _BH−σ_[O III]  relation up to   z ∼ 0.8  . A significant correlation between the [O  iii ] luminosity and broad-line region (BLR) luminosity is found. When transferring the [O  iii ] luminosity to narrow-line region (NLR) luminosity, the BLR luminosity is, on average, larger than the NLR one by about one order of magnitude. We found a strong correlation between the synchrotron peak luminosity and NLR luminosity, which implies a tight relation between the jet physics and accretion process.     

Searching for large-scale structure in deep radio surveys

We calculate the expected amplitude of the dipole and higher spherical harmonics in the angular distribution of radio galaxies. The median redshift of radio sources in existing catalogues is z  ∼ 1, which allows us to study large-scale structure on scales between those accessible to present optical and infrared surveys, and that of the cosmic microwave background (CMB). The dipole is a result of two effects which turn out to be of comparable magnitude: (i) our motion with respect to the CMB, and (ii) large-scale structure, parametrized here by a family of cold dark matter power-spectra. We make specific predictions for the Green Bank 1987 (87GB) and Parkes–MIT–NRAO (PMN) catalogues, which in our combined catalogue include ∼ 40 000 sources brighter than 50 mJy at 4.85 GHz, over about 70 per cent of the sky. For these relatively sparse catalogues both the motion and large-scale structure dipole effects are expected to be smaller than the Poisson shot noise. However, we detect dipole and higher harmonics in the combined 87GB–PMN_raw catalogue which are far larger than expected. We attribute this to a 2 per cent flux mismatch between the two catalogues. Ad hoc corrections made in an effort to match the catalogues may suggest a marginal detection of a dipole. To detect a dipole and higher harmonics unambiguously, a catalogue with full sky coverage and ∼ 10⁶ sources is required. We also investigate the existence and extent of the supergalactic plane in the above catalogues. In a strip of ± 10° of the standard supergalactic equator, we find a 3 σ detection in PMN_raw, but only 1 σ in 87 GB_raw. We briefly discuss the implications of ongoing surveys such as FIRST and NVSS and follow-up redshift surveys.     

Analysis of a sample of RC catalog objects in the region overlapping with the areas covered by FIRST and SDSS surveys. II: Optical identification with the SDSS survey and USNO-B1 and 2MASS catalogs

We report the results of optical identification of a sample of RC catalog radio sources with the FIRST and SDSS surveys. For 320 sources identified with NVSS and FIRST objects we perform optical identification with the SDSS survey. When selecting optical candidates we make maximum use of the information about the structure of radio sources as provided by the FIRST survey images. We find optical candidates for about 70% of all radio sources.     

A pilot survey for KX QSOs in the UKIDSS Ultra Deep Survey Field

We have undertaken a pilot survey for faint quasi-stellar objects (QSOs) in the UKIRT Infrared Deep Survey (UKIDSS) Ultra Deep Survey (UDS) Field using the KX selection technique. These observations exploit the very deep near-infrared and optical imaging of this field from United Kingdom Infrared Telescope (UKIRT) and Subaru to select candidate QSOs based on their VJK colours and morphologies. We determined redshifts for 426 candidates using the AAOmega spectrograph on the Anglo-Australian Telescope in service time. We identify 17 QSOs  ( M _B ≲−23)  in this pilot survey at z = 1.57–3.29. We combine our sample with an X-ray-selected sample of QSOs in the same field (a large fraction of which also comply with our KX selection) to constrain the surface density of QSOs with K ≤ 20, deriving limits on the likely surface density of 85–150 deg⁻². We use the good image quality available from our near-infrared imaging to detect a spatially extended component of the QSO light which probably represents the host galaxies. We also use our sample to investigate routes to improve the selection of KX QSOs at faint limits in the face of the significant contamination by compact, foreground galaxies. The brightest examples from our combined QSO sample will be used in conjunction with a large Very Large Telescope VIMOS spectroscopic survey of high-redshift galaxies in this region to study the structures inhabited by gas, galaxies and growing supermassive black holes at high redshifts in the UKIDSS UDS.     

Emission-line outflows in PKS1549−79: the effects of the early stages of radio-source evolution?

We present new spectroscopic observations of the southern radio galaxy         . Despite the flat-spectrum character of the radio emission from this source, our optical spectra show no sign of the broad permitted lines and non-stellar continuum characteristic of quasar nuclei and broad-line radio galaxies. However, the high-ionization forbidden lines, including [O  iii ] λλ 5007, 4959, are unusually broad for a narrow-line radio galaxy     , and are blueshifted by 600 km s⁻¹ relative to the low-ionization lines such as [O  ii ] λλ 3726,3729. The [O  ii ] lines are also considerably narrower     than the [O  iii ] lines, and have a redshift consistent with that of the recently detected H  i 21-cm absorption-line system. Whereas the kinematics of the [O  iii ] emission lines are consistent with outflow in an inner narrow-line region, the properties of the [O  ii ] emission lines suggest that they are emitted by a more extended and quiescent gaseous component. We argue that, given the radio properties of the source, our line of sight is likely to be lying close to the direction of bulk outflow of the radio jets. In this case it is probable that the quasar nucleus is entirely obscured at optical wavelengths by the material responsible for the H  i absorption-line system. The unusually broad [O  iii ] emission lines suggest that the radio source is intrinsically compact. Overall, our data are consistent the idea that     is a radio source in an early stage of evolution.     

XMM–Newton and Chandra observations of SHEEP sources

We present Chandra and XMM–Newton observations of 12 bright  [ f (2–10 keV) > 10⁻¹³ erg cm⁻² s⁻¹]  sources from the ASCA search for the High Energy Extragalactic Population (SHEEP) survey. Most of these have been either not observed or not detected previously with the ROSAT mission, and therefore they constitute a sample biased towards hard sources. The Chandra observations are important in locating the optical counterpart of the X-ray sources with accuracy. Optical spectroscopic observations show that our sample is associated with both narrow-line (NL) (six objects) and broad-line (BL) active galactic nuclei (AGN) (five objects), with one source remaining unidentified. Our sources cover the redshift range 0.04–1.29, spanning luminosities from 10⁴² to  10⁴⁵ erg s⁻¹  (2–10 keV). The NL sources have preferentially lower redshift (and luminosity) compared to the BL ones. This can be most easily explained in a model where the NL AGN are intrinsically less luminous than the BL ones in line with the results of Steffen et al. The X-ray spectral fittings show a roughly equal number of obscured  ( N _H > 10²² cm⁻²)  and unobscured  ( N _H < 10²² cm⁻²)  sources. There is a clear tendency for obscured sources to be associated with NL AGN and unobscured sources with BL ones. However, there is a marked exception with the highest obscuring column observed at a BL AGN at a redshift of z = 0.5.