The VVDS type-1 AGN sample: the faint end of the luminosity function

¹ Università di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127, Bologna, Italy e-mail: angela.bongiorno@oabo.inaf.it
² INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Astrophysical Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Integral Science Data Centre, Ch. d'Écogia 16, 1290 Versoix, Switzerland
⁵ Geneva Observatory, University of Geneva, Ch. des Maillettes 51, 1290 Sauverny, Switzerland
⁶ Laboratoire d'Astrophysique de Toulouse/Tabres (UMR5572), CNRS, Université Paul Sabatier - Toulouse III, Observatoire Midi-Pyrénées, 14 Av. E. Belin, 31400 Toulouse, France
⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁸ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
⁹ IASF-INAF, via Bassini 15, 20133 Milano, Italy
¹⁰ Laboratoire d'Astrophysique de Marseille, UMR 6110 CNRS - Université de Provence, BP 8, 13376 Marseille Cedex 12, France
¹¹ IRA-INAF, via Gobetti 101, 40129 Bologna, Italy
¹² INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
¹³ Max Planck Institut fur Astrophysik, 85741 Garching, Germany
¹⁴ Institut d'Astrophysique de Paris, UMR 7095, 98bis Bd. Arago, 75014 Paris, France
¹⁵ School of Physics & Astronomy, University of Nottingham, University Park, Nottingham NG72RD, UK
¹⁶ INAF - Osservatorio Astronomico di Brera, via Brera 28, Milan, Italy
¹⁷ Institute for Astronomy, 2680 Woodlawn Dr., University of Hawaii, Honolulu 96822, Hawaii
¹⁸ Observatoire de Paris, LERMA, 61 avenue de l'Observatoire, 75014 Paris, France
¹⁹ Centre de Physique Théorique, UMR 6207 CNRS-Université de Provence, 13288 Marseille France
²⁰ Astronomical Observatory of the Jagiellonian University, ul Orla 171, 30-244 Kraków, Poland
²¹ INAF - Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
²² Center for Astrophysics & Space Sciences, University of California, San Diego, La Jolla, CA 92093-0424, USA
²³ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
²⁴ Universitá di Milano-Bicocca, Dipartimento di Fisica - Piazza delle Scienze 3, 20126 Milano, Italy
²⁵ Università di Bologna, Dipartimento di Fisica, via Irnerio 46, 40126 Bologna, Italy

Received: 4 April 2007
Accepted: 26 June 2007

Abstract

In a previous paper (Gavignaud et al. 2006, A&A, 457, 79), we presented the type-1 Active Galactic Nuclei (AGN) sample obtained from the first epoch data of the VIMOS-VLT Deep Survey (VVDS). The sample consists of 130 faint, broad-line AGN with redshift up to and , selected on the basis of their spectra. The sample is thus free of the morphological and color selection biases, that lead to significant incompleteness in the optical surveys of faint AGN.
In this paper we present the measurement of the Optical Luminosity Function up to derived from this sample, we compare our results with previous results from brighter samples both at low and at high redshift and finally, through the estimate of the bolometric luminosity function, we compare them also with the results from X-ray and mid-IR selected samples.
Our data, more than one magnitude fainter than previous optical surveys, allow us to constrain the faint part of the luminosity function up to high redshift. A comparison of our data with the 2dF sample at low redshift () shows that the VVDS data can not be well fitted with the PLE models derived by previous samples. Qualitatively, this appears to be due to the fact that our data suggest the presence of an excess of faint objects at low redshift () with respect to these models.
By combining our faint VVDS sample with the large sample of bright AGN extracted from the SDSS DR3 (Richards et al. 2006b, AJ, 131, 2766) and testing a number of different evolutionary models, we find that the model which better represents the combined luminosity functions, over a wide range of redshift and luminosity, is a luminosity dependent density evolution (LDDE) model, similar to those derived from the major X-surveys. Such a parameterization allows the redshift of the AGN space density peak to change as a function of luminosity and explains the excess of faint AGN that we find at . On the basis of this model we find, for the first time from the analysis of optically selected samples, that the peak of the AGN space density shifts significantly towards lower redshift going to lower luminosity objects. The position of this peak moves from for to for . This result, already found in a number of X-ray selected samples of AGN, is consistent with a scenario of “AGN cosmic downsizing”, in which the density of more luminous AGN, possibly associated to more massive black holes, peaks earlier in the history of the Universe (i.e. at higher redshift), than that of low luminosity ones, which reaches its maximum later (i.e. at lower redshift).