Cosmological evolution of compact AGN at 15 GHz

Free access article

Issue		A&A Volume 458, Number 2, November I 2006


Page(s)		397 - 404
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20065733

A&A 458, 397-404 (2006)
DOI: 10.1051/0004-6361:20065733

Cosmological evolution of compact AGN at 15 GHz

T. G. Arshakian, E. Ros and J. A. Zensus

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: [tigar;ros;azensus]@mpifr-bonn.mpg.de

(Received 5 October 2005 / Accepted 26 July 2006)

Abstract
Aims. We study the uniformity of the distribution of compact flat-spectrum active galactic nuclei (AGN) on the sky and the evolution of their relativistic jets with cosmic epoch.
Methods.A complete sample of compact extragalactic radio sources at 2 cm (15 GHz) was recently compiled to conduct the MOJAVE (Monitoring Of Jets in AGN with VLBA Experiments) program (Lister & Homan 2005, AJ, 130, 1389). The MOJAVE sample comprises 133 radio-loud flat-spectrum AGN with compact relativistic outflows detected at parsec scales. We use a two-point angular correlation function to test the isotropy of the distribution of radio sources on the sky. The generalized and banded versions of $V/V_{\rm max}$ statistic are used to investigate the cosmological evolution of compact AGN.
Results.The survey sources are distributed uniformly on the sky. The source counts of compact AGN shows that the MOJAVE sample represents a flux-limited complete sample. Analysis of the population of flat-spectrum quasars of the sample reveals that the pc-scale jets of quasars have intrinsic luminosities in the range between ~ $10^{24}\,{\rm W\,Hz^{-1}}$ and ~ $10^{27}\,{\rm W\,Hz^{-1}}$ and Lorentz factors distributed between $3\la\gamma \la30$ . We find that the apparent speed (or Lorentz factor) of jets evolves with redshift, increasing from $z\sim0$ to $z\sim1$ and then falling at higher redshifts ( $z\sim2.5$ ) by a factor of 2.5. The evolution of apparent speeds does not affect significantly the evolution of the beamed luminosity function of quasars, which is most likely to be dependent on the evolution of radio luminosity. Furthermore, the beamed radio luminosity function suggests that the intrinsic luminosity function of quasars has a double power-law form: it is flat at low luminosities and steep at high luminosities. There is a positive evolution of quasars at low redshifts (z<0.5) and strong negative evolution at redshifts >1.7 with space density decline up to $z\sim2.5$ . This implies that the powerful jets were more populous at redshifts between 0.5 and 1.7. We show that the evolution of compact quasars is luminosity dependent and it depends strongly on the speed of the jet suggesting that there are two distinct populations of quasars with slow and fast jets which evolve differently with redshift.

Key words: surveys -- quasars: general -- galaxies: active -- galaxies: nuclei -- galaxies: jets -- BL Lacertae objects: general

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