arXiv subjects

M. J. Hardcastle

Publications and source records attributed to M. J. Hardcastle.

At least 253 records · Page 14Linked to original sources

arXiv2007

Hot and cold gas accretion and feedback in radio-loud active galaxies

We have recently shown that X-ray observations of the population of `low-excitation' radio galaxies, which includes most low-power, Fanaroff-Riley class I sources as well as some more powerful Fanaroff-Riley class II objects, are consistent with a model in which the active nuclei of these objects are not radiatively efficient at any waveband. In another recent paper Allen et al. have shown that Bondi accretion of the hot, X-ray emitting phase of the intergalactic medium (IGM) is sufficient to power the jets of several nearby, low-power radio galaxies at the centres of clusters. In this paper we combine these ideas and suggest that accretion of the hot phase of the IGM is sufficient to power all low-excitation radio sources, while high-excitation sources are powered by accretion of cold gas that is in general unrelated to the hot IGM. This model explains a number of properties of the radio-loud active galaxy population, and has important implications for the energy input of radio-loud active galactic nuclei into the hot phase of the IGM: the energy supply of powerful high-excitation sources does not have a direct connection to the hot phase.

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arXiv2007

A Radio through X-ray Study of the Hot Spots, Active Nucleus, and Environment of the Nearby FR II Radio Galaxy 3C 33

We present results from {\em Chandra}/ACIS-S, {\em Spitzer}, {\em XMM-Newton}, {\em HST}, and VLA observations of the radio hot spots, extended environment, and nucleus of the nearby ($z$=0.0597) FR II radio galaxy 3C 33. This is a relatively low-power FR II radio galaxy, and so we expect, {\it a priori}, to detect a significant X-ray synchrotron component to the emission from the hot spots. We detect X-ray emission coincident with the two knots of polarized optical emission from the southern hot spot (SHS), as well as along the northwest arm of this hot spot. We also detect X-ray emission from two compact regions of the northern hot spot (NHS), as well as diffuse emission behind the radio peak. The X-ray flux density of the region at the tip of the southern hot spot, the most compact radio feature of the southern lobe, is consistent with the synchrotron self-Compton (SSC) process. The X-ray flux densities of the other three regions of the SHS and the two compact regions of the NHS are an order of magnitude or more above the predictions from either the SSC and inverse-Compton scattering of the CMB (IC/CMB) mechanisms, thus strongly disfavoring these scenarios unless they are far from equipartition ($B$ $\sim$4-14 times smaller than the equipartition values). The X-ray flux from the diffuse region behind the NHS is consistent with the IC/CMB prediction assuming a small departure from equipartition. We conclude that the X-ray emission is synchrotron emission from multiple populations of ultrarelativistic electrons unless these regions are far from equipartition. There must therefore be unresolved substructure within each knot, similar to that which is found in Chandra observations of nearby FR I jets such as Centaurus A and 3C 66B.

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arXiv2006

AGN heating in galaxy groups: A statistical study

We present gas temperature, density, entropy and cooling time profiles for the cores of a sample of 15 galaxy groups observed with {\it Chandra}. We find that the entropy profiles follow a power-law profile down to very small fractions of $R_{500}$. Differences between the gas profiles of groups with radio loud and radio quiet BGGs are only marginally significant, and there is only a small difference in the $L_X:T_X$ relations, for the central regions we study with {\it Chandra}, between the radio-loud and radio-quiet objects in our sample, in contrast to the much larger difference found on scales of the whole group in earlier work. However, there is evidence, from splitting the sample based on the mass of the central black holes, that repeated outbursts of AGN activity may have a long term cumulative effect on the entropy profiles. We argue that, to first-order, energy injection from radio sources does not change the global structure of the gas in the cores of groups, although it can displace gas on a local level. In most systems, it appears that AGN energy injection serves primarily to counter the effects of radiative cooling, rather than being responsible for the similarity breaking between groups and clusters.

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arXiv2006

Shock heating by nearby AGN

Shock heating by radio jets is potentially an important process in a range of environments, as it will increase the entropy of the heated gas. Although this process is expected to occur in the most powerful radio-loud AGN, strong shocks have so far only been detected in nearby low-power radio galaxies. Here we discuss X-ray detections of strong shocks in nearby galaxies, including a new detection of shocked gas around both lobes of the nearby radio galaxy NGC 3801 with inferred Mach numbers of 3 - 6 and a total injected energy comparable to the thermal energy of the ISM within 11 kpc. We discuss possible links between shock heating, AGN fuelling and galaxy mergers and the role of this type of system in feedback models.

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arXiv2006

On the Environment of Powerful Radio Galaxies at z>0.5

Active galaxies are the most powerful engines in the Universe for converting gravitational energy into radiation, and radio galaxies and radio-loud quasars are highly luminous and can be detected across the Universe. The jets that characterise them need a medium to propagate into, and thus radio galaxies at high redshift point to gaseous atmospheres on scales of at least the radio source diameter, which in many cases can reach hundreds of kpc. The variation with redshift of X-ray properties of radio-selected clusters provides an important test of structure formation theories as, unlike X-ray selection, this selection is not biased towards the most luminous clusters in the Universe. We present new results from a sample of 19 luminous radio galaxies at redshifts between 0.5 and 1. The properties of the gaseous atmosphere around these sources as mapped by Chandra and XMM-Newton observations are discussed. By combining these with observations at radio frequency, we will be able to draw conclusions on cluster size, density, and pressure balance between the radio source and the environment in which it lies.

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arXiv2006

A relativistic model of the radio jets in 3C 296

We present new, deep 8.5-GHz VLA observations of the nearby, low-luminosity radio galaxy 3C 296 at resolutions from 0.25 to 5.5 arcsec. These show the intensity and polarization structures of the twin radio jets in detail. We also derive the spectral-index and rotation-measure distributions. We apply our intrinsically symmetrical, decelerating relativistic jet model to the new observations. An optimized model accurately fits the data in both total intensity and linear polarization. We infer that the jets are inclined by 58 deg to the line of sight. On-axis, they decelerate from a (poorly-constrained) initial velocity of 0.8c to approximately 0.4c around 5 kpc from the nucleus, the velocity thereafter remaining constant. The speed at the edge of the jet is low everywhere. The longitudinal profile of proper emissivity has three principal power-law sections and the emission is centre-brightened. Our observations rule out a globally-ordered, helical magnetic-field configuration. Instead, we model the field as random on small scales but anisotropic, with toroidal and longitudinal components only. The ratio of longitudinal to toroidal field falls with distance along the jet, qualitatively but not quantitatively as expected from flux freezing, so that the field is predominantly toroidal far from the nucleus. The toroidal component is relatively stronger at the edges of the jet. A simple adiabatic model fits the emissivity evolution only in the outer region after the jets have decelerated and recollimated; closer to the nucleus, it predicts far too steep an emissivity decline with distance. We also interpret the morphological differences between brightness enhancements ("arcs") in the main and counter-jets as an effect of relativistic aberration. (Abridged)

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arXiv2006

The X-ray nuclei of intermediate-redshift radio sources

We present a Chandra and XMM-Newton spectral analysis of the nuclei of the radio galaxies and radio-loud quasars from the 3CRR sample in the redshift range 0.1 < z < 0.5. In the range of radio luminosity sampled by these objects, mostly FRIIs, it has been clear for some time that a population of radio galaxies (`low-excitation radio galaxies') cannot easily participate in models that unify narrow-line radio galaxies and broad-line objects. We show that low-excitation and narrow-line radio galaxies have systematically different nuclear X-ray properties: while narrow-line radio galaxies universally show a heavily absorbed nuclear X-ray component, such a heavily absorbed component is rarely found in sources classed as low-excitation objects. Combining our data with the results of our earlier work on the z<0.1 3CRR sources, we discuss the implications of this result for unified models, for the origins of mid-infrared emission from radio sources, and for the nature of the apparent FRI/FRII dichotomy in the X-ray. The lack of direct evidence for accretion-related X-ray emission in FRII LERGs leads us to argue that there is a strong possibility that some, or most, FRII LERGs accrete in a radiatively inefficient mode. However, our results are also consistent with a model in which the accretion mode is the same for low- and high-excitation FRIIs, with the lower accretion luminosities in FRII LERGs attributed instead to more efficient radio luminosity production in those objects.

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arXiv2006

The infrared jet in Centaurus A: multiwavelength constraints on emission mechanisms and particle acceleration

We report on Spitzer and Gemini observations of the jet of Centaurus A in the infrared, which we combine with radio, ultraviolet and X-ray data. Spitzer detects jet emission from about 2 arcmin from the nucleus, becoming particularly bright after the jet flare point at ~3.4 arcmin. Where X-ray and infrared emission are seen together the broad-band data strongly support a synchrotron origin for the X-rays. The jet flare point is marked by a broad, diffuse region of X-rays which may be associated with a shock: we discuss possible physical mechanisms for this. The infrared jet persists after the flare point region although X-ray emission is absent; it is plausible that here we are seeing the effects of particle acceleration followed by downstream advection with synchrotron losses. Gemini data probe the inner regions of the jet, putting limits on the mid-infrared flux of jet knots.

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arXiv2006

The complex X-ray morphology of NGC 7618: A major group-group merger in the local Universe?

We present results from a short {\em Chandra}/ACIS-S observation of NGC 7618, the dominant central galaxy of a nearby ($z$=0.017309, d=74.1 Mpc) group. We detect a sharp surface brightness discontinuity 14.4 kpc N of the nucleus subtending an angle of 130$^\circ$ with an X-ray tail extending $\sim$ 70 kpc in the opposite direction. The temperature of the gas inside and outside the discontinuity is 0.79$\pm$0.03 and 0.81$\pm$0.07 keV, respectively. There is marginal evidence for a discontinuous change in the elemental abundance ($Z_{inner}$=0.65$\pm$0.25,$Z_{outer}$=0.17$\pm$0.21 at 90% confidence), suggesting that this may be an `abundance' front. Fitting a two-temperature model to the ASCA/GIS spectrum of the NGC 7618/UGC 12491 pair shows the presence of a second, much hotter ($T$=$\sim$2.3 keV) component. We consider several scenarios for the origin of the edge and the tail including a radio lobe/IGM interaction, non-hydrostatic `sloshing', equal-mass merger and collision, and ram-pressure stripping. In the last case, we consider the possibility that NGC 7618 is falling into UGC 12491, or that both groups are falling into a gas poor cluster potential. There are significant problems with the first two models, however, and we conclude that the discontinuity and tail are most likely the result of ram pressure stripping of the NGC 7618 group as it falls into a larger dark matter potential.

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arXiv2005

Chandra and XMM-Newton Observations of a Sample of Low-Redshift FRI and FRII Radio-Galaxy Nuclei

We present spectral results, from Chandra and XMM-Newton observations, of a sample of 22 low-redshift (z < 0.1) radio galaxies, and consider whether the core emission originates from the base of a relativistic jet, an accretion flow, or contains contributions from both. We find correlations between the unabsorbed X-ray, radio, and optical fluxes and luminosities of FRI-type radio-galaxy cores, implying a common origin in the form of a jet. On the other hand, we find that the X-ray spectra of FRII-type radio-galaxy cores is dominated by absorbed emission, with $N_{\rm H} > 10^{23}$ atoms cm$^{-2}$, that is likely to originate in an accretion flow. We discuss several models which may account for the different nuclear properties of FRI- and FRII-type cores, and also demonstrate that both heavily obscured, accretion-related, and unobscured, jet-related components may be present in all radio-galaxy nuclei. Any absorbed, accretion-related, components in FRI-type galaxies have low radiative efficiencies.

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arXiv2005

Testing the beamed inverse-Compton model for jet X-ray emission: velocity structure and deceleration?

By considering a small sample of core-dominated radio-loud quasars with X-ray jets, I show, as has been argued previously by others, that the observations require bulk jet deceleration if all of the X-ray emission is to be explained using the widely adopted beamed inverse-Compton model, and argue that jets even in these powerful objects must have velocity structure in order to reconcile their radio and X-ray properties. I then argue that the deceleration model has several serious weaknesses, and discuss the viability of alternative models for the decline in X-ray/radio ratio as a function of position. Although inverse-Compton scattering from the jets is a required process and must come to dominate at high redshifts, adopting an alternative model for the X-ray emission of some nearby, well-studied objects can greatly alleviate some of the problems posed by these observations for the beamed inverse-Compton model.

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arXiv2005

The Hot Gas Environment of the Radio Galaxy 3C 388: Quenching the Accumulation of Cool Gas in a Cluster Core by a Nuclear Outburst

We present results from a 35 ks {\em Chandra}/ACIS-I observation of the hot ICM around the FR II radio galaxy 3C 388. 3C 388 resides in a cluster environment with an ICM temperature of $\sim$3.5 keV. We detect cavities in the ICM coincident with the radio lobes. The enthalpy of these cavities is $\sim1.2\times10^{60}$ ergs. The work done on the gas by the inflation of the lobes is $\sim3\times10^{59}$ ergs, or $\sim$0.87 keV per particle out to the radius of the lobes. The radiative timescale for gas at the center of the cluster at the current temperature is a few Gyrs. The gas in the core was probably cooler and denser before the outburst, so the cooling time was considerably shorter. We are therefore likely to be witnessing the quenching of a cluster cooling flow by a radio galaxy outburst. The mechanical power of the lobes is at least 20 times larger than the radiative losses out to the cooling radius. Outbursts of similar power with a $\sim$5% duty cycle would be more than sufficient to continually reheat the cluster core over the Hubble time and prevent the cooling of any significant amount of gas. The mechanical power of the outburst is also roughly two orders of magnitude larger than either the X-ray luminosity of the active nucleus or the radio luminosity of the lobes. The equipartition pressure of the radio lobes is more than an order of magnitude lower than that of the ambient medium, indicating that the pressure of the lobe is dominated by something other than the relativistic electrons radiating at GHz frequencies.

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arXiv2005

FRII sources at z>0.5: X-ray properties of the core and extended emission

Active galaxies are the most powerful engines in the Universe for converting gravitational energy into radiation, and their study at all epochs of evolution is therefore important. Powerful radio-loud quasars and radio galaxies have the added advantage that, since their radio jets need X-ray-emitting gas as a medium in which to propagate, the sources can be used as cosmological probes to trace significant atmospheres at high redshift. The radio emission can be used as a measure of source orientation, and sensitive X-ray measurements, especially when used in combination with multi-wavelength data, can be used to derive important results on the physical structures on a range of sizes from the cores to the large-scale components. In this paper we present new results on a significant sample of powerful radio galaxies and quasars at z > 0.5, drawn from the 3CRR catalogue and selected to sample a full range of source orientation. Using high-quality observations from XMM-Newton and Chandra, we discuss the X-ray properties of the cores, jets, lobes and cluster gas, and, through the incorporation of multi-wavelength data, draw conclusions about the nature of the emission from the different components.

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arXiv2005

High redshift FRII radio galaxies: X-ray properties of the cores

We present an extensive X-ray spectral analysis of the cores of 19 FRII sources in the redshift range 0.5<z<1.0 which were selected to be matched in isotropic radio power. The sample consists of 10 radio galaxies and 9 quasars. We compare our results with the expectations from a unification model that ascribes the difference between these two types of sources to the viewing angle to the line of sight, beaming and the presence of a dust and gas torus. We find that the spectrum of all the quasars can be fitted with a single power law, and that the spectral index flattens with decreasing angle to the line of sight. We interpret this as the effect of increasingly dominant inverse Compton X-ray emission, beamed such that the jet emission outshines other core components. For up to 70 per cent of the radio galaxies we detect intrinsic absorption; their core spectra are best fitted with an unabsorbed steep power law of average spectral index $Γ=2.1$ and an absorbed power law of spectral index Gamma=1.6, which is flatter than that observed for radio-quiet quasars. We further conclude that the presence of a jet affects the spectral properties of absorbed nuclear emission in AGN. In radio galaxies, any steep-spectrum component of nuclear X-ray emission, similar to that seen in radio-quiet quasars, must be masked by a jet or by jet-related emission.

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arXiv2005

The Chandra view of extended X-ray emission from Pictor A

We discuss the extended X-ray emission seen in three archival Chandra observations, and one archival XMM-Newton observation, of the FRII radio galaxy Pictor A. The overall properties of the X-ray lobes are consistent with the conclusions of earlier workers that the extended X-ray emission is largely due to the inverse-Compton process, and the implied departure from equipartition is in the range seen by us in other sources. In detail, we show that the X-ray/radio flux ratio varies quite strongly as a function of position throughout the source, and we discuss possible implications of this observation for the spatial variation of electron energy spectra and magnetic field strength through the lobe. We show that the radio and X-ray properties of the lobe are not consistent with a simple model in which variations in the magnetic field strength alone are responsible for the observed differences between emission at different frequencies. We also discuss the origins of the extended emission seen around the eastern hotspot, arguing that it may be diffuse synchrotron radiation tracing a region of distributed particle acceleration, and the implications of a possible weak X-ray counterjet detection which, taken together with the other properties of the bright X-ray jet, leads us to suggest that the X-ray jet and possible counterjet are also produced by synchrotron emission.

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arXiv2005

The cool wake around 4C 34.16 as seen by XMM-Newton

We present XMM-Newton observations of the wake-radiogalaxy system 4C34.16, which shows a cool and dense wake trailing behind 4C34.16's host galaxy. A comparison with numerical simulations is enlightening, as they demonstrate that the wake is produced mainly by ram pressure stripping during the galactic motion though the surrounding cluster. The mass of the wake is a substantial fraction of the mass of an elliptical galaxy's X-ray halo. This observational fact supports a wake formation scenario similar to the one demonstrated numerically by Acreman et al (2003): the host galaxy of 4C34.16 has fallen into its cluster, and is currently crossing its central regions. A substantial fraction of its X-ray halo has been stripped by ram pressure, and remains behind to form the galaxy wake.

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arXiv2005

An X-ray study of magnetic field strengths and particle content in FRII radio sources

We present a Chandra and XMM-Newton study of X-ray emission from the lobes of 33 classical double radio galaxies and quasars. We report new detections of lobe-related X-ray emission in 11 sources. Together with previous detections we find that X-ray emission is detected from at least one radio lobe in ~75 percent of the sample. For all of the lobe detections, we find that the measured X-ray flux can be attributed to inverse-Compton scattering of the cosmic microwave background radiation, with magnetic field strengths in the lobes between (0.3 - 1.3) B_eq, where the value B_eq corresponds to equipartition between the electrons and magnetic field assuming a filling factor of unity. There is a strong peak in the magnetic field strength distribution at B ~ 0.7 B_eq. We find that > 70 percent of the radio lobes are either at equipartition or electron dominated by a small factor. The distribution of measured magnetic field strengths differs for narrow-line and broad-line objects, in the sense that broad-line radio galaxies and quasars appear to be further from equipartition; however, this is likely to be due to a combination of projection effects and worse systematic uncertainty in the X-ray analysis for those objects. Our results suggest that the lobes of classical double radio sources do not contain an energetically dominant proton population, because this would require the magnetic field energy density to be similar to the electron energy density rather than the overall energy density in relativistic particles.

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arXiv2005

Chandra and XMM-Newton Observations of NGC 6251

We present new X-ray observations of the nucleus, jet and extended emission of the nearby radio galaxy NGC 6251 using the Chandra/ACIS-S camera, together with a reanalysis of archival Chandra/ACIS-I and XMM-Newton/EPIC data. We find that the nuclear X-ray spectrum is well-fitted with an absorbed power-law, and that there is tentative, but not highly significant, evidence for Fe K$α$ emission. We argue that the observed nuclear X-ray emission is likely to originate in a relativistic jet, based on the double-peaked nature, and our synchrotron self-Compton modelling, of the radio-to-X-ray spectral energy distribution. However, we cannot rule out a contribution from an accretion flow. We resolve X-ray jet emission in three distinct regions, and argue in favour of a synchrotron origin for all three; inverse-Compton emission models are possible but require extreme parameters. We detect thermal emission on both galaxy and group scales, and demonstrate that hot gas can confine the jet, particularly if relativistic beaming is important. We show evidence that the radio lobe has evacuated a cavity in the X-ray-emitting gas, and suggest that the lobe is close to the plane of the sky, with the jet entering the lobe close to the surface nearest to the observer.

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