Publications by Katherine Blundell

Inverse Compton X-ray halos around high-z radio galaxies: A feedback mechanism powered by far-infrared starbursts or the CMB?

ArXiv (0)

I Smail, KM Blundell, BD Lehmer, DM Alexander

We report the detection of extended X-ray emission around two powerful high-z radio galaxies (HzRGs) at z~3.6 (4C03.24 & 4C19.71) and use these to investigate the origin of extended, Inverse Compton (IC) powered X-ray halos at high z. The halos have X-ray luminosities of Lx~3e44 erg/s and sizes of ~60kpc. Their morphologies are broadly similar to the ~60-kpc long radio lobes around these galaxies suggesting they are formed from IC scattering by relativistic electrons in the radio lobes, of either CMB or FIR photons from the dust-obscured starbursts in these galaxies. These observations double the number of z>3 HzRGs with X-ray detected IC halos. We compare the IC X-ray to radio luminosity ratios for these new detections to the two previously detected z~3.8 HzRGs. Given the similar redshifts, we would expect comparable X-ray IC luminosities if CMB mm photons are the seed field for the IC emission. Instead the two z~3.6 HzRGs, which are ~4x fainter in the FIR, also have ~4x fainter X-ray IC emission. Including a further six z>2 radio sources with IC X-ray halos from the literature, we suggest that in the more compact (lobe sizes <100-200kpc), majority of radio sources, the bulk of the IC emission may be driven by scattering of locally produced FIR photons from luminous, dust-obscured starbursts within these galaxies, rather than CMB photons. The resulting X-ray emission can ionise the gas on ~100-200-kpc scales around these systems and thus form their extended Ly-alpha emission line halos. The starburst and AGN activity in these galaxies are thus combining to produce an effective and wide-spread "feedback" process, acting on the long-term gas reservoir for the galaxy. If episodic radio activity and co-eval starbursts are common in massive, high-z galaxies, then this IC-feedback mechanism may affect the star-formation histories of massive galaxies. [Abridged]

The X-ray luminous cluster underlying the z = 1.04 quasar PKS1229-021

ArXiv (0)

HR Russell, AC Fabian, GB Taylor, JS Sanders, KM Blundell, CS Crawford, RM Johnstone, E Belsole

We present a 100 ks Chandra observation studying the extended X-ray emission around the powerful z=1.04 quasar PKS1229-021. The diffuse cluster X-ray emission can be traced out to ~15 arcsec (~120 kpc) radius and there is a drop in the calculated hardness ratio inside the central 5 arcsec consistent with the presence of a cool core. Radio observations of the quasar show a strong core and a bright, one-sided jet leading to the SW hot spot and a second hot spot visible on the counter-jet side. Although the wings of the quasar PSF provided a significant contribution to the total X-ray flux at all radii where the extended cluster emission was detected, we were able to accurately subtract off the PSF emission using ChaRT and marx simulations. The resulting steep cluster surface brightness profile for PKS1229-021 appears similar to the profile for the FRII radio galaxy 3C444, which has a similarly rapid surface brightness drop caused by a powerful shock surrounding the radio lobes (Croston et al.). Using a model surface brightness profile based on 3C444, we estimated the total cluster luminosity for PKS1229-021 to be L_X ~ 2 x 10^{44} erg/s. We discuss the difficulty of detecting cool core clusters, which host bright X-ray sources, in high redshift surveys.

When Microquasar Jets and Supernova Collide: Hydrodynamically Simulating the SS433-W50 Interaction

ArXiv (0)

PT Goodall, F Alouani-Bibi, KM Blundell

We present investigations of the interaction between the relativistic, precessing jets of the microquasar SS433 with the surrounding, expanding Supernova Remnant (SNR) shell W50, and the consequent evolution in the inhomogeneous Interstellar Medium (ISM). We model their evolution using the hydrodynamic FLASH code, which uses adaptive mesh refinement. We show that the peculiar morphology of the entire nebula can be reproduced to a good approximation, due to the combined effects of: (i) the evolution of the SNR shell from the free-expansion phase through the Sedov blast wave in an exponential density profile from the Milky Way disc, and (ii) the subsequent interaction of the relativistic, precessing jets of SS 433. Our simulations reveal: (1) Independent measurement of the Galaxy scale-height and density local to SS433 (as n_0 = 0.2 cm^{-3}, Z_d = 40 pc), with this scale-height being in excellent agreement with the work of Dehnen & Binney. (2) A new mechanism for hydrodynamic refocusing of conical jets. (3) The current jet precession characteristics do not simply extrapolate back to produce the lobes of W50 but a history of episodic jet activity having at least 3 different outbursts with different precession characteristics would be sufficient to produce the W50 nebula. A history of intermittent episodes of jet activity from SS433 is also suggested in a kinematic study of W50 detailed in a companion paper (Goodall et al, MNRAS submitted). (4) An estimate of the age of W50, and equivalently the age of SS433's black hole created during the supernova explosion, in the range of 17,000 - 21,000 years.

Multiwavelength study of Cygnus A II. X-ray inverse-Compton emission from a relic counterjet and implications for jet duty-cycles

ArXiv (0)

KC Steenbrugge, KM Blundell, P Duffy

The duty-cycle of powerful radio galaxies and quasars such as the prototype Cygnus A is poorly understood. X-ray observations of inverse-Compton scattered Cosmic Microwave Background (ICCMB) photons probe lower Lorentz-factor particles than radio observations of synchrotron emission. Comparative studies of the nearer and further lobes, separated by many 10s of kpc and thus 10s of thousands of years in light-travel time, yield additional temporal resolution in studies of the lifecycles. We have co-added all archival Chandra ACIS-I data and present a deep 200 ks image of Cygnus A. This deep image reveals the presence of X-ray emission from a counterjet i.e. a jet receding from Earth and related to a previous episode of jet activity. The non-thermal X-ray emission, we interpret as ICCMB radiation. There is an absence of any discernible X-ray emission associated with a jet flowing towards Earth. We conclude that: (1) The emission from a relic jet, indicates a previous episode of jet activity, that took place earlier than the current jet activity appearing as synchrotron radio emission. (2) The presence of X-ray emission from a relic counterjet of Cygnus A and the absence of X-ray emission associated with any relic approaching jet constrains the timescale between successive episodes of jet activity to ~10^6 years. (3) Transverse expansion of the jet causes expansion losses which shifts the energy distribution to lower energies. (4) Assuming the electrons cooled due to adiabatic expansion, the required magnetic field strength is substantially smaller than the equipartition magnetic field strength. (5) A high minimum Lorentz factor for the distribution of relativistic particles in the current jet, of a few 10^3, is ejected from the central nucleus of this active galaxy. Abridged.

The prevalence of FRI radio quasars

ArXiv (0)

I Heywood, KM Blundell, S Rawlings

We present deep, multi-VLA-configuration radio images for a set of 18 quasars, having redshifts between 0.36 and 2.5, from the 7C quasar survey. Approximately one quarter of these quasars have FRI-type twin-jet structures and the remainder are a broad range of wide angle tail, fat double, classical double, core-jet and hybrid sources. These images demonstrate that FRI quasars are prevalent in the universe, rather than non-existent as had been suggested in the literature prior to the serendipitous discovery of the first FRI quasar a few years ago, the optically powerful "radio quiet" quasar E1821+643. Some of the FRI quasars have radio luminosities exceeding the traditional FRI / FRII break luminosity, however we find no evidence for FRII quasars with luminosities significantly below the break. We consider whether the existence of such high luminosity FRI structures is due to the increasingly inhomogeneous environments in the higher redshift universe.

The inevitable youthfulness of known high-redshift radio galaxies

ArXiv (0)

KM Blundell, S Rawlings

Radio galaxies can be seen out to 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 10^7 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 helps to explain many observed trends of radio-galaxy properties with redshift [(i) the `alignment effect' of optical emission along radio-jet axes, (ii) the increased distortion in radio structure, (iii) the decrease in physical sizes, (iv) the increase in radio depolarisation, and (v) the increase in dust emission] 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 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.

The central engines of radio-quiet quasars

ArXiv (0)

KM Blundell, AJ Beasley

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 complete sample of quasars from the 7C Redshift Survey


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.

New results from complete samples of faint radio galaxies and quasars

IAU SYMP (1996) 567-568

KM Blundell, S Rawlings, SA Eales, M Lacy

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

Spectral index asymmetries in double radio sources and the unified model

Vistas in Astronomy 40 (1996) 179-184

KM Blundell

A model is presented which reconciles the conflicting implications of the Garrington-Laing effect and the Liu-Pooley effect, which had caused difficulties for the projection model and hence for unified schemes. I also show that DPM asymmetries are significantly larger for quasars than for radio galaxies, adding further weight to the suggestion that quasars are closer to the line of sight. Copyright ©1996 Elsevier Science Ltd.

Evidence for widely separated primary and secondary hotspots in 3C 171

Monthly Notices of the Royal Astronomical Society 283 (1996) 538-542

KM Blundell

A multi-radio-frequency study of the unusual structure of the radio galaxy 3C 171 is presented. A spectral index study is used to make deductions about the origins of the plumes which emanate from the hotspots. Optical images are overlaid on radio maps which give support to the inference that the plumes are diverted backflow from the hotspots. Evidence is presented which supports the interpretation of the region of enhanced brightness in the north-west plume as a quasi-hotspot, and thus that there is a widely separated primary and secondary hotspot pair in 3C 171.








(1993) 261-262