Extremely red galaxy counterparts to 7C radio sources

We present RIJHK imaging of seven radio galaxies from the 7C Redshift Survey (7CRS) which lack strong emission lines and we use these data to investigate their spectral energy distributions (SEDs) with models which constrain their redshifts. Six of these seven galaxies have extremely red colours (R-K>5.5) and we find that almost all of them lie in the redshift range 1<z<2. We also present near-infrared spectroscopy of these galaxies which demonstrate that their SEDs are not dominated by emission lines, although tentative lines, consistent with H-alpha at z=1.45 and z=1.61, are found in two objects. Although the red colours of the 7CRS galaxies can formally be explained by stellar populations which are either very old or young and heavily reddened, independent evidence favours the former hypothesis. At z~1.5 at least 1/4 of powerful radio jets are triggered in massive (>L*) galaxies which formed the bulk of their stars several Gyr earlier, that is at epochs corresponding to redshifts z>5. If a similar fraction of all z~1.5 radio galaxies are old, then extrapolation of the radio luminosity function shows that, depending on the radio source lifetimes, between 10-100% of the near-IR selected extremely red object (ERO) population undergo a radio outburst at epochs corresponding to 1<z<2. An ERO found serendipitously in the field of one of the 7CRS radio sources appears to be a radio-quiet analogue of the 7CRS EROs with an emission line likely to be [OII] at z=1.20. The implication is that some of the most massive elliptical galaxies formed the bulk of their stars at z>5 and these objects probably undergo at least two periods of AGN activity: one at high redshift during which the black hole forms and another one at an epoch corresponding to z~1.5.Comment: 20 pages, 10 figures, accepted for publication in MNRA

The Nature and Evolution of Classical Double Radio Sources from Complete Samples

We present a study of the trends in luminosity, linear size, spectral index, and redshift of classical double radio sources from three complete samples selected at successively fainter low radio-frequency flux-limits. We have been able to decouple the effects of the tight correlation between redshift and luminosity (inherent in any single flux-limited sample) which have hitherto hindered interpretation of the relationships between these four source properties. The major trends found are that (i) spectral indices increase with linear size, (ii) rest-frame spectral indices have a stronger dependence on luminosity than on redshift except at high (GHz) frequencies, and that (iii) the linear sizes are smaller at higher redshifts. We reproduce the observed dependences in a model for radio sources (born throughout cosmic time according to a radio-source birth function) whose lobes are fed with a synchrotron-emitting population (whose energy distribution is governed by compact hotspots), and which suffer inverse Compton, synchrotron and adiabatic expansion losses. In simulating the basic observed dependences, we find that there is no need to invoke any systematic change in the environments of these objects with redshift if the consequences of imposing a survey flux-limit on our simulated datasets are properly included in the model. We present evidence that for a radio survey there is an unavoidable `youth--redshift degeneracy', even though radio sources are short-lived relative to the age of the Universe; it is imperative to take this into account in studies which seemingly reveal correlations of source properties with redshift such as the `alignment effect'.Comment: 48 pages, 19 figures, uses aas2pp4.sty. To appear in AJ. Also available at http://www-astro.physics.ox.ac.uk/~kmb References updated and minor typos correcte

The emission line - radio correlation for radio sources using the 7C Redshift Survey

We have used narrow emission line data from the new 7C Redshift Survey to investigate correlations between the narrow-line luminosities and the radio properties of radio galaxies and steep-spectrum quasars. The 7C Redshift Survey is a low-frequency (151 MHz) selected sample with a flux-density limit about 25-times fainter than the 3CRR sample. By combining these samples, we can for the first time distinguish whether the correlations present are controlled by 151 MHz radio luminosity L_151 or redshift z. We find unequivocal evidence that the dominant effect is a strong positive correlation between narrow line luminosity L_NLR and L_151, of the form L_NLR proportional to L_151 ^ 0.79 +/- 0.04. Correlations of L_NLR with redshift or radio properties, such as linear size or 151 MHz (rest-frame) spectral index, are either much weaker or absent. We use simple assumptions to estimate the total bulk kinetic power Q of the jets in FRII radio sources, and confirm the underlying proportionality between jet power and narrow line luminosity first discussed by Rawlings & Saunders (1991). We make the assumption that the main energy input to the narrow line region is photoionisation by the quasar accretion disc, and relate Q to the disc luminosity, Q_phot. We find that 0.05 < Q / Q_phot < 1 so that the jet power is within about an order of magnitude of the accretion disc luminosity. The most powerful radio sources are accreting at rates close to the Eddington limit of supermassive black holes (~ 10^9 - 10^10 solar masses), whilst lower power sources are accreting at sub-Eddington rates.Comment: 20 pages, 7 figures, to be published in MNRA

No evidence for a `redshift cut-off' for the most powerful classical double radio sources

We use three samples (3CRR, 6CE and 6C*) to investigate the radio luminosity function (RLF) for the `most powerful' low-frequency selected radio sources. We find that the data are well fitted by a model with a constant co-moving space density at high redshift as well as by one with a declining co-moving space density above some particular redshift. This behaviour is very similar to that inferred for steep-spectrum radio quasars by Willott et al (1998) in line with the expectations of Unified Schemes. We conclude that there is as yet no evidence for a `redshift cut-off' in the co-moving space densities of powerful classical double radio sources, and rule out a cut-off at z < 2.5.Comment: To appear in `The Hy-redshift universe: Galaxy formation and evolution at high redshift' eds. A.J. Bunker and W.J.M. van Breuge

Optical spectroscopy of radio galaxies in the 7C Redshift Survey

We present optical spectroscopy of all 49 radio galaxies in the 7C-I and 7C-II regions of the 7C Redshift Survey (7CRS). The low-frequency (151 MHz) selected 7CRS sample contains all sources with flux-densities S_151 > 0.5 Jy in three regions of the sky; 7C-I and 7C-II were chosen to overlap with the 5C6 and 5C7 surveys respectively, and cover a total sky area of 0.013 sr. The sample has been completely identified and spectroscopy of the quasars and broad-lined radio galaxies has been presented in Willott et al. (1998). Only seven of the radio galaxies do not have redshift determinations from the spectroscopy, giving a redshift completeness for the sample of >90%. The median redshift of the 7CRS is 1.1. We present a composite 0.2<z<0.8 7CRS radio galaxy spectrum and investigate the strengths of the 4000 Angstrom breaks in these radio galaxies. We find an anti-correlation between the 4000 Angstrom break strength and emission line luminosity, indicating that departures from old elliptical galaxy continuum shapes are most likely due to non-stellar emission associated with the active nucleus.Comment: 14 pages, 6 figures, MNRAS in pres

Radio galaxy evolution: what you can learn from a Brief Encounter

We describe the pitfalls encountered in deducing from classical double radio source observables (luminosity, spectral index, redshift and linear size) the essential nature of how these objects evolve. We discuss the key role played by hotspots in governing the energy distribution of the lobes they feed, and subsequent spectral evolution. We present images obtained using the new 74 MHz receivers on the VLA and discuss constraints which these enforce on models of the backflow and ages in classical doubles.Comment: invited talk at `Lifecycles of Radio Galaxies' workshop; eds John Biretta et a

The radio luminosity function from the low-frequency 3CRR, 6CE & 7CRS complete samples

We measure the radio luminosity function (RLF) of steep-spectrum radio sources using three redshift surveys of flux-limited samples selected at low (151 & 178 MHz) radio frequency, low-frequency source counts and the local RLF. The redshift surveys used are the new 7C Redshift Survey (7CRS) and the brighter 3CRR and 6CE surveys totalling 356 sources with virtually complete redshift information. This yields unprecedented coverage of the radio luminosity versus z plane for steep-spectrum sources, and hence the most accurate measurements of the steep-spectrum RLF yet made. We find that a simple dual-population model for the RLF fits the data well, requiring differential density evolution (with z) for the two populations. The low-luminosity population can be associated with radio galaxies with weak emission lines, and includes sources with both FRI and FRII radio structures; its comoving space density $\rho$ rises by about one dex between z~0 and z~1 but cannot yet be meaningfully constrained at higher redshifts. The high-luminosity population can be associated with FRII radio galaxies and quasars with strong emission lines; its $\rho$ rises by nearly three dex between z~0 and z~2. These results mirror the situation seen in X-ray and optically-selected AGN. The integrated radio luminosity density of the combination of the two populations is controlled by the value of $\rho$ at the low-luminosity end of the RLF of the high-luminosity population, a quantity which has been directly measured at z~1 by the 7CRS. We argue that robust determination of this quantity at higher redshifts requires a new redshift survey based on a large (~1000 source) sample about five times fainter than the 7CRS.Comment: 20 pages, 16 figures, accepted for publication in MNRA

The mass of radio galaxies from low to high redshift

Using a new radio sample, 6C* designed to find radio galaxies at z > 4 along with the complete 3CRR and 6CE sample we extend the radio galaxy K-z relation to z~4.5. The 6C* K-z data significantly improve delineation of the K-z relation for radio galaxies at high redshift (z >2). In a spatially flat universe with a cosmological constant (Omega_M=0.3 and Omega_Lambda=0.7), the most luminous radio sources appear to be associated with galaxies with a luminosity distribution with a high mean (~5L*), and a low dispersion (~0.5 mag) which formed their stars at epochs corresponding to z >~ 2.5.Comment: 6 pages, 3 figures. To appear in ESO conference "The mass of galaxies from low to high redshift", Venice Oct 24-26, 200

The quasar fraction in low-frequency selected complete samples and implications for unified schemes

Low-frequency radio surveys are ideal for selecting orientation-independent samples of extragalactic sources because the sample members are selected by virtue of their isotropic steep-spectrum extended emission. We use the new 7C Redshift Survey along with the brighter 3CRR and 6C samples to investigate the fraction of objects with observed broad emission lines - the `quasar fraction' - as a function of redshift and of radio and narrow emission line luminosity. We find that the quasar fraction is more strongly dependent upon luminosity (both narrow line and radio) than it is on redshift. Above a narrow [OII] emission line luminosity of log L_[OII] > 35 W (or radio luminosity log L_151 > 26.5 W/Hz/sr), the quasar fraction is virtually independent of redshift and luminosity; this is consistent with a simple unified scheme with an obscuring torus with a half-opening angle theta_trans approx 53 degrees. For objects with less luminous narrow lines, the quasar fraction is lower. We show that this is not due to the difficulty of detecting lower-luminosity broad emission lines in a less luminous, but otherwise similar, quasar population. We discuss evidence which supports at least two probable physical causes for the drop in quasar fraction at low luminosity: (i) a gradual decrease in theta_trans and/or a gradual increase in the fraction of lightly-reddened (0 < A(V) < 5) lines-of-sight with decreasing quasar luminosity; and (ii) the emergence of a distinct second population of low luminosity radio sources which, like M87, lack a well-fed quasar nucleus and may well lack a thick obscuring torus.Comment: 10 pages, 4 figures, accepted for publication in MNRA