arXiv subjects

J. V. Wall

Publications and source records attributed to J. V. Wall.

22 records · Page 2Linked to original sources

arXiv1999

Constraints on the Clustering, Biasing and Redshift Distribution of Radio Sources

We discuss how different theoretical predictions for the variance $σ^2$ of the distribution of radio sources can be matched to measurements from the FIRST survey at different flux limits. The predictions are given by the integration of models for the angular correlation function $w(θ)$ for three different functional forms of the redshift distribution $N(z)$, different spatial correlation functions and by different evolutions of the bias $b(z)$ with redshift. We also consider the two cases of open and flat Universes. Although the predicted $w(θ)$ show substantial differences due to differences in the $N(z)$'s, these differences are not significant compared to the uncertainties in the current observations. It turns out that the best fit is provided by models with constant biasing at all times, although the difference between models with epoch-independent bias and models with bias that evolves linearly with redshift is not very large. All models with strong evolution of bias with epoch are ruled out. As a further step we directly calculated $w_{obs}(θ)$ at 3mJy from the catalogue and matched it with our models for the angular correlation function in the hypothesis that the clustering signal comes from two different populations, namely AGN-powered sources and starbursting galaxies. The results are consistent with a scenario for hierarchical clustering where the fainter starbursting galaxies trace the mass at all epochs, while brighter AGN's are strongly biased, with $b(z)$ evolving linearly with redshift, as suggested by some theories of galaxy formation and evolution.

astro-ph↗

arXiv1998

Variance and Skewness in the FIRST survey

We investigate the large-scale clustering of radio sources in the FIRST 1.4-GHz survey by analysing the distribution function (counts in cells). We select a reliable sample from the the FIRST catalogue, paying particular attention to the problem of how to define single radio sources from the multiple components listed. We also consider the incompleteness of the catalogue. We estimate the angular two-point correlation function $w(θ)$, the variance $Ψ_2$, and skewness $Ψ_3$ of the distribution for the various sub-samples chosen on different criteria. Both $w(θ)$ and $Ψ_2$ show power-law behaviour with an amplitude corresponding a spatial correlation length of $r_0 \sim 10 h^{-1}$Mpc. We detect significant skewness in the distribution, the first such detection in radio surveys. This skewness is found to be related to the variance through $Ψ_3=S_3(Ψ_2)^α$, with $α=1.9\pm 0.1$, consistent with the non-linear gravitational growth of perturbations from primordial Gaussian initial conditions. We show that the amplitude of variance and skewness are consistent with realistic models of galaxy clustering.

astro-ph↗

arXiv1998

The Redshift Cutoff and the Quasar Epoch

We have obtained complete redshift information for a sample of 442 radio-loud quasars with flux densities S(11cm) >= 0.25Jy. These come from a completely-identified sample of 878 flat-spectrum radio sources, so there is no optical magnitude limit. With these quasars, therefore, we can map out the entire quasar epoch, unhindered by optical selection effects and any intervening dust. The rapid decline in true space density above redshifts of about 2.5 is confirmed. It agrees closely with that obtained from optically-selected samples, indicating that these are also not significantly affected by dust. The evolution of the quasar space density is similar to that deduced for the global star formation rate, implying that the quasar epoch may also be that of star formation. This sample of radio-selected quasars can now be used in a variety of other studies, including unbiased searches for damped Ly-alpha absorbers, studies of the phenomenon of ``red quasars'', and searches for high-redshift molecular absorption systems.

astro-ph↗

arXiv1996

The correlation function of radio sources

We investigate the large-scale clustering of radio sources in the Green Bank and Parkes-MIT-NRAO 4.85 GHz surveys by measuring the angular two-point correlation function w(θ). Excluding contaminated areas, the two surveys together cover 70 per cent of the whole sky. We find both surveys to be reasonably complete above 50 mJy. On the basis of previous studies, the radio sources are galaxies and radio-loud quasars lying at redshifts up to z \sim 4, with a median redshift z \sim 1. This provides the opportunity to probe large-scale structures in a volume far larger than that within the reach of present optical and infrared surveys. We detect a clustering signal w(θ) \approx 0.01 for θ= 1\degr. By assuming an evolving power-law spatial correlation function in comoving coordinates ξ(r_c,z) = ( r_c / r_0 )^{-γ} (1+z)^{γ-(3+ε)}, where γ= 1.8, and the redshift distribution N(z) of the radio galaxies, we constrain the r_0--εparameter space. For `stable clustering' (ε= 0), we find the correlation length r_0 \approx 18 Mpc/h, larger than the value for nearby normal galaxies and comparable to the cluster-cluster correlation length.

astro-ph↗