EDP Sciences
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Volume 445, Number 2, January II 2006
Page(s) 465 - 469
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20053797

A&A 445, 465-469 (2006)
DOI: 10.1051/0004-6361:20053797

High-frequency radio observations of the Kühr sample and the epoch-dependent luminosity function of flat-spectrum quasars

R. Ricci1, I. Prandoni2, C. Gruppioni3, R. J. Sault1 and G. De Zotti4, 5

1  Australia Telescope National Facility, CSIRO, PO Box 76, Epping, NSW 2121, Australia
    e-mail: Roberto.Ricci@csiro.au
2  INAF, Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
3  INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40126 Bologna, Italy
4  INAF, Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
    e-mail: dezotti@pd.astro.it
5  SISSA/ISAS, via Beirut 2-4, 34014 Trieste, Italy

(Received 8 July 2005 / Accepted 6 September 2005 )

We discuss our ATCA 18.5 and 22 GHz flux density measurements of Southern extragalactic sources in the complete 5 GHz sample of Kühr et al. (1981, A&AS, 45, 367). The high frequency (5-18.5 GHz) spectral indices of steep-spectrum sources for which we have 18.5 GHz data (66% of the complete sample) are systematically steeper than the low frequency (2.7-5 GHz) ones, with median $\alpha^5_{2.7} = 0.76$, median $\alpha^{18.5}_{5} = 1.18$ ( $S_{\nu}\propto \nu^{-\alpha}$), and median steepening $\Delta\alpha = 0.32$, and there is evidence of an anti-correlation of $\Delta\alpha^{18.5}_{5}$ with luminosity. The completeness of 18.5 GHz data is much higher (89%) for flat-spectrum sources (mostly quasars), which also exhibit a spectral steepening: median $\alpha^5_{2.7}=-0.14$, median $\alpha^{18.5}_{5}=0.16$ ( $S_{\nu}\propto \nu^{-\alpha}$), and median $\Delta\alpha = 0.19$. Taking advantage of the almost complete redshift information on flat-spectrum quasars, we have estimated their 5 GHz luminosity function in several redshift bins. The results confirm that their radio luminosity density peaks at $z_{\rm peak} \simeq 2.5$ but do not provide evidence for deviations from pure luminosity evolution as hinted at by other data sets. A comparison of our 22 GHz flux densities with WMAP K-band data for flat-spectrum sources suggests that WMAP flux densities may be low by a median factor of $\simeq$1.2. The extrapolations of 5 GHz counts and luminosity functions of flat-spectrum radio quasars using the observed distribution of the 5-18.5 GHz spectral indices match those derived directly from WMAP data, indicating that the high frequency WMAP survey does not detect any large population of FSRQs with anomalous spectra.

Key words: radio continuum: general -- galaxies: nuclei -- quasars: general -- galaxies: luminosity function, mass function

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