arXiv subjects

Steve Rawlings

Publications and source records attributed to Steve Rawlings.

At least 91 records · Page 5Linked to original sources

arXiv2001

Detection of a CMB decrement towards a cluster of mJy radiosources

We present the results of radio, optical and near-infrared observations of the field of TOC J0233.3+3021, a cluster of milliJansky radiosources from the TexOx Cluster survey. In an observation of this field with the Ryle Telescope (RT) at 15 GHz, we measure a decrement in the cosmic microwave background (CMB) of $-675 \pm 95 μ$Jy on the RT's $\approx$ 0.65 k$λ$ baseline. Using optical and infrared imaging with the McDonald 2.7-m Smith Reflector, Calar Alto 3.5-m telescope and UKIRT, we identify the host galaxies of five of the radiosources and measure magnitudes of $R \approx 24$, $J \approx 20$, $K \approx 18$. The CMB decrement is consistent with the Sunyaev-Zel'dovich (SZ) effect of a massive cluster of galaxies, which if modelled as a spherical King profile of core radius $θ_C = 20^{\prime\prime}$ has a central temperature decrement of $900 μ$K. The magnitudes and colours of the galaxies are consistent with those of old ellipticals at $z \sim 1$. We therefore conclude that TOC J0233.3+3021 is a massive, high redshift cluster. These observations add to the growing evidence for a significant population of massive clusters at high redshift, and demonstrate the effectiveness of combining searches for AGN `signposts' to clusters with the redshift-independence of the SZ effect.

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arXiv2001

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 spatially-flat cosmologies with Omega_Lambda = 0 and Omega_Lambda = 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.

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arXiv2001

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

(Abridged) This is the second in a series of three papers which present and interpret basic observational data on the 6C* 151-MHz radio sample: a low-frequency selected sample which exploits filtering criteria based on radio properties (steep spectral index and small angular size) to find radio sources at redshift z > 4 within a 0.133sr patch of sky. We present results of a programme of optical spectroscopy which has yielded redshifts in the range 0.5 < z < 4.4 for the 29 sources in the sample, all but six of which are secure. We find that the fil tering criteria used for 6C* are very effective in excluding the low-redshift, low-luminosity radio sources: the median redshift of 6C* is z~1.9 compared to z~1.1 for a complete sample matched in 151-MHz flux density. By combining the emission-line dataset for the 6C* radio sources with those for the 3CRR, 6CE and 7CRS samples we establish that z > 1.75 radio galaxies follow a rough proportionality between Lyalpha- and 151 MHz-luminosity which, like similar correlations seen in samples of lower-redshift radio sources, are indicative of a primary link between the power in the source of the photoionising 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. 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.

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arXiv2001

A sample of 6C radio sources designed to find objects at redshift > 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, the most luminous radio sources appear to be associated with galaxies with a luminosity distribution with a high mean (~5 Lstar), and a low dispersion (sigma ~ 0.5 mag) 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.

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arXiv2001

Extremely red radio galaxies

At least half the radio galaxies at z>1 in the 7C Redshift Survey have extremely red colours (R-K>5), consistent with stellar populations which formed at high redshift (z>5). We discuss the implications of this for the evolution of massive galaxies in general and for the fraction of near-IR-selected EROs which host AGN, a result which is now being tested by deep, hard X-ray surveys. The conclusion is that many massive galaxies undergo at least two active phases: one at z~5 when the black hole and stellar bulge formed and another at z~1-2 when activity is triggered by an event such as an interaction or merger.

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arXiv2001

The radio galaxy K-z relation to z ~ 4.5

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). Accounting for non-stellar contamination, and for correlations between radio luminosity and estimates of 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. This indicates that we are not probing into the formation epoch until at least z > 3.

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arXiv2000

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 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 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.

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arXiv2000

A sample of 6C radio sources with virtually complete redshifts. II - optical spectroscopy

This is the second of two papers presenting basic observational data on the 6CE sample of extragalactic radio sources. It presents the results of optical spectroscopy which has yielded virtually complete redshift information for the 6CE sample: 56 of the 59 sample members have spectroscopic redshifts which, with the exception of seven cases, are secure. The redshift distribution N(z) is fairly flat over the redshift range 0 < z < 2, with a median redshift of 1.1 and a high-redshift tail reaching to z = 3.4. The highest-redshift (z > 1.75) members of the 6CE sample have similar optical spectra and a tight (less than one dex) spread in narrow-Lyalpha emission line luminosity.

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arXiv2000

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 $ρ$ 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 $ρ$ 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 $ρ$ 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.

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arXiv2000

Cosmological Studies from Radio Source Samples

I review some recent cosmological studies based on redshift surveys of radio sources selected at low frequencies. The accretion rate onto the central black hole is identified as the basis of a crude physical division of the low-frequency population into two sub-populations: the first comprises `Eddington-tuned' (high accretion rate) quasars and their torus-hidden counterparts; the second comprises `starved quasars' like M87. There exist remarkable similarities between the shapes and evolutionary behaviours of the luminosity functions of radio sources and radio-quiet quasars; all luminous AGN are $\sim300-$times rarer at epochs corresponding to $z \sim 0$ than at $z \sim 2.5$. I argue that any evidence that quasars were intrinsically rarer at $z \sim 5$ than at $z \sim 2.5$ is as yet both tentative and indirect. A simple calculation suggests that the radio source population has been over-looked as a potentially important contributor to the entropy budget of the Universe. A recent sub-mm survey of radio sources is used to demonstrate a connection between the events which trigger jets and intense bursts of star formation, and a close link between the histories of star formation and AGN activity is proposed. I discuss the aims and methods of future large redshift surveys of radio sources, emphasising the importance of dovetailing these with the development of robust physical models for radio sources and their epoch-dependent environments.

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arXiv2000

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

We use data from the Parkes Half-Jansky Flat-Spectrum (PHJFS) sample (Drinkwater et al. 1997) to constrain the cosmic evolution in the co-moving space density of radio sources in the top decade of the flat-spectrum radio luminosity function (RLF). A consistent picture for the high-redshift evolution is achieved using both simple parametric models, which are the first to allow for distributions in both radio luminosity and spectral index, and variants of the V / V_max test, some of which incorporate the effects of radio spectral curvature. For the most luminous flat-spectrum objects, the PHJFS sample is extremely similar to that used by Shaver et al. (1996, 1998) to argue for an abrupt `redshift cut-off': a decrease by a factor ~30 in space density between a peak redshift z~2.5 and z~5. Our analysis finds that the observable co-moving volume is too small to make definitive statements about any redshift cut-off for the most luminous flat-spectrum sources, although both constant-space density (no cut-off) models and models with cut-offs as abrupt as those envisaged by Shaver et al. are outside the 90% confidence region. The inference that the decline in space density is most likely to be gradual, by a factor ~4 between z~2.5 and z~5, is in accordance with previous work on the RLF by Dunlop & Peacock (1990), but different to the abrupt decline favoured by studies of optically-selected quasars. Dust obscuration provides one explanation for this difference. We show that a significant fraction of the most radio-luminous flat-spectrum objects are Giga-Hertz Peaked Spectrum (GPS) rather than Doppler-Boosted (DB) sources, complicating any interpretation of the redshift cut-off.

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arXiv2000

Infrared photometry of z~1 3C quasars

We present JHKL' photometry of a complete sample of steep-spectrum radio-loud quasars from the revised 3CR catalogue in the redshift range 0.65 < z < 1.20. After correcting for contributions from emission lines and the host galaxies, we investigate their spectral energy distributions (SEDs) around 1 micron. About 75% of the quasars are tightly grouped in the plane of optical spectral index, alpha_lo, versus near-infrared spectral index, alpha_hi, with the median value of alpha_lo close to the canonical value, and the median alpha_hi slightly flatter. We conclude that the fraction of moderately-obscured, red quasars decreases with increasing radio power, in accordance with the `receding torus' model which can also explain the relatively flat median near-infrared spectra of the 3CR quasars. Two of the red quasars have inverted infrared spectral indices, and we suggest that their unusual SEDs might result from a combination of dust-scattered and transmitted quasar light.

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arXiv2000

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.

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arXiv2000

The spectra and energies of classical double radio lobes

We compare two temporal properties of classical double radio sources: i) radiative lifetimes of synchrotron-emitting particles and ii) dynamical source ages. We discuss how these can be quite discrepant from one another, rendering use of the traditional spectral ageing method inappropriate: we contend that spectral ages give meaningful estimates of dynamical ages only when these ages are << 10^7 years. In juxtaposing the fleeting radiative lifetimes with source ages which are significantly longer, a refinement of the paradigm for radio source evolution is required. The changing spectra along lobes are explained, not predominantly by synchrotron ageing but, by gentle gradients in a magnetic field mediated by a low-gamma matrix which illuminates an energy-distribution of particles, controlled largely by classical synchrotron loss in the high magnetic field of the hotspot. The energy in the particles is an order of magnitude higher than that inferred from the minimum-energy estimate, implying that the jet-power is of the same order as the accretion luminosity produced by the quasar central engine. This refined paradigm points to a resolution of the findings of Rudnick et al (1994) and Katz-Stone & Rudnick (1994) that both the Jaffe-Perola and Kardashev-Pacholczyk model spectra are invariably poor descriptions of the curved spectral shape of lobe emission, and indeed that for Cygnus A all regions of the lobes are characterised by a `universal spectrum'. [abridged]

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arXiv1999

The hyperluminous infrared quasar 3C 318 and its implications for interpreting sub-mm detections of high-redshift radio galaxies

We present near-infrared spectroscopy and imaging of the compact steep- spectrum radio source 3C 318 which shows it to be a quasar at redshift z=1.574 (the z=0.752 value previously reported is incorrect). 3C 318 is an IRAS, ISO and SCUBA source so its new redshift makes it the most intrinsically luminous far-infrared (FIR) source in the 3C catalogue (there is no evidence of strong gravitational lensing effects). Its bolometric luminosity greatly exceeds the 10^13 solar luminosity level above which an object is said to be hyperluminous. Its spectral energy distribution (SED) requires that the quasar heats the dust responsible for the FIR flux, as is believed to be the case in other hyperluminous galaxies, and contributes (at the >10% level) to the heating of the dust responsible for the sub-mm emission. We cannot determine whether a starburst makes an important contribution to the heating of the coolest dust, so evidence for a high star-formation rate (SFR) is circumstantial being based on the high dust mass required by its sub-mm detection. We show that the current sub-mm and FIR data available for high-z radio galaxies are consistent with SEDs similar to that of 3C 318. This indicates that at least some of this population may be detected in the sub-mm because of dust heated by the quasar nucleus, and that interpreting sub-mm detection as evidence for very high SFRs may not always be valid. We show that the 3C318 quasar is slightly reddened (A(V) ~ 0.5), the most likely cause of which is SMC-type dust in the host galaxy. If very distant radio galaxies are reddened in a similar way then we show that only slightly greater amounts of dust could obscure the quasars in these sources, thus explaining the low fraction of quasars amongst the very high redshift (z > 3) objects in low-frequency radio-selected samples.

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arXiv1999

High-z radio galaxies and the `Youth-Redshift Degeneracy'

We discuss a unifying explanation for many `trends with redshift' of radio galaxies which includes the relevance of their ages (time since their jet triggering event), and the marked dependence of their ages on redshift due to the selection effect of imposing a flux-limit. We briefly describe some important benefits which this `youth-redshift degeneracy' brings.

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arXiv1999

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.

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arXiv1999

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.

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