The Population of KPC-Scale Flat-Spectrum Radio Sources

In this paper we present a subsample of 55 flat-spectrum radio sources dominated by (∼ 100 mas) kpc-scale structure, selected from a parent sample of 1665 VLA sources. Most are core-jets and 23 are CSO/MSO candidates. Properties of the subsample are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

The 6C** sample of steep-spectrum radio sources – II. Redshift distribution and the space density of high-redshift radio galaxies

We use the 6C** sample to investigate the comoving space density of powerful, steep-spectrum radio sources. This sample, consisting of 68 objects, has virtually complete K -band photometry and spectroscopic redshifts for 32 per cent of the sources. In order to find its complete redshift distribution, we develop a method of redshift estimation based on the K – z diagram of the 3CRR, 6CE, 6C* and 7CRS radio galaxies. Based on this method, we derive redshift probability density functions for all the optically identified sources in the 6C** sample. Using a combination of spectroscopic and estimated redshifts, we select the most radio luminous sources in the sample. Their redshift distribution is then compared with the predictions of the radio luminosity function of Jarvis et al. We find that, within the uncertainties associated with the estimation method, the data are consistent with a constant comoving space density of steep-spectrum radio sources beyond z ≳ 2.5, and rule out a steep decline.     

The large-scale distribution of quasars identified with strong radio sources at 5 GHz

The large-scale angular distribution of quasars from a complete sample of extragalactic radio sources is examined at different redshifts. The sample contains 264 quasars which have been found so far among the complete sample of 518 radio sources stronger than. 1 Jy at 5 GHz. Of these, 19 quasars have redshift z > 2. Dividing the entire sky into three separate declination zones of equal area, the counts of quasars seem to indicate a deficit of high redshift quasars in the northernmost declination zone. On the other hand, the Iow-redshift quasars (z < 1) appear fairly uniformly distributed. We discuss some possible selection effects that might have led to the apparent anomaly at high redshifts and estimate the expected number of high-redshift quasars amongst the radio sources in the sample for which redshifts are presently not available.     

MERLIN Images of Five Compact Symmetric Object Candidates at 1.6 GHz

1 INTRODUCTIONCompact Symmetric Objects (CSOs) are powerful and compact sources (overall size <1 bpc) with lobe emission on both sides of the central engine. The small size of these sourcesis almost certainly to be attributed to the youth of the sources themselves (ages < 104 yr) andnot due to a dense coallning medium (Readhead 1996). The unification scenario assumes thatCSOs evolve into compact steep spectrum (CSS) sotirces and then into Fanaroff-Riley type 11objects (Fanti 1995). …     

A sample of mJy radio sources at 1.4 GHz in the Lynx and Hercules fields – I. Radio imaging, multicolour photometry and spectroscopy

With the goal of identifying high-redshift radio galaxies with Fanaroff–Riley class I (FR I) classification, here are presented high-resolution, wide-field radio observations, near-infrared and optical imaging and multi-object spectroscopy of two fields of the Leiden–Berkeley Deep Survey. These fields, Hercules.1 and Lynx.2, contain a complete sample of 81 radio sources with   S _1.4 GHz > 0.5 mJy  within 0.6 deg². This sample will form the basis for a study of the population and cosmic evolution of high-redshift, low-power, FR I radio sources which will be presented in Paper II. Currently, the host galaxy identification fraction is 86 per cent with 11 sources remaining unidentified at a level of   r '≥ 25.2 mag  (Hercules; four sources) or   r '≥ 24.4 mag  (Lynx; seven sources) or   K ≳ 20 mag  . Spectroscopic redshifts have been determined for 49 per cent of the sample and photometric redshift estimates are presented for the remainder of the sample.     

Giant radio sources

We present multifrequency Very Large Array (VLA) observations of two giant quasars, 0437−244 and 1025−229, from the Molonglo Complete Sample. These sources have well-defined FR II radio structure, possible one-sided jets, no significant depolarization between 1365 and 4935 MHz and low rotation measure (|RM|<20 rad m⁻²). The giant sources are defined to be those with overall projected size 1 Mpc. We have compiled a sample of about 50 known giant radio sources from the literature, and have compared some of their properties with a complete sample of 3CR radio sources of smaller sizes to investigate the evolution of giant sources, and test their consistency with the unified scheme for radio galaxies and quasars. We find an inverse correlation between the degree of core prominence and total radio luminosity, and show that the giant radio sources have similar core strengths to smaller sources of similar total luminosity. Hence their large sizes are unlikely to be caused by stronger nuclear activity. The degree of collinearity of the giant sources is also similar to that of the sample of smaller sources. The luminosity–size diagram shows that the giant sources are less luminous than our sample of smaller sized 3CR sources, consistent with evolutionary scenarios in which the giants have evolved from the smaller sources, losing energy as they expand to these large dimensions. For the smaller sources, radiative losses resulting from synchrotron radiation are more significant while for the giant sources the equipartition magnetic fields are smaller and inverse Compton loss owing to microwave background radiation is the dominant process. The radio properties of the giant radio galaxies and quasars are consistent with the unified scheme.     

The LBDS Hercules sample of mJy radio sources at 1.4 GHz – I. Multicolour photometry

The results are presented of an extensive programme of optical and infrared imaging of radio sources in a complete subsample of the Leiden–Berkeley Deep Survey. The LBDS Hercules sample consists of 72 sources observed at 1.4 GHz, with flux densities S _1.41.0 mJy, in a 1.2 deg² region of Hercules. This sample is almost completely identified in the g , r , i and K bands, with some additional data available at J and H . The magnitude distributions peak at r ≃22 mag, K ≃16 mag and extend down to r ≃26 mag, K ≃21 mag. The K -band magnitude distributions for the radio galaxies and quasars are compared with those of other radio surveys. At S _1.4 GHz≲1 Jy, the K -band distribution does not change significantly with radio flux density. The sources span a broad range of colours, with several being extremely red ( r − K ≳6). Though small, this is the most optically complete sample of mJy radio sources available at 1.4 GHz, and is ideally suited for studying the evolution of the radio luminosity function out to high redshifts.     

The Phoenix survey: the pairing fraction of faint radio sources

The significance of tidal interactions in the evolution of the faint radio population (sub-mJy) is studied using a deep and homogeneous radio survey (1.4 GHz), covering an area of 3.14 deg² and complete to a flux density of 0.4 mJy. Optical photometric and spectroscopic data are also available for this sample. A statistical approach is employed to identify candidate physical associations between radio sources and optically selected 'field' galaxies. We find an excess of close pairs around optically identified faint radio sources, albeit at a low significance level, implying that the pairing fraction of the sub-mJy radio sources is similar to that of 'field' galaxies (at the same magnitude limit) but higher than that of local galaxies.     

A unifying view of the spectral energy distributions of blazars

We collect data at well-sampled frequencies from the radio to the γ-ray range for the following three complete samples of blazars: the Slew survey, the 1-Jy samples of BL Lacs and the 2-Jy sample of flat-spectrum radio-loud quasars (FSRQs). The fraction of objects detected in γ-rays ( E  ≳ 100 MeV) is ∼ 17, 26 and 40 per cent in the three samples respectively. Except for the Slew survey sample, γ-ray detected sources do not differ either from other sources in each sample, or from all the γ-ray detected sources, in terms of the distributions of redshift, radio and X-ray luminosities or of the broad-band spectral indices (radio to optical and radio to X-ray). We compute average spectral energy distributions (SEDs) from radio to γ-rays for each complete sample and for groups of blazars binned according to radio luminosity, irrespective of the original classification as BL Lac or FSRQ. The resulting SEDs show a remarkable continuity in that (i) the first peak occurs in different frequency ranges for different samples/luminosity classes, with most luminous sources peaking at lower frequencies; (ii) the peak frequency of the γ-ray component correlates with the peak frequency of the lower energy one; (iii) the luminosity ratio between the high and low frequency components increases with bolometric luminosity. The continuity of properties among different classes of sources and the systematic trends of the SEDs as a function of luminosity favour a unified view of the blazar phenomenon: a single parameter, related to luminosity, seems to govern the physical properties and radiation mechanisms in the relativistic jets present in BL Lac objects as well as in FSRQs. The general implications of this unified scheme are discussed while a detailed theoretical analysis, based on fitting continuum models to the individual spectra of most γ-ray blazars, is presented in a separate paper.     

Investigating statistical and spectral characteristics of the radio sources from the RadioAstron catalog at millimetric wavelengths

We consider some statistical characteristics of radio sources at 22 GHz in the framework of the preparatory works on the RadioAstron space mission. The expected density of radio sources with inverted spectra is estimated. Based on the observations of the radio sources from the preliminary RadioAstron sample, we compare the expected and measured angular densities of the radio sources with inverted spectra. The optical characteristics of the objects with inverted spectra and of the sources from the complete sample are compared as well. We present some spectral parameters of the radio sources observed at two different frequencies. Some conclusions about the completeness and reliability of the preliminary RadioAstron catalog are made.     

A MERLIN pilot study of faint variable VLA FIRST radio sources

We present the results of a pilot study in which we obtained Multi-Element Radio Linked Interferometer Network (MERLIN) L-band snapshot images of the 20 strongest radio sources from a sample of 123 radio objects that exhibit significant flux density variations at 1.4 GHz on a seven year base-line. The sample was created using FIRST VLA B-array data from 1995 and 2002 on a strip around zero declination near the south Galactic cap. The primary purpose is to assess the presence of radio jets and provide direct evidence between long-term variability and jet structures, as several models suggest. The radio structure of all the sources in this subsample appears very compact and our high resolution MERLIN observations reveal for the first time the presence of small milli-arcsecond (mas) jets and/or jet-like extensions in 17 of the 20 variable sources (literature data show core-jet structures or hints of extended structure in the other three objects which are point sources in our MERLIN observations). In the future we will use the radio results from the complete study, in combination with their optical Sloan Digital Sky Survey (SDSS) data, in order to investigate trends and possible differences or similarities between the quasars and galaxies that host these radio sources, and so examine existing unification schemes or open up new aspects in AGN unification.     

The redshift dependence of spectral index in powerful radio galaxies

We present and discuss in this paper the rest frame radio spectra (1–25 GHz) of a sample of fourteen radio galaxies atz >2 from the newly defined MRC/1Jy complete sample of 558 radio sources. These galaxies are among the most powerful radio sources known and range in luminosity from 1028-1028·8 watt Hz-1 at 1 GHz. We find that the median rest frame spectral index of this sample of galaxies atz >2 is significantly steeper than that of a matched luminosity sample of 3CRR galaxies which are at a much lower redshift (0.85 <z < 1.7). This indicates that spectral index correlates primarily with redshift, at least in the luminosity range considered here. The difference between the distributions of rest frame spectral curvatures for the two samples does not appear to be statistically significant. We suggest a new explanation for the steeper spectra of radio galaxies at high redshift involving steeper electron energy spectra at injection. Electron energy spectra are expected to steepen in a first-order Fermi acceleration process, at both non-relativistic and relativistic shock fronts, as the upstream fluid velocity decreases. This may well be the case at high redshifts: the hotter and denser circum-galactic medium at high redshifts could result in slower speeds for the hotspot and the jet material behind it. The smaller sizes of radio sources at higher redshifts provide support to this scenario. Since deceased.     

Optical and radio studies of radio sources

We present the classification of optical identifications and radio spectra of six radio sources from a complete (in flux density) sample in the declination range 10° to 12°30′ (J2000.0). The observations were carried out with the 6-m Special Astrophysical Observatory telescope (Russia) in the wavelength range 3600–10000 Å, the 2.1-m GHAO telescope (Mexico) in the range 4200–9000 Å, and the RATAN-600 radio telescope in the frequency range 0.97–21.7 GHz. Three of the six objects under study are classified as quasars, one is a BL Lac object, one is an absorption-line radio galaxy, and one is an emission-line radio galaxy. Five objects have flat radio spectra, and one object has a power-law radio spectrum. All of the radio sources identified as quasars or BL Lac objects show variable radio flux densities. The spectra of three objects were separated into extended and compact components.     

A sharper view of the outer Galaxy at 1420 and 408 MHz from the Canadian Galactic Plane Survey – I. Revisiting the KR catalogue and new Gigahertz Peaked Spectrum sources

Arcminute-resolution radio continuum images at 408 and 1420 MHz from the Canadian Galactic Plane Survey (CGPS) have been used to re-examine radio sources listed in the Kallas & Reich catalogue. This catalogue is of particular interest to Galactic studies as it lists both extended and compact radio sources found in the second Galactic quadrant. We have determined the nature (extended versus compact, Galactic versus extragalactic) of all of these bright radio sources. A number of large H  ii regions with no optical counterparts are highlighted along with a sample of large radio galaxies. Many sources previously thought to be extended Galactic objects are shown to be point sources. A sample of point sources with flat or rising spectra between 408 and 1420 MHz has been compiled, and within this sample likely Gigahertz Peaked Spectrum sources have been identified.     

PKS 1117+146: a new large compact symmetric object

We present 23-GHz MERLIN observations of the high-luminosity radio galaxy PKS 1117+146. The radio image obtained from these data shows, for the first time, a central weak component (designated C) that we identify as the core of the radio galaxy, and two outer components (designated N and S) which are the hotspots of the extended lobes observed at lower frequencies. Extended emission in components C and S is an indication of a possible jet connecting the core to the strongest component. The overall optical and radio properties of PKS 1117+146 are consistent with the source being classified as a compact symmetric object (CSO). We discuss this hypothesis, which would make PKS 1117+146 the largest CSO known so far.     

Optical spectroscopy of two overlapping, flux-density-limited samples of radio sources in the North Ecliptic Cap, selected at 38 and 151 MHz

We present the results of optical spectroscopy of two flux-density-limited samples of radio sources selected at frequencies of 38 and 151 MHz in the same region around the North Ecliptic Cap, the 8C-NEC and 7C- iii samples respectively. Both samples are selected at flux density levels ≈20 times fainter than samples based on the 3C catalogue. They are amongst the first low-frequency selected samples with no spectral or angular size selection for which almost complete redshift information has been obtained, and contain many of the lowest-luminosity z >2 radio galaxies so far discovered. They will therefore provide a valuable resource for understanding the cosmic evolution of radio sources and their hosts and environments. The 151-MHz 7C- iii sample is selected to have S ₁₅₁≥0.5 Jy and is the more spectroscopically complete; out of 54 radio sources fairly reliable redshifts have been obtained for 44 objects. The 8C sample has a flux limit of S ₃₈≥1.3 Jy and contains 58 sources of which 46 have fairly reliable redshifts. We discuss possible biases in the observed redshift distribution, and some interesting individual objects, including a number of cases of probable gravitational lensing. Using the 8C-NEC and 7C- iii samples in conjunction, we form the first sample selected on low-frequency flux in the rest-frame of the source, rather than the usual selection on flux density in the observed frame. This allows us to remove the bias associated with an increasing rest-frame selection frequency with redshift. We investigate the difference this selection makes to correlations of radio source properties with redshift and luminosity by comparing the results from traditional flux-density selection with our new method. We show in particular that flux-density-based selection leads to an overestimate of the steepness of the correlation of radio source size with redshift.     

The redshift distribution of FIRST radio sources at 1 mJy

We present spectra for a sample of radio sources from the FIRST survey, and use them to define the form of the redshift distribution of radio sources at mJy levels. We targeted 365 sources and obtained 46 redshifts (13 per cent of the sample). We find that our sample is complete in redshift measurement to R ∼18.6, corresponding to z ∼0.2. Galaxies were assigned spectral types based on emission-line strengths. Early-type galaxies represent the largest subset (45 per cent) of the sample and have redshifts 0.15≲ z ≲0.5; late-type galaxies make up 15 per cent of the sample and have redshifts 0.05≲ z ≲0.2; starbursting galaxies are a small fraction (∼6 per cent), and are very nearby ( z ≲0.05). Some 9 per cent of the population have Seyfert 1/quasar-type spectra, all at z ≳0.8, and 4 per cent are Seyfert 2 type galaxies at intermediate redshifts ( z ∼0.2).
Using our measurements and data from the Phoenix survey (Hopkins et al.), we obtain an estimate for N ( z ) at S _1.4 GHz≥1 mJy and compare this with model predictions. At variance with previous conclusions, we find that the population of starbursting objects makes up ≲5 per cent of the radio population at S ∼1 mJy.     

Galaxy Evolution in the Reddest Possible Filter

We describe an observational programme aimed at understanding the radio emission from distant, rapidly evolving galaxy populations. These observations were carried out at 1.4 and 8.5 GHz with the VLA, centred on the Hubble Deep Field, obtaining limiting flux densities of 40 and 8 μJy respectively. The differential count of the radio sources is marginally sub-Euclidean to the completeness limits(γ = − 2.4 ± 0.1) and fluctuation analysis suggests nearly 60 sources per arcmin² at the 1 μJy level. Using high-resolution 1.4 GHz observations obtained with MERLIN, we resolve all radio sources detected in the VLA complete sample and measure a median angular size for the microjansky radio population of 1-2". This clue, coupled with the steep spectral index of the 1.4 GHz selected sample, suggests diffuse synchrotron radiation in z ∼ 1 galactic discs. The wide-field HST and ground-based optical exposures show that the radio sources are identified primarily with disc systems composed of irregulars, peculiars, interacting/merging galaxies and a few isolated field spirals. Only 20% of the radio sources can be attributed to AGN – the majority are probably associated with starburst activity. The available redshifts range from 0.1 to 3, with a mean of about 0.8. We are plrobably witnessing a major episode of starburst activity in these luminous (L > L _*) systems, occasionallyaccompanied by an embedded AGN.About 20% of the radio sources remain unidentified to I = 26-28 inthe HDF and flanking fields. Several of these objectshave extremely red counterparts. We suggestthat these are high-redshift dusty protogalaxies. This revised version was published online in July 2006 with corrections to the Cover Date.     

A new sample of gigahertz-peaked-spectrum, extra-galactic radio sources

A new sample of extra-Galactic radio sources having a peak in the continuous spectrum near 1 GHz has been compiled, using data available in the literature on their radio flux densities, and some characteristic parameters of radio sources in this sample have been calculated. The median value of the calculated spectral indices of radio sources in the sample is -0.95, the median flux density at the peak (S_m) is 465 mJy, and almost half the radio sources (14 out of 30) have ultrasteep spectra, for which the spectral indices in the high-frequency range are α <-1.0 (S ∝ v^α). Translated from Astrofizika, Vol. 41, No. 3, pp. 377–388, July–September, 1998.     

More redshifts of powerful equatorial radio sources from the Best, Röttgering & Lehnert sample

A new sample of very powerful radio sources, defined from the Molonglo Reference Catalogue, was recently compiled by Best, Röttgering & Lehnert. These authors provided redshifts for 174 of the 178 objects in the sample, making the sample 98 per cent spectroscopically complete. Here, redshifts for three of the remaining galaxies are presented, confirming the optical identifications and raising the spectroscopic completeness of the sample to 99.5 per cent; only 1059−010 (3C 249) is currently without redshift.