Radio Source Counts Research Articles

On the expected anisotropy of radio source counts

On the expected anisotropy of radio source counts

The Ursa Major supercluster - II. A statistical analysis of the radio survey

A statistical analysis of the 5C 10 radio survey of the Ursa Major supercluster is used to derive estimates for the radio luminosity distribution of the central cluster (Abell 1318) and the whole supercluster. Galaxies within the supercluster only form about 12 per cent of the total number of radio sources, and only 3 per cent of the galaxies were detected. The overall radio source counts and spectral-index distribution are compatible with previous 5C surveys, but there is a significant difference between the spectral-index distribution and that found for more intense sources. This difference has implications for the radio source counts.

Evidence for Local Anisotropy of the Hubble Flow

The Universe is accurately isotropic when measured by radio source counts and deep galaxy counts, or by the integrated microwave and X-ray backgrounds. On the other hand, galaxies are distributed in a decidedly clumpy way on scales of a few tens of megaparsecs, and it is natural therefore to ask whether galaxy redshifts show systematic departures from Hubble's law when averaged over similar scales. Discussions of this question have tended to develop along lines fixed by ideas for what might be reasonable patterns of deviations from Hubble's law. If the coherence length of the effect is large enough, it makes sense to expand the redshift field in a power series in distance, just as Oort (55) did for the velocity field in the Galaxy. This has been a convenient approach in theoretical discussions, but so far has proved to be only moderately useful as a framework for analysis of the data, perhaps because one coherence length contains too few independent groups of galaxies. A second framework is based on the thought that the mass concentration in a cluster of galaxies might induce a more or less radial flow toward the cluster in the surrounding field (24). In

Deep Radio Source Counts and Small Scale Fluctuations of the Microwave Background Radiation

Source counts which now extend to surface densities of ∼105 sources/sr make possible a direct evaluation of the radio source contribution to the small-scale fluctuations in the microwave background on scales larger than ∼10′, at wavelengths cm. Comprehensive radio spectral data permit a straightforward and largely model-independent extrapolation of the N(S) relation to shorter wavelengths. On the other hand, Peacock and Gull (1981, hereafter PG) have constructed a set of models which incorporate a wealth of additional data, such as local luminosity functions, luminosity/redshift distributions, luminosity-spectral index correlations; they can therefore be exploited to optimize the extrapolations both to higher frequencies and to fainter flux densities. Only one of these models, however, namely No. 4, is consistent with the recent P(D) results (Wall et al. 1982; Ledden et al. 1980) which provide information on the areal density of sources at s~1mJy; therefore, in the following we shall focus on it.

New Limits to the Small Scale Fluctuations in the Cosmic Background Radiation

The VLA has been used at 4.9 GHz to observe a small region of sky in order to extend the radio source count to low flux density (Fomalont et al., these proceedings) and to look for small scale fluctuations in the 2.7 K cosmic microwave background radiation.

A 6-CM Deep Sky Survey

In order to extend radio source counts to lower flux density, we have used the VLA to survey a small region of sky at 4.885 GHz (6 cm) to a limiting flux density of 50 μJy. Details of this deep survey are given in the paper by Kellermann et al. (these proceedings). In addition, we have observed 10 other nearby fields to a limiting flux density of 350 μJy in order to provide better statistics on sources of intermediate flux density.

Mapping the Universe

Mapping the Universe

X-ray quasars and the X-ray background

The Einstein X-ray observations of a sample of 202 radio- and optically-selected quasars due to Ku, Helfand & Lucy and to Zamorani et al. are analysed. Correlations between X-ray, optical and radio luminosities are examined. The contribution of radio-loud quasars to the 2-keV X-ray background is estimated using high-frequency radio-source counts, and the contribution due to radio-quiet, optically bright quasars using optical counts. It is shown that radio-loud quasars and radio-quiet optically bright quasars together contribute ∼ 15 per cent of the observed 2-keV X-ray background. The contribution of optically faint radio-quiet quasars is uncertain, but may be limited to a maximum of ∼ 30 per cent if recent indications of a flattening in optical counts at faint magnitudes are correct.

A revised statistical estimate of the counts of faint radio sources at 5 GHz

A revised statistical estimate of the counts of faint radio sources at 5 GHz

Right-handed and left-handed neutrinos and the two galactic populations of the universe additional evidence for the neutrino mass

There is astrophysical evidence in favour of the right-handed and left-handed nature of the neutrinos: the existence of our recent galactic population could be associated with a recent clustering of cosmological left-handed neutrinos, while a primordial galactic population could be created by a corresponding clustering of a cosmological right-handed neutrinos. This latter galactic population could be associated with an anomalous excess in the radiosource counts at a large redshift which is consistent with the range of red-shifts predicted by our estimate, based on presently known elementary-particle physics and thermodynamics.

Models of the cosmological evolution of extragalactic radio sources – I. The 408-MHz source count

A simple numerical procedure for analysing the counts of extra-galactic radio sources is described. The scheme makes efficient use of the extensive data base which is now available for studying the spatial distribution of radio sources. The method is first applied to data at 408 MHz. We show that, if proper statistical testing is incorporated, the technique provides strong support for the qualitative conclusions of previous analyses: the comoving density of the more powerful radio sources with extended structure is now less than at earlier epochs by a factor of ≳ 103. However, several evolution models proposed earlier are quantitatively unsatisfactory, and new models are derived which adequately describe the current data. These new models make contrasting predictions about redshift distributions amongst the faint radio sources (S408 ≽ 10 mJy), and identification data at such levels can therefore provide powerful constraints upon the models. Our results are compared with results from recent investigations of the spatial distribution of steep-spectrum radio sources.

Cosmological tests of the Hoyle-Narlikar conformal gravity

For the first time the Hoyle-Narlikar theory with creation of matter and a variable gravitational constant G, is subjected to the following cosmological tests: (1) the magnitude versus z relation, (2) the N(m) versus m relation for quasars, (3) the metric angular diameters versus z relation, (4) the isophotal angles versus z relation, (5) the log N-log S radio source count, and finally (6) the 3 K radiation. It is shown that the theory passes all these tests just as well as the standard cosmology, with the additional advantage that the geometry of the universe is uniquely determined, with a curvature parameter equal to zero. It is also interesting to note that the variability of G affects the log N-log S curve in a way similar to the density evolution introduced in standard cosmologies. The agreement with the data is therefore achieved without recourse to an ad hoc density evolution.

The Space Distribution of Radio Sources Based on Short Wavelength Surveys

The counts of radio sources at short wavelengths are discussed and are compared with the results of the V/Vm analysis. Although the space distributions of compact and extended radio sources appear different, the former are not necessarily more uniformly distributed as suggested in earlier discussions.

Frequency dependence of radio-source counts and spectral index distributions

Frequency dependence of radio-source counts and spectral index distributions

An estimate of the counts of faint radio sources at 5 GHz

A recent search for 5-GHz emission from nearby E and S0 galaxies produced a frequency distribution of flux densities dominated by background fluctuations. Analysis of this distribution provides an estimate of the number–flux density relation for extragalactic radio sources at levels corresponding to ∼ 106/sr. The results indicate a very slow convergence for the 5-GHz count in comparison with N(S) relations at lower frequencies. This suggests that a flat-spectrum population dominates the 5-GHz count at levels below ∼ 20 mJy, and calculations have been carried out to determine whether the recognized populations of objects with appropriate spectra, i.e. ‘flat-spectrum’ quasars, or ‘active’ galaxies, could be responsible. If it is the former, the cosmological evolution of the objects must be very strong and there can be no redshift cut-off for z < 10; if the latter, this low-luminosity population also must undergo strong evolution, in contradistinction to low-luminosity sources with steep spectra. Finally, if a previously unrecognized population is responsible, the sources must resemble the ‘active’ galaxies in having flat spectra and relatively low luminosities, and considerable evolution is again required.

Statistical investigation of compact emitting regions in extragalactic radio sources

EXTRAGALACTIC radio sources of high total luminosity usually contain compact emitting regions. These may be coincident with the optical galaxy or quasar, or they may be situated at the outer edges of extended double structures. Radio interferometers sensitive to structures with an angular scale ≤1 arc s have been used at Jodrell Bank to investigate the frequency of occurrence of such compact regions. Large samples of sources, covering a wide range of flux densities, were studied. This work extends to lower flux densities than previous investigations based on observations of sources showing interplanetary scintillations1,2. The frequency of occurrence of compact regions varies as a function of flux density. This is best explained as a cosmological effect involving the strong evolution of source properties with redshift. Several forms of this evolution, similar to those deduced from the radio source counts (see ref. 3), and the relationship between largest angular size and flux density4,5 are discussed elsewhere6.

The cosmological evolution of flat-spectrum quasars

The spatial distribution and luminosity functions of a complete sample of quasars from the ±4° declination strip of the Parkes 2.7-GHz survey have been investigated. The luminosity–volume (V/Vm) test strongly suggests that the flat-spectrum quasars which dominate the sample have an evolutionary history different from that of the steep-spectrum quasars which dominate the 3CR and 4C samples investigated previously. The bivariate luminosity function for the flat-spectrum quasars shows the same strong correlation between radio and optical luminosities exhibited by steep-spectrum quasars. The results of the luminosity–volume test considered in conjunction with radio structures, spectra, and source counts, lead to the tentative suggestion that the cosmological evolution of radio-active quasars is confined to those with extended radio structures.

The Molonglo Deep Sky Survey of Radio Sources. III. Source Counts

Number-flux density counts of radio sources are given for the first two deep surveys made with the Molonglo Mills Cross at 408 MHz, extending to lower limits of 84 and 88 mJy. Practical techniques are developed for the calculation of corrections to the counts when confusion errors are significant. The resulting corrections due to noise, confusion and other effects are given. Counts are also given for the MC2 and MC3 catalogues in the range 2-10 Jy to reduce the statistical uncertainties there. Results are shown on a composite plot, using relevant Molonglo surveys and an all-sky catalogue of strong sources. The existence of a convergence in the number of weak sources is confirmed.

Young Galaxies, Quasars and the Cosmological Evolution of Extragalactic Radio Sources

The V/Vmax test for quasars and the counts of radio sources show that the most powerful extragalactic radio sources exhibit strong evolutionary changes with cosmological epoch (see M. Schmidt and J.V. Wall et al., this volume). It should be emphasised that these are very large changes indeed. Schmidt, for example, has shown that for the world model with Ω = 0, evolution functions of the form F(t)∝exp(-10t/to) can account for the observations, to being the present epoch and t cosmic time. Since the quasar population from which this law is derived extends at least to Z = 2.5, corresponding to t = to/3.5 if Ω = 0, the comoving space density of quasars at Z = 2.5 must have been about 1300 times greater than it is at the present epoch. It is not known whether or not this law continues to hold at larger redshifts but even if it does, the increase in comoving space density from Z = 2.5 to infinity is only a further factor of 17. Therefore the bulk of the evolution occurs within the range of observationally accessible redshifts. Notice that a characteristic time-scale for the decay of the quasar population of ≈ to/10 ≈ 109 years comes out of this analysis.

Effects of a rotation of the universe on the number counts of radio sources - Goedel's universe

By using Goedel's (1949) rotating universe model, it is shown that vorticity alone in a homogeneous universe could have an effect on the (number-count)/(flux density) relation. This effect is the same as evolutionary effects. The effect also shows in expanding models, where a spatial gradient also appears, generating an anisotropy in the number counts. The possible existence of a global vorticity of the universe is discussed.