Katherine M. Blundell

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

ABSTRA C T 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 L151 or redshift z. We find unequivocal evidence that the dominant effect is a strong positive correlation between narrow-line luminosity LNLR and L151, of the form LNLR/ L 0:79^0:04 151 . Correlations of LNLR 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 FR II radio sources, and confirm the underlying proportionality between jet power and narrow-line luminosity first discussed by Rawlings & Saunders. We make the assumption that the main energy input to the narrow-line region is photoionization by the quasar accretion disc, and relate Q to the disc luminosity, Qphot. 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. Values of Q=Qphot , 1 require the volume filling factor h of the synchrotron-emitting material to be of the order of unity, and in addition require one or more of the following: (i) an important contribution to the energy budget from protons; (ii) a large reservoir of mildly relativistic electrons; and (iii) a substantial departure from the minimumenergy condition in the lobe material. The most powerful radio sources are accreting at rates close to the Eddington limit of supermassive black holes OMBH * 10 9 M(U, whilst lower power sources are accreting at sub-Eddington rates.

The radio luminosity function from the low-frequency 3CRR, 6CE and 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 and 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 z 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 ρ rises by about one dex between z∼0 and 1 but cannot yet be meaningfully constrained at higher redshifts. The high-luminosity population can be associated with radio galaxies and quasars with strong emission lines, and consists almost exclusively of sources with FRII radio structure; its ρ rises by nearly three dex between z∼0 and 2. These results mirror the situation seen in X-ray and optically selected samples of AGN where: (i) low-luminosity objects exhibit a gradual rise in ρ with z that crudely matches the rises seen in the rates of global star formation and galaxy mergers; and (ii) the density of high-luminosity objects rises much more dramatically. The integrated radio luminosity density of the combination of the two populations is controlled by the value of ρ 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.

Near-infrared imaging and the K z relation for radio galaxies in the 7C Redshift Survey

We present K-band imaging 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 1 5 1 > 0.5 Jyin three regions of the sky. We combine the K-band magnitudes of the 7CRS radio galaxies with those from the 3CRR, 6CE and 6C* samples to investigate the nature of the relationship between K-magnitude and redshift and whether there is any dependence upon radio luminosity. We find that radio galaxies appear to belong to a homogeneous population that formed the bulk of their stars at high redshifts (z f > 5) and evolved passively from then until they reach a mean present-day luminosity of 3 L * . We find a significant difference between the K-magnitudes of the 7CRS and 3CRR radio galaxies with the 7CRS galaxies being 0.55 mag fainter at all redshifts. The cause of this weak correlation between stellar and radio luminosities probably lies in mutual correlations of these properties with the central black hole mass. We compare the evolution-corrected host luminosities at a constant radio luminosity and find that the typical host luminosity (mass) increases by approximately 1 L * from z ∼ 2 to ∼0.5 which, although a much smaller factor than predicted by semi-analytic models of galaxy formation, is in line with results on optically selected quasars. Our study has therefore revealed that the small dispersion in stellar luminosity of radio galaxies around 3 L * includes subtle but significant differences between the host galaxies of extreme- and moderate-power radio sources at fixed redshift, and between hose of high- and low-redshift radio sources at fixed radio luminosity.

The properties of the X-ray holes in the intracluster medium of the Perseus cluster

Abstract : High-resolution X-ray and low-frequency radio imaging now allow us to examine in detail the interaction and physical properties of the radio source 3C 84 and the surrounding thermal gas. The radiative and dynamical properties of the inner X-ray holes, which coincide with the radio lobes, indicate that the ratio of the energy factor k to the filling factor f is in the range 180 < k/f < 500. We define k to be the ratio of the total particle energy to that of the electrons radiating above a fiducial frequency of 10 MHz. The relativistic plasma and magnetic field are not in equipartition, since the field must be a factor of 4 or more lower than required for pressure balance. Unexpected steep-spectrum spurs in the low-frequency radio maps point to outer X-ray holes, which are plausibly buoyant old radio lobes. The evidence that the inner lobes are currently expanding subsonically, yet have not detached due to buoyancy, and the requirement that the synchrotron cooling time must exceed the age of the hole enable us to constrain the jet power of the nucleus to between 10(exp 44) and 10(exp 45) erg s (exp-1), depending on the filling factor of the relativistic plasma.

A sample of 6C radio sources designed to find objects at redshift z > 4 - III. Imaging and the radio galaxy K-z relation

In this paper, the third and final of a series, we present complete K− band imaging and some complementary I−band imaging of the filtered 6C* sample. We find no systematic differences between the K − z relation of 6C* radio galaxies and those from complete samples, so the near-infrared properties of luminous radio galaxies are not obviously biased by the additional 6C* radio selection criteria (steep spectral index and small angular size). The 6C* K − z data significantly improve delineation of the K − z relation for radio galaxies at high-redshift (z > 2). Accounting for non-stellar contamination, and for correlations between radio luminosity and stellar mass, we find little support for previous claims that the underlying scatter in the stellar luminosity of radio galaxies increases significantly at z > 2. In a particular spatially-flat universe with a cosmological constant (M = 0.3 and � = 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 (σ ∼ 0.5mag) which formed their stars at epochs corresponding to z >2.5. This result is in line with recent sub-mm studies of high-redshift radio galaxies and the inferred ages of extremely red objects from faint radio samples.

RADIO VARIABILITY OF RADIO-QUIET AND RADIO-LOUD QUASARS

The majority of quasars are weak in their radio emission, with flux densities comparable to those in the optical, and energies far lower. A small fraction, about 10%, are hundreds to thousands of times stronger in the radio. Conventional wisdom holds that there are two classes of quasars, the radio-quiets and radio-louds, with a deficit of sources having intermediate power. Are there really two separate populations, and if so, is the physics of the radio emission fundamentally different between them? This paper addresses the second question, through a study of radio variability across the full range of radio power, from quiet to loud. The VLA was used during 10 epochs to study three carefully selected samples of 11 radio-quiet quasars, 11 radio-intermediate quasars, and 8 radio-loud quasars. A fourth sample consists of 20 VLA calibrators used for phase correction during the observations, all of which are radio-loud. The basic findings are that the root mean square amplitude of variability is independent of radio luminosity or radio-to-optical flux density ratio and that fractionally large variations can occur on timescales of months or less in both radio-quiet and radio-loud quasars. Combining this with similarities in other indicators, such as radio spectral index and the presence of VLBI-scale components, leads to the suggestion that the physics of radio emission in the inner regions of all quasars is essentially the same, involving a compact, partially opaque core together with a beamed jet. It is possible that differences in large-scale radio structures between radio-loud and radio-quiet quasars could stem from disruption of the jets in low-power sources before they can escape their host galaxies.

The inevitable youthfulness of known high-redshift radio galaxies

Some galaxies are very luminous in the radio part of the spectrum. These ‘radio galaxies’ have extensive (hundreds of kiloparsecs) lobes of emission powered by plasma jets originating at a central black hole. Some radio galaxies can be seen at very high redshifts, where in principle they can serve as probes of the early evolution of the Universe. Here we show that, for any model of radio-galaxy evolution in which the luminosity decreases with time after an initial rapid increase (that is, essentially all reasonable models), all observable high-redshift radio galaxies must be seen when the lobes are less than 107 years old. This means that high-redshift radio galaxies can be used as a high-time-resolution probe of evolution in the early Universe. Moreover, this result explains many observed trends of radio-galaxy properties with redshift, without needing to invoke explanations based on cosmology or strong evolution of the surrounding intergalactic medium with cosmic time, thereby avoiding conflict with current theories of structure formation.

The central engines of radio-quiet quasars

Two rival hypotheses have been proposed for the origin of the compact radio flux observed in radio-quiet quasars (RQQs). It has been suggested that the radio emission in these objects, typically some two or three orders of magnitude less powerful than in radio-loud quasars (RLQs), represents either emission from a circumnuclear starburst or is produced by radio jets with bulk kinetic powers 10^3 times lower than those of RLQs with similar luminosity ratios in other wavebands. We describe the results of high resolution (parsec-scale) radio-imaging observations of a sample of 12 RQQs using the Very Long Baseline Array (VLBA). We find strong evidence for jet-producing central engines in 8 members of our sample.

A sample of 6C radio sources designed to find objects at redshift z>4: II - spectrophotometry and emission line properties

This is the second in a series of three papers that present and interpret basic observational data on the 6C* 151-MHz radio sample: a low-frequency selected sample that exploits filtering criteria based on radio properties (steep spectral index and small angular size) to find radio sources at redshift inline image within a 0.133-sr patch of sky. We present results of a programme of optical spectroscopy that has yielded redshifts in the range inline image for the 29 sources in the sample, all but six of which are secure. We find that the filtering criteria used for 6C* are very effective in excluding the low-redshift, low-luminosity radio sources: the median redshift of 6C* is inline image compared with inline image for a complete sample matched in 151-MHz flux density. By combining the emission-line data set for the 6C* radio sources with those for the 3CRR, 6CE and 7CRS samples we establish that inline image radio galaxies follow a rough proportionality between Lyα and 151-MHz luminosity, which, like similar correlations seen in samples of lower redshift radio sources, is indicative of a primary link between the power in the source of the photoionizing photons (most likely a hidden quasar nucleus) and the power carried by the radio jets. We argue that radio sources modify their environments and that the range of emission-line properties seen is determined more by the range of source age than by the range in ambient environment. The smallest inline image radio galaxies have all the properties expected if the size distribution of luminous high-redshift steep-spectrum radio sources reflects a broad range (∼2 dex) of source ages with a narrower range (≲ 1.5 dex) of environmental densities, namely: (1) high-ionization lines, e.g. Lyα, of relatively low luminosity; (2) boosted low-ionization lines, e.g. C ii]; (3) spatially compact emission-line regions; and (4) H i-absorbed Lyα profiles. This is in accord with the idea that all high-redshift, high-luminosity radio sources are triggered in similar environments, presumably recently collapsed massive structures.

On the redshift cut-off for steep-spectrum radio sources

We use three samples (3CRR, 6CE and 6C*) selected at low radio frequency to constrain the cosmic evolution in the radio luminosity function (RLF) for the ‘most luminous’ steep-spectrum radio sources. Although intrinsically rare, such sources give the largest possible baseline in redshift for the complete flux-density-limited samples currently available. Using parametric models to describe the RLF which incorporate distributions in radio spectral shape and linear size as well as the usual luminosity and redshift, we find that the data are consistent with a constant comoving space density between z ∼ 2.5 and z ∼ 4.5. We find this model is favoured over a model with similar evolutionary behaviour to that of optically-selected quasars (i.e. a roughly Gaussian distribution in redshift) with a probability ratio of ∼ 25:1 and ∼ 100:1 for spatiallyflat cosmologies with � = 0 and � = 0.7 respectively. Within the uncertainties, this evolutionary behaviour may be reconciled with the shallow decline preferred for the comoving space density of flat-spectrum sources by Dunlop & Peacock (1990) and Jarvis & Rawlings (2000), in line with the expectations of Unified Schemes.