Publications by Katherine Blundell


Extremely red radio galaxies

(2001) 113-118

CJ Willott, S Rawlings, KM Blundell

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 greater than or similar to 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 similar to 5 when the black hole and stellar bulge formed and another at z similar to 1 - 2 when activity is triggered by an event such as an interaction or merger.


Images of an equatorial outflow in SS 433

Astrophysical Journal 562 (2001)

KM Blundell

We have imaged the X-ray binary SS 433 with unprecedented Fourier plane coverage at 6 cm using simultaneously the Very Long Baseline Array (VLBA), MERLIN, and the Very Large Array and also at 20 cm using the VLBA. At both wavelengths we have securely detected smooth, low surface brightness emission having the appearance of a "ruff" or collar attached perpendicularly to the well-studied knotty jets in this system and extending over at least a few hundred AU. We interpret this smooth emission as a windlike outflow from the binary and discuss its implications for the present evolutionary stage of this system. © 2001. The American Astronomical Society. All rights reserved.


The radio galaxy K-z relation to z similar to 4.5

(2001) 333-338

MJ Jarvis, S Rawlings, S Eales, KM Blundell, CJ Willott

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 greater than or similar to 3.


The optically powerful quasar E1821+643 is associated with a 300 kiloparsec-scale FR I radio structure

Astrophysical Journal 562 (2001)

KM Blundell, S Rawlings

We present a deep image of the optically powerful quasar E1821+643 at 18 cm made with the Very Large Array. This image reveals radio emission, over 280 h-1 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 FR I sources. Its radio luminosity at 5 GHz falls in the classification for "radio-quiet" quasars (it is only 1023.9 W Hz-1 sr-1). Its radio luminosity at 151 MHz (which is 1025.3 W Hz-1 sr-1) is at the transition luminosity observed to separate FR I and FR II structures. Hitherto, no optically powerful quasar had been found to have a conventional FR I 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 FR I radio structures being seriously underestimated by existing targeted follow-up observations of other optically selected quasars, which are typically short exposures of z > 0.3 objects, and we discuss the implications for the purported radio bimodality in quasars. The nature of the inner arcsecond-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 SS 433). 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. © 2001. The American Astronomical Society. All rights reserved.


Images of an equatorial outflow in SS433

Astrophys.J. 562 (2001) L79-L82

K Blundell, A Mioduszewski, T Muxlow, P Podsiadlowski, M Rupen

We have imaged the X-ray binary SS433 with unprecedented Fourier-plane coverage at 6cm using simultaneously the VLBA, MERLIN, and the VLA, and also at 20cm with the VLBA. At both wavelengths we have securely detected smooth, low-surface brightness emission having the appearance of a `ruff' or collar attached perpendicularly to the well-studied knotty jets in this system, extending over at least a few hundred AU. We interpret this smooth emission as a wind-like outflow from the binary, and discuss its implications for the present evolutionary stage of this system.


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

Astrophys.J. 562 (2001) L5-L8

K Blundell, S Rawlings

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.


The spectra and energies of classical double radio lobes

Astron.J. 119 (2000) 1111-1122

K Blundell, S Rawlings

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]


The evolution of classical doubles: clues from complete samples

Proceedings of Perspectives on Radio Astronomy (2000)

K Blundell, S Rawlings, C Willott

We describe the inter-dependence of four properties of classical double radio sources - spectral index, linear size, luminosity and redshift - from an extensive study based on spectroscopically-identified complete samples. We use these relationships to discuss aspects of strategies for searching for radio galaxies at extreme redshifts, in the context of possible capabilities of the new generation of proposed radio telescopes.


The spectra and energies of classical double radio lobes

Astronomical Journal 119 (2000) 1111-1122

KM Blundell, S Rawlings

We compare two temporal properties of classical double radio sources: (1) radiative lifetimes of synchrotron-emitting particles and (2) dynamical source ages. We discuss how these can be quite discrepant from one another, rendering use of the traditional spectral aging method inappropriate: we contend that spectral ages give meaningful estimates of dynamical ages only when these ages are ≪107 yr. In juxtaposing the fleeting radiative lifetimes with source ages that are significantly longer, a refinement of the paradigm for radio source evolution is required. We move beyond the traditional bulk backflow picture and consider alternative means of the transport of high Lorentz factor (γ) particles, which are particularly relevant within the lobes of low-luminosity classical double radio sources. The changing spectra along lobes are explained, not predominantly by synchrotron aging but by gentle gradients in a magnetic field frozen into a low-γ matrix that illuminates an energy distribution of particles, N(γ), controlled largely by classical synchrotron loss in the high magnetic field of the hot spot. A model of a magnetic field whose strength decreases with increasing distance from the hot spot and in so doing becomes increasingly different from the equipartition value in the head of the lobe is substantiated by constraints from different types of inverse Compton scattered X-rays. 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 1994 findings of Rudnick et al. and Katz-Stone & Rudnick 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 characterized by a "universal spectrum.".


High-z radio galaxies and the 'youth-redshift degeneracy'

ASTR SOC P 193 (1999) 75-78

KM Blundell, S Rawlings

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.


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

ASTR SOC P 193 (1999) 90-93

MJ Jarvis, S Rawlings, CJ Willott, KM Blundell, S Eales, M Lacy

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 ca-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 cutoff' in the co-moving space densities of powerful classical double radio sources, and rule out a art-off at z less than or similar to 2.5.


Quasars from the 7C Survey - I:sample selection and radio maps

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (1999)

J Riley, S Rawlings, R McMahon, K Blundell, P Miller, M Lacy, E Waldram

We describe the selection of candidate radio-loud quasars obtained by cross-matching radio source positions from the low-frequency (151 MHz) 7C survey with optical positions from five pairs of EO POSS-I plates scanned with the Cambridge Automatic Plate-measuring Machine (APM). The sky region studied is centred at RA 10 h 28 m, Dec +41 and covers 0.057 sr. We present VLA observations of the quasar candidates, and tabulate various properties derived from the radio maps. We discuss the selection criteria of the resulting `7CQ' sample of radio-loud quasars. The 70 confirmed quasars, and some fraction of the 36 unconfirmed candidates, constitute a filtered sample with the following selection criteria: 151-MHz flux density S151 > 100 mJy; POSS-I E-plate magnitude E approx R < 20; and POSS-I colour (O - E) < 1.8; the effective area of the survey drops significantly below S151 approx 200 mJy. We argue that the colour criterion excludes few if any quasars, but note, on the basis of recent work by Willott et al. (1998b), that the E magnitude limit probably excludes more than 50 per cent of the radio-loud quasars.


Quasars from the 7C survey - I. Sample selection and radio maps

Monthly Notices of the Royal Astronomical Society 307 (1999) 293-314

JM Riley, S Rawlings, RG McMahon, KM Blundell, P Miller, M Lacy, EM Waldram

We describe the selection of candidate radio-loud quasars obtained by cross-matching radio source positions from the low-frequency (151-MHz) 7C survey with optical positions from five pairs of EO POSS-I plates scanned with the Cambridge Automatic Plate-measuring Machine (APM). The sky region studied is centred at RA 10h 28m, Dec.+41° and covers ≈0.057 sr. We present VLA observations of the quasar candidates, and tabulate various properties derived from the radio maps. We discuss the selection criteria of the resulting '7CQ' sample of radio-loud quasars. The 70 confirmed quasars, and some fraction of the 36 unconfirmed candidates, constitute a filtered sample with the following selection criteria: 151-MHz flux density S151 > 100 mJy; POSS-I E-plate magnitude E ≈ R < 20; POSS-I colour (O - E) < 1.8; the effective area of the survey drops significantly below S151 ≈ 200 mJy. We argue that the colour criterion excludes few if any quasars, but note, on the basis of recent work by Willott et al., that the E magnitude limit probably excludes more than 50 per cent of the radio-loud quasars.


The 7C Redshift Survey - understanding radio-loud quasars and radio galaxies

ASTR SOC P 162 (1999) 135-144

CJ Willott, S Rawlings, KM Blundell

Orientation-based unified schemes for radio-loud quasars and radio galaxies are discussed in the light of a new complete sample of identified radio sources - the 7C Redshift Survey. Selected at the low radio frequency of 151 MHz this sample is free of orientation biases and together with the 3CRR sample allows a direct comparison of the properties of radio galaxies and quasars. The fraction of quasars in complete samples is used to estimate the opening angle of the putative obscuring torus. Correlations between the extended radio luminosity and the optical continuum and narrow emission line luminosities are presented and discussed in terms of the physical processes occurring in radio-loud AGN.


The evolution of radio sources from complete samples

ASTROPHYS SPACE SC L 226 (1998) 179-184

KM Blundell, S Rawlings, CJ Willott, M Lacy

From new complete samples of radio sources selected at low flux limits in low radio frequency, which give significantly improved coverage of the luminosity-redshift plane, we are able to decouple dependencies of source properties on redshift, from those depending on luminosity. We describe, with particular reference to the unification of radio galaxies and quasars, trends in linear size and also discuss core properties of these objects.


A complete sample of quasars from the 7C Redshift Survey

OBSERVATIONAL COSMOLOGY 226 (1998) 209-214

CJ Willott, S Rawlings, K Blundell, M Lacy


Cosmology with redshift surveys of radio sources

ASTROPHYS SPACE SC L 226 (1998) 171-178

S Rawlings, KM Blundell, M Lacy, CJ Willott, SA Eales

We use the K - z relation for radio galaxies to illustrate why it has proved difficult to obtain definitive cosmological results from studies based entirely on catalogues of the brightest radio sources, e.g. 3C. To improve on this situation we have been undertaking redshift surveys of complete samples drawn from the fainter 6C and 7C radio catalogues. We describe these surveys, and illustrate the new studies they are allowing. We also discuss our 'filtered' 6C redshift surveys: these have led to the discovery of a radio galaxy at z = 4.4, and are sensitive to similar objects at higher redshift provided the space density of these objects, rho, is not declining too rapidly with z. There is currently no direct evidence for a sharp decline in the rho of radio galaxies for z > 4, a result only barely consistent with the observed decline of flat-spectrum radio quasars.


A multi-radio-frequency study of a RQQ

IAU SYMP (1996) 185-186

KM Blundell, M Lacy


A radio galaxy at redshift 4.41

Nature 383 (1996) 502-505

S Rawlings, M Lacy, KM Blundell, SA Eales, AJ Bunker, ST Garrington

THE most distant astronomical objects observed are quasars at redshifts of z ≃ 4.9 (ref. 1), corresponding to a time when the Universe was less than a billion years old. This leaves little time during which the quasars and their host galaxies could form. In principle, the evolutionary state of the host galaxies can be probed by determining how many stars have formed, but this task is not straightforward because light from the quasar itself overwhelms any accompanying starlight. High-redshift radio galaxies-the likely progenitors of luminous elliptical galaxies-provide better targets for such studies, as optical emissions from their active nuclei are observed to be faint. Here we report the discovery of a radio galaxy (6C0140 + 326) at z = 4.41 which shows no evidence for either a stellar continuum or an unobscured quasar nucleus. We conclude that the galaxy associated with the radio source is neither fully formed nor obviously in the process of forming stars. This implies that at least some giant elliptical galaxies are still immature at z ≃ 4.5, and that if the intense bursts of star formation thought to produce the bulk of their stellar populations occur during the radio-bright phase, these star-forming regions are obscured by dust and gas.


Radio source environments at redshifts > 0.5

IAU SYMP (1996) 321-322

M Lacy, S Rawlings, M Wold, A Bunker, KM Blundell, SA Eales, PB Lilje

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