The luminosity-dependent high-redshift turnover in the steep spectrum radio luminosity function: clear evidence for downsizing in the radio-AGN population

Mon. Not. R. Astron. Soc. 416, 1900–1915 (2011) doi:10.1111/j.1365-2966.2011.19167.x The luminosity-dependent high-redshift turnover in the steep spectrum radio luminosity function: clear evidence for downsizing in the radio-AGN population E. E. Rigby,1,2 P. N. Best,2 M. H. Brookes,2 J. A. Peacock,2 J. S. Dunlop,2 H. J. A. Röttgering,3 J. V. Wall4 and L. Ker2 1 School of Physics & Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD 2 SUPA†, Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ 3 Leiden Observatory, PO Box 9513, 2300 RA, Leiden, the Netherlands 4 Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Rd, Vancouver, BC V6T 1Z1, Canada Accepted 2011 June 1. Received 2011 May 25; in original form 2011 April 23 This paper presents a new grid-based method for investigating the evolution of the steepspectrum radio luminosity function, with the aim of quantifying the high-redshift cut-off suggested by previous work. To achieve this, the Combined EIS–NVSS Survey of Radio Sources (CENSORS) has been developed; this is a 1.4-GHz radio survey, containing 135 sources complete to a flux density of 7.2 mJy, selected from the NRAO VLA Sky Survey (NVSS) over 6 deg2 of the ESO Imaging Survey (EIS) Patch D. The sample is currently 73 per cent spectroscopically complete, with the remaining redshifts estimated via the K–z or I–z magnitude–redshift relation. CENSORS is combined with additional radio data from the Parkes All-Sky, Parkes Selected Regions, Hercules and Very Large Array (VLA) COSMOS samples to provide comprehensive coverage of the radio power versus redshift plane. The redshift distributions of these samples, together with radio source count determinations, and measurements of the local luminosity function, provide the input to the fitting process. The modelling reveals clear declines, at >3σ significance, in comoving density at z > 0.7 for lower luminosity sources (log P = 25–26); these turnovers are still present at log P > 27, but move to z  3, suggesting a luminosity-dependent evolution of the redshift turnover, similar to the ‘cosmic downsizing’ seen for other active galactic nucleus populations. These results are shown to be robust to the estimated redshift errors and to increases in the spectral index for the highest redshift sources. Analytic fits to the best-fitting steep spectrum grid are provided so that the results presented here can be easily accessed by the reader, as well as allowing plausible extrapolations outside of the regions covered by the input data sets. Key words: galaxies: active – galaxies: evolution – galaxies: high-redshift – galaxies: luminosity function, mass function. 1 I N T RO D U C T I O N It has become increasingly apparent in recent years that radio-loud active galactic nuclei (AGN) play a key role in galaxy evolution; the interplay of their expanding radio jets and the surrounding intergalactic and intracluster medium acts to provide part, or possibly all, of the heat required to prevent both large-scale cluster cooling flows and the continued growth of massive ellipticals (e.g. Best et al. 2006,  E-mail: †Scottish Universities Physics Alliance 2007; Bower et al. 2006; Croton et al. 2006; Fabian et al. 2006). Determining the evolution of the radio luminosity function (RLF) is therefore important for understanding the time-scales on which they impose these effects. Also, since radio-loud AGN are powered by the most massive black holes, their RLF can be used to investigate the behaviour of the upper end of the black hole mass function and hence the build-up of these objects in the early Universe. The work of Sandage (1972), Osmer (1982), Peacock (1985), Schmidt, Schneider & Gunn (1988) and, in particular, Dunlop & Peacock (1990, hereafter DP90) has shown that the comoving number density of both flat- and steep-spectrum powerful radio galaxies, selected at 2.7 GHz, is greater by 2 to 3 orders of magnitude at a  C 2011 The Authors C 2011 RAS Monthly Notices of the Royal Astronomical Society  ABSTRACT RLF evolution modelling  C 2011 The Authors, MNRAS 416, 1900–1915 C 2011 RAS Monthly Notices of the Royal Astronomical Society  In this paper the CENSORS data set, combined with additional samples, is used to investigate the nature of the high-redshift evolution of radio sources, via a new grid-based modelling technique in which no prior assumptions are made about the behaviour of the luminosity function. This is an improvement on previous investigations which have either used functional forms, or only considered pure luminosity or density evolution, or a combination of both [although Dye & Eales (2010) have recently developed a similar method to study the evolution of submm galaxies]. The layout of the paper is as follows. Section 2 describes both CENSORS and the additional data sets needed. Section 3 presents the modelling technique. Section 4 describes the results from the best-fitting model and investigates their robustness. Finally, Section 5 summarizes the findings. Throughout this paper, values for the cosmological parameters of H 0 = 70 km s−1 Mpc−1 , m = 0.3 and  = 0.7 are used and the spectral index, α, is defined as Sν ∝ ν −α . 2 I N P U T DATA As discussed above, several data sets are needed to constrain the radio source cosmic evolution. In addition to the CENSORS sample, therefore, four other radio samples, along with determinations of the local RLF (LRLF), and measurements of the radio source counts are used; these are described in this section. Fig. 1 illustrates the coverage of the P–z plane obtained using these radio samples. 2.1 CENSORS The full CENSORS sample contains 150 sources with S1.4 GHz > 3.8 mJy in a 3 × 2 deg2 field of the ESO Imaging Survey (EIS) Patch D, centred on 09h 51m 36.s 0, −21◦ 00 00 (J2000). Paper I presents the radio data, along with the optical host galaxy identifications, with additional K-band imaging presented by Brookes et al. (2006, hereafter Paper II). Spectroscopic data for a subset of the sample can Figure 1. The Wall & Peacock (1985, WP85), Parkes Selected Regions (PSR; Downes et al. 1986; Dunlop et al. 1989), CENSORS, Hercules (Waddington et al. 2001) and VLA-COSMOS (for z ≤ 1.3 only; Smolčić et al. 2008) samples plotted on a radio luminosity versus redshift plane to illustrate how they efficiently cover a large part of the plane without much overlap. Radio luminosities were calculated using previously published spectral indices for the WP85, PSR and Hercules samples; α = 0.8 was assumed for the sources in VLA-COSMOS. The spectral indices for the CENSORS sample are taken from Ker et al. (in preparation). The PSR, WP85 and COSMOS samples are restricted to steep-spectrum sources only. See text for full details of sample selection. redshift of 2 compared with the present-day Universe. This density increase is expected to peak at some point simply because sufficient t (...truncated)