arXiv subjects

Steve Rawlings

Publications and source records attributed to Steve Rawlings.

At least 73 records · Page 4Linked to original sources

arXiv2003

Implications for unified schemes from the quasar fraction and emission-line luminosities in radio-selected samples

We use a principal components analysis of radio-selected (3CRR, 6CE and 7CRS) AGN datasets to define two parameters related to low-frequency (151 MHz) radio luminosity L_151 and [OIII] luminosity L_[OIII]: a parameter alpha encoding the L_151 - L_[OIII] correlation and a parameter beta encoding scatter about this correlation. We describe methods for constructing generalized luminosity functions (GLFs) based on alpha, beta, redshift and schemes for unifying quasars and radio galaxies. Luminosity-dependent unified schemes (e.g. a receding-torus scheme) have been invoked to explain the low quasar to radio galaxy fraction at low alpha and the differences in emission-line luminosities of radio quasars and radio galaxies. With the constraints of the 3CRR, 6CE and 7CRS datasets and radio source counts, our GLF approach was used to determine whether a receding-torus-like scheme is required if there are two populations of radio sources: one at low alpha, consisting of 'starved AGN'; the other at high alpha consisting of 'Eddington-tuned AGN'. Because of the overlap between these two populations and the effects of the beta parameter, schemes with or without a receding torus can produce a low quasar fraction at low alpha and differences in [OIII] luminosity between radio galaxies and quasars. The receding torus may be a physical process important in one or more populations of radio sources, but this is not yet proved either by the quasar fraction or the emission-line properties of radio-selected samples.

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arXiv2003

Broad absorption line quasars have the same cool dust emission as quasars without BALs

The results of a sub-millimeter survey of SDSS broad CIV absorption line quasars is discussed. It is found that the sub-millimeter flux distribution of BAL quasars is similar to that of non-BAL quasars. This is consistent with the idea that all quasars contain broad absorption line regions, but only a fraction of them are visible along our line-of-sight. The observations are inconsistent with BAL quasars being observed at a special evolutionary epoch co-inciding with a high star-formation rate and dust mass.

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arXiv2003

A Search for Radio-loud Quasars within the Epoch of Reionization

The Universe became fully reionized, and observable optically, at a time corresponding to redshift z~6.5, so it is only by studying the HI and molecular absorption lines against higher-redshift, radio-loud sources that one can hope to make detailed studies of the earliest stages of galaxy formation. At present no targets for such studies are known. In these proceedings we describe a survey which is underway to find radio-loud quasars at z > 6.5.

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arXiv2003

Near-infrared spectroscopy of powerful compact steep-spectrum radio sources

We have obtained near-infrared spectroscopy of a small sample of powerful Compact Steep-Spectrum (CSS) radio sources mainly, but not exclusively, from the 3CR sample. We find no differences between the distributions in the equivalent width and luminosity of the [OIII] 5007A line for our sample and other larger, presumably older, high-redshift 3C objects, suggesting that the underlying quasar luminosity remains roughly constant as quasars age. We also find a possible broad line in 3C241, adding to recent evidence for broad lines in some radio galaxies.

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arXiv2003

The sub-millimeter properties of broad absorption line quasars

We have carried out the first systematic survey of the sub-millimeter properties of broad absorption line (BAL) quasars. 30 BAL quasars drawn from a homogeneously selected sample from the Sloan Digital Sky Survey at redshifts 2 2 sigma significance, four of which are at > 3 sigma significance. The far-infrared luminosities of these quasars are > 10^{13} L_solar. There is no correlation of sub-millimeter flux with either the strength of the broad absorption feature or with absolute magnitude in our sample. We compare the sub-millimeter flux distribution of the BAL quasar sample with that of a sample of quasars which do not show BAL features in their optical spectra and find that the two are indistinguishable. BAL quasars do not have higher sub-millimeter luminosities than non-BAL quasars. These findings are consistent with the hypothesis that all quasars would contain a BAL if viewed along a certain line-of-sight. The data are inconsistent with a model in which the BAL phenomenon indicates a special evolutionary stage which co-incides with a large dust mass in the host galaxy and a high sub-millimeter luminosity. Our work provides constraints on alternative evolutionary explanations of BAL quasars.

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arXiv2003

Two 100 Mpc-scale structures in the 3-D distribution of radio galaxies and their implications

We present unequivocal evidence for a huge (~80 x 100 x 100 Mpc^3) super-structure at redshift z=0.27 in the 3-D distribution of radio galaxies from the TONS08 sample, confirming tentative evidence for such a structure from the 7C redshift survey (7CRS). A second, newly discovered super-structure is also less securely found at redshift 0.35 (of dimensions ~100 x 100 x 100 Mpc^3). We present full observational details on the TONS08 sample which was constructed to probe structures in the redshift range 0 < z < 0.5 by matching NVSS sources with objects in APM catalogues to obtain a sample of optically bright (E ~ R < 19.83), radio faint (1.4-GHz flux density S_1.4 > 3 mJy) radio galaxies in the same 25 deg^2 area as part-II of the 7CRS. Out of the total sample size of 84 radio galaxies, at least 25 are associated with the two ~100 Mpc-scale super-structures. We use quasi-linear structure formation theory to estimate the number of such structures expected in the TONS08 volume if the canonical value for radio galaxy bias is assumed. Under this assumption, the structures represent ~ 4-5 sigma peaks in the primordial density field and their expected number is low (~10^-2 - 10^-4). Fortunately, there are several plausible explanations (many of which are testable) for these low probabilities in the form of potential mechanisms for boosting the bias on large scales. These include: the association of radio galaxies with highly biased rich clusters in super-structures, enhanced triggering by group/group mergers, and enhanced triggering and/or redshift space distortion in collapsing systems as the growth of super-structures moves into the non-linear regime (abridged).

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arXiv2003

High-redshift radio galaxies: at the crossroads

The next generation of surveys of extragalactic radio sources will be dominated by different types of objects than the jetted-AGN that dominate surveys like 3C, 6C and 7C. Before radio astronomy becomes concentrated on the new types of object, it is vital that we understand the cosmological importance of the jetted-AGN that have been studied for many years. I argue that, as observational manifestations of Eddington-tuned outflow events, these objects may have more significance for galaxy/cluster formation and evolution than is typically appreciated. Outstanding problems in galaxy formation may be solved by cosmological simulations in which this type of outflow, as well as other types associated with weaker jets, are properly taken into account. I will highlight areas of ignorance which are currently hindering attempts to do this.

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arXiv2003

Using radio galaxies to find super-structures

Radio galaxies are excellent at tracing large-scale structure due to their high bias. We present new results from the TONS08 radio galaxy redshift survey. We find unequivocal evidence for a huge (at least 80 x 80 x 100 Mpc^3) super-structure at redshift z=0.27, confirming tentative evidence for such a structure from the 7C redshift survey (7CRS). A second, newly discovered super-structure is also tentatively found at redshift 0.35 (of dimensions at least 100 x 100 x 100 Mpc^3). Out of the total sample size of 84 radio galaxies, at least 25 are associated with the two super-structures. We use quasi-linear structure formation theory to estimate the number of such structures expected in the TONS08 volume if the canonical value for radio galaxy bias is assumed. Under this assumption, the structures represent ~ 4-5 sigma peaks in the primordial density field and their expected number is low (10^{-2}-10^{-4}). Fortunately, there are several plausible explanations (many of which are testable) for these low probabilities in the form of potential mechanisms for boosting the bias on large scales. These include: the association of radio galaxies with highly biased rich clusters in super-structures, enhanced triggering by group/group mergers, and enhanced triggering and/or redshift space distortion in collapsing systems as the growth of super-structures moves into the non-linear regime. Similar structures could have been missed in previous surveys because of the effects of Poisson-sampling fluctuations.

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arXiv2003

The TOOT Survey: status and early results

The TexOx-1000 (TOOT) radio source redshift survey is designed to find and study typical radio-loud active galaxies to high redshift. They are typical in the same sense that L* galaxies are typical of galaxies in the optical. Previous surveys have only included the most luminous, rare objects at and beyond the peak of activity at z~2, but in going a factor of 100 fainter than the 3C survey, and in assembling a large sample, TOOT probes for the first time the objects that dominate the radio luminosity density of the universe at high redshift. Here we describe the current status of the TOOT survey and draw preliminary conclusions about the redshift distribution of the radio sources. So far, ~520 of the 1000 radio sources have redshifts, with ~440 of those in well-defined, complete, sub-regions of the survey. For these we find a median redshift of z=1, but the measured redshift distribution has a deficit of objects with z~2, when compared to predictions based on extrapolating luminosity functions constrained by higher-flux-density samples. These are the more luminous objects that usually show emission lines, and which should not be missed in the survey unless they are heavily reddened. The deficit may be real, but it would not be too surprising to find a population of faint, reddened radio sources at z~2-3 among the TOOT sources yet to have accurate redshifts.

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arXiv2003

High-redshift clusters from NVSS: The TexOx Cluster (TOC) Survey

The TexOx Cluster (TOC) Survey uses overdensities of radiosources in the NVSS to trace clusters of galaxies. The links between radiosources and rich environments make this a powerful way to find clusters which may potentially be overlooked by other selection techniques. By including constraints from optical surveys, TOC is an extremely efficient way to find clusters at high redshift. One such field, TOC J0233.3+3021, contains at least one galaxy cluster (at z ~ 1.4) and has been detected using the Sunyaev-Zel'dovich (SZ) effect. Even in targetted deep optical observations, however, distinguishing the cluster galaxies from the background is difficult, especially given the tendency of TOC to select fields containing multiple structures at different redshifts.

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arXiv2002

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_151 > 0.5 Jy in 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 which 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 3L*. 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 1L* from z~2 to z~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 3L* includes subtle but significant differences between the host galaxies of extreme- and moderate-power radio sources at fixed redshift, and between those of high- and low-redshift radio sources at fixed radio luminosity.

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arXiv2002

Spectral ageing: a new age perspective

We present an up-to-date critique of the physical basis for the spectral ageing method. We find that the number of cases where this method may be meaningfully applied to deduce the ages of classical double radio sources is small indeed. This critique is much more than merely a re-expression of anxieties about the calibration of spectral ageing (which have been articulated by others in the past).

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arXiv2002

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.

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arXiv2002

The radio galaxy 3C 356 and clues to the trigger mechanisms for powerful radio sources

We present deep near-infrared images, taken with Subaru Telescope, of the region around the z=1.08 radio source 3C 356 which show it to be associated with a poor cluster of galaxies. We discuss evidence that this cluster comprises two subclusters traced by the two galaxies previously proposed as identifications for 3C 356, which both seem to harbour AGN, and which have the disturbed morphologies expected if they underwent an interpenetrating collision at the time the radio jets were triggered. We explain the high luminosity and temperature of the diffuse X-ray emission from this system as the result of shock-heating of intracluster gas by the merger of two galaxy groups. Taken together with the results on other well-studied powerful radio sources, we suggest that the key ingredient for triggering a powerful radio source, at least at epochs corresponding to z~1, is a galaxy--galaxy interaction which can be orchestrated by the merger of their parent subclusters. This provides an explanation for the rapid decline in the number density of powerful radio sources since z~1. We argue that attempts to use distant radio-selected clusters to trace the formation and evolution of the general cluster population must address ways in which X-ray properties can be influenced by the radio source, both directly, by mechanisms such as inverse-Compton scattering, and indirectly, by the fact that the radio source may be preferentially triggered at a specific time during the formation of the cluster.

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arXiv2002

A submillimetre difference between radio galaxies and radio quasars: evidence for quasar-heated dust and synchronized submillimetre and radio source activity

We present submillimetre photometry of 11 3CR and 6CE radio quasars of similar 151-MHz radio luminosity and redshifts to the radio galaxies studied at 1.3 ~2-times brighter accounting for possible synchrotron contamination. This difference is in quantitative agreement with a receding-torus unified scheme for radio sources in which the torus opening angle depends on quasar optical luminosity, provided that there is a close relationship between the optical and submillimetre luminosities. The implication is that quasar-heated dust dominates the submillimetre emission for powerful radio quasars at z~1.5. We find a significant anti-correlation between submillimetre/far-infrared luminosity L_FIR and radio source age in the sense that hyperluminous quasars tend to be associated with young (<10^7 yr) sources. This suggests that the processes controlling L_FIR are synchronized with the jet-triggering event and that L_FIR declines on a ~10^7 yr timescale. There is evidence for a small amount of obscuration in the hyperluminous quasars from reddening and associated or broad absorption lines. We speculate that shortly after jet triggering, dust is more widely distributed than at later times and is heated by the quasar nucleus and/or a short-lived synchronized starburst.

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arXiv2001

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.

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arXiv2001

The optically-powerful quasar E1821+643 is associated with a 300-kpc scale FRI radio structure

We present a deep image of the optically-powerful quasar E1821+643 at 18cm made with the Very Large Array (VLA). This image reveals radio emission, over 280 kpc in extent, elongated way beyond the quasar's host galaxy. Its radio structure has decreasing surface brightness with increasing distance from the bright core, characteristic of FRI sources (Fanaroff & Riley 1974). Its radio luminosity at 5GHz falls in the classification for `radio-quiet' quasars (it is only 10^23.9 W/Hz/sr; see e.g. Kellermann et al 1994). Its radio luminosity at 151MHz (which is 10^25.3 W/Hz/sr) is at the transition luminosity observed to separate FRIs and FRIIs. Hitherto, no optically-powerful quasar had been found to have a conventional FRI radio structure. For searches at low-frequency this is unsurprising given current sensitivity and plausible radio spectral indices for radio-quiet quasars. We demonstrate the inevitability of the extent of any FRqI radio structures being seriously under-estimated by existing targetted follow-up observations of other optically-selected quasars, which are typically short exposures of z > 0.3 objects, and discuss the implications for the purported radio bimodality in quasars. The nature of the inner arcsec-scale jet in E1821+643, together with its large-scale radio structure, suggest that the jet-axis in this quasar is precessing (cf. Galactic jet sources such as SS433). A possible explanation for this is that its central engine is a binary whose black holes have yet to coalesce. The ubiquity of precession in `radio-quiet' quasars, perhaps as a means of reducing the observable radio luminosity expected in highly-accreting systems, remains to be established.

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arXiv2001

The TexOx Survey of Radio-Selected Galaxy Clusters

We present some initial results from the TexOx (Texas-Oxford) Cluster (TOC) survey - a new method of selecting distant galaxy clusters. The cosmic evolution of the radio source population suggests that some massive clusters at high redshift will contain several radio-loud AGN. We searched for extreme over-densities at ~mJy levels in 7' x 7' boxes within the NVSS radio catalogue, covering a large (~1100 square degree) sky area. We have acquired optical images for ~130 cluster candidates, and followed up a subset of these with the VLA, and with Calar Alto near-IR imaging. Ryle Telescope observations have yielded at least one Sunyaev-Zel'dovich (SZ) detection of a massive z~1 system. Spectroscopic follow-up with 8-m class telescopes is in progress.

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