arXiv subjects

M. J. Hardcastle

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

At least 181 records · Page 10Linked to original sources

arXiv2014

Numerical modelling of the lobes of radio galaxies in cluster environments II: Magnetic field configuration and observability

We describe three-dimensional magnetohydrodynamical modelling of powerful radio galaxies in realistic poor cluster environments. This modelling extends our earlier work on the hydrodynamics of radio galaxies as a function of their cluster environment to consider the magnetic field configuration in the lobes and its observational consequences, using a realistic model for the magnetic field in the intracluster medium, very high density contrast in the lobes and high numerical resolution. We confirm, now with a realistic magnetic field model, that lobes have characteristic trajectories in the radio power/linear size diagram which depend strongly on their environment. We investigate the detailed evolution of polarized emission, showing that the lobes evolve from the initially ordered field configuration imposed by our boundary conditions to one in which the longitudinal field comes to dominate. We obtain simulated observations of polarization whose properties are quantitatively consistent with observations. The highly spatially intermittent magnetic field also reproduces the observation that inverse-Compton emission from lobes is much smoother than synchrotron. Our simulations allow us to study the depolarizing effect of the external medium on the lobes, and so to demonstrate that Faraday depolarization from environments of the type we consider can reproduce the integrated fractional polarization properties of large samples and the observed preferential depolarization of the receding lobe.

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arXiv2014

FRII RGs: jet dynamics and episodic behaviour

Radio galaxies are episodic in nature. In our recent work, our study of jet properties and their dynamics in different episodes of activity has revealed various hitherto unknown aspects of the extragalactic jets. We discover that the injection spectral indices are similar in the two different episodes for most of the episodic radio galaxies in our sample. We argue that in order to produce the similar injection indices in two episodes (i) the jet power in different episodes have to be similar and (ii) the Lorentz factor of the spine of the jet should be > 10. We further argue from particle acceleration physics that (iii) the jet fluid is made of electron-positron plasma and (iv) the inner jets of double-double radio galaxies are capable of forming hotspots even when propagating through the tenuous, nonthermal electron-positron plasma of the outer cocoon without any thermal matter in it. The episodic nature of the FR\,II radio galaxies we have studied appears to be unrelated to AGN feedback on the ambient medium.

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arXiv2014

The spectral structure and energetics of powerful radio sources

Determining the energy spectrum of an electron population can give key insights into the underlying physics of a radio source; however, the lack of high resolution, broad-bandwidth observations has left many ambiguities in our understanding of radio galaxies. The improved capabilities of telescopes such as the JVLA and LOFAR mean that within the bandwidth of any given observation, a detailed spectral shape can now be produced. We present recent investigations of powerful FR-II radio galaxies at GHz and MHz frequencies and show for the first time their small-scale spectral structure. We highlight problems in traditional methods of analysis and demonstrate how these issues can now be addressed. We present the latest results from low frequency studies which suggest a potential increase in the total energy content of radio galaxy lobes with possible implications for the energetics of the population as a whole.

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arXiv2014

Herschel-ATLAS: Far-infrared properties of radio-loud and radio-quiet quasars

We have constructed a sample of radio-loud and radio-quiet quasars from the Faint Im- ages Radio Sky at Twenty-one centimetres (FIRST) and the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), over the H-ATLAS Phase 1 Area (9h, 12h and 14.5h). Using a stacking analysis we find a significant correlation between the far-infrared luminosity and 1.4-GHz luminosity for radio-loud quasars. Partial correlation analysis confirms the intrinsic correlation after removing the redshift contribution while for radio-quiet quasars no partial correlation is found. Using a single-temperature grey-body model we find a general trend of lower dust temperatures in the case of radio-loud quasars comparing to radio-quiet quasars. Also, radio-loud quasars are found to have almost constant mean values of dust mass along redshift and optical luminosity bins. In addition, we find that radio-loud quasars at lower optical luminosities tend to have on average higher FIR and 250-micron luminosity with respect to radio-quiet quasars with the same optical luminosites. Even if we use a two-temperature grey-body model to describe the FIR data, the FIR luminosity excess remains at lower optical luminosities. These results suggest that powerful radio jets are associated with star formation especially at lower accretion rates.

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arXiv2014

VLBA Observations of Mrk 6: Probing the Jet-Lobe Connection

We present the results of high resolution VLBI observations at 1.6 and 4.9 GHz of the radio-loud Seyfert galaxy, Mrk 6. These observations are able to detect a compact radio core in this galaxy for the first time. The core has an inverted spectral index ($α^{1.6}_{4.9}$=+1.0$\pm$0.2) and a brightness temperature of $1\times10^8$ K. Three distinct radio components which resemble jet elements and/or hot spots, are also detected. The position angles of these elongated jet elements point, not only to a curved jet in Mrk 6, but also towards a connection between the AGN and the kpc-scale radio lobes/bubbles in this galaxy. Firmer constraints on the star formation rate provided by new Herschel observations (SFR $<0.8$ M$_\sun$ yr$^{-1}$) make the starburst-wind powered bubble scenario implausible. From plasma speeds obtained via prior Chandra X-ray observations, and ram pressure balance arguments for the ISM and radio bubbles, the north-south bubbles are expected to take $7.5\times10^6$ yr to form, and the east-west bubbles $1.4\times10^6$ yr. We suggest that the jet axis has changed at least once in Mrk 6 within the last $\approx10^7$ yr. A comparison of the nuclear radio-loudness of Mrk 6 and a small sample of Seyfert galaxies with a subset of low-luminosity FRI radio galaxies reveals a continuum in radio properties.

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arXiv2014

An X-ray survey of the 2Jy sample. I: is there an accretion mode dichotomy in radio-loud AGN?

We carry out a systematic study of the X-ray emission from the active nuclei of the 0.02<z<0.7 2Jy sample, using Chandra and XMM-Newton observations. We combine our results with those from mid-IR, optical emission line and radio observations, and add them to those of the 3CRR sources. We show that the low-excitation objects in our samples \redit{show signs} of radiatively inefficient accretion. We study the effect of the jet-related emission on the various luminosities, confirming that it is the main source of soft X-ray emission for our sources. We also find strong correlations between the accretion-related luminosities, and identify several sources whose optical classification is incompatible with their accretion properties. We derive the bolometric and jet kinetic luminosities for the samples and find a difference in the total Eddington rate between the low and high-excitation populations, with the former peaking at ~1 per cent and the latter at ~20 per cent Eddington. Our results are consistent with a simple Eddington switch when the effects of environment on radio luminosity and black hole mass calculations are considered. The apparent independence of jet kinetic power and radiative luminosity in the high-excitation population in our plots supports a model in which jet production and radiatively efficient accretion are not strongly correlated in high-excitation objects, though they have a common underlying mechanism.

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arXiv2013

The particle content of low-power radio galaxies in groups and clusters

The synchrotron-radiating particles and magnetic fields in low-power radio galaxies (including most nearby cluster-centre sources), if at equipartition, can provide only a small fraction of the total internal energy density of the radio lobes or plumes, which is now well constrained via X-ray observations of their external environments. We consider the constraints on models for the dominant energy contribution in low-power radio-galaxy lobes obtained from a detailed comparison of how the internal equipartition pressure and external pressure measured from X-ray observations evolve with distance for two radio galaxies, 3C 31 and Hydra A. We rule out relativistic-lepton dominance of the radio lobes, and conclude that models in which magnetic field or relativistic protons/ions carried up the jet dominate lobe energetics are unlikely. Finally, we argue that entrainment of material from the jet surroundings can provide the necessary pressure, and construct a simple self-consistent model of the evolution of the entrainment rate required for pressure balance along the 100-kpc scale plumes of 3C 31. Such a model requires that the entrained material is heated to temperatures substantially above that of the surrounding intra-group or cluster medium, and that the temperature of the thermal component of the jet increases with distance, though remaining sub-relativistic.

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arXiv2013

The WISE properties of complete samples of radio-loud AGN

We present an analysis of four complete samples of radio-loud AGN (3CRR, 2Jy, 6CE and 7CE) using near- and mid-IR data taken by the Wide-Field Infrared Survey Explorer (WISE). The combined sample consists of 79 quasars and 273 radio galaxies, and covers a redshift range 0.003<z<3.395. The dichotomy in the mid-IR properties of low- and high-excitation radio galaxies (LERGs - HERGs) is analysed for the first time using large complete samples. Our results demonstrate that a division in the accretion modes of LERGs and HERGs clearly stands out in the mid-IR-radio plane (L_(22 μm) = 5x10^(43) erg s^(-1)). This means that WISE data can be effectively used to diagnose accretion modes in radio-loud AGN. The mid-IR properties of all objects were analysed to test the unification between quasars and radio galaxies, consistent with earlier work and we argue that smooth torus models best reproduce the observation. Quasars are found to have higher mid-IR luminosities than radio galaxies. We also studied all the sources in the near-IR to gain insights into evolution of AGN host galaxies. A relation found between the near-IR luminosity and redshift, well-known in the near-IR, is apparent in the two near-IR WISE bands, supporting the idea that radio sources are hosted by massive elliptical galaxies that formed their stars at high redshifts and evolved passively thereafter. Evaluation of the positions of the sample objects in WISE colour-colour diagrams shows that widely used WISE colour cuts are not completely reliable in selecting AGN.

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arXiv2013

Isothermal dust models of Herschel-ATLAS galaxies

We use galaxies from the Herschel-ATLAS survey, and a suite of ancillary simulations based on an isothermal dust model, to study our ability to determine the effective dust temperature, luminosity and emissivity index of 250um selected galaxies in the local Universe (z < 0.5). As well as simple far-infrared SED fitting of individual galaxies based on chi^2 minimisation, we attempt to derive the best global isothermal properties of 13,826 galaxies with reliable optical counterparts and spectroscopic redshifts. Using our simulations, we highlight the fact that applying traditional SED fitting techniques to noisy observational data in the Herschel Space Observatory bands introduces artificial anti-correlation between derived values of dust temperature and emissivity index. This is true even for galaxies with the most robust detections in our sample, making the results hard to interpret. We apply a method to determine the best-fit global values of isothermal effective temperature and emissivity index for z < 0.5 galaxies in H-ATLAS, deriving T = 22.3 +/- 0.1K and Beta = 1.98 +/- 0.02 (or T = 23.5 +/- 0.1K and Beta = 1.82 +/- 0.02 if we attempt to correct for bias by assuming that T and Beta are independent and normally distributed). We use our technique to test for an evolving emissivity index, finding only weak evidence. The median dust luminosity of our sample is log(Ldust/Lsolar) = 10.72 +/- 0.05 which (unlike T) shows little dependence on the choice of Beta used in our analysis, including whether it is variable or fixed. We use a further suite of simulations to emphasise the importance of the H-ATLAS PACS data for deriving dust temperatures at these redshifts, even though they are less sensitive than the SPIRE data. The majority of galaxies detected by H-ATLAS are normal star-forming galaxies, though a substantial minority (~31%) fall in the Luminous Infrared Galaxy category. (Abridged)

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arXiv2013

The Dynamics of Radio Galaxies and Double-Double Radio Galaxies

Relativistic and magnetised plasma ejected by radio loud AGNs through jets form the diffuse lobes of radio galaxies. The radiating particles (electron/electron-positron) in lobes emit in radio via the synchrotron process and X-ray via inverse-Compton scattering of cosmic microwave background photons. The thermal environment around radio galaxies emits X-rays via the thermal bremsstrahlung process. By combining information from these processes we can measure physical conditions in and around the radio lobes and thus study the dynamics of radio galaxies, including double-double radio galaxies.

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arXiv2013

Rejuvenated radio galaxies J0041+3224 and J1835+6204: how long can the quiescent phase of nuclear activity last?

We present radio observations of two well-known double-double radio galaxies, J0041+3224 and J1835+6204, at frequencies ranging from 150 to 8460 MHz, using both the Giant Metrewave Radio Telescope and the Very Large Array. These observations, over a large radio frequency range, enable us to determine the spectra of the inner and outer lobes. Our detailed spectral ageing analysis of their inner and outer lobes demonstrates that the outer doubles of double-double radio galaxies are created by the previous cycle of activity, while the inner doubles are due to the present cycle of activity. The (core subtracted) spectra of the inner doubles of both sources are power laws over a large frequency range. We found that the duration of the quiescent phase of J0041+3224 is between 4 and 28 per cent of the active phase of the previous activity. The outer north-western lobe of J1835+6204 has a compact hotspot and the regions of both the outer hotspots have close to power-law (rather than curved) spectra, which indicates that the outer lobes are still fed by jet material ejected in the previous episode just before the central engine stopped powering the jet. We estimate that the duration of the quiescent phase of J1835+6204 is $\lapp$5 per cent of the duration of the active phase of the previous activity. Therefore, we conclude that the duration of the quiescent phase can be as short as a few per cent of the active phase in radio galaxies of this type.

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arXiv2013

Episodic radio galaxies J0116$-$4722 and J1158+2621: can we constrain the quiescent phase of nuclear activity?

We present multifrequency radio observations of two well known episodic FR\,II radio galaxies (J0116$-$4722 and J1158+2621) and a detailed investigation of the life-cycle of episodic radio galaxies from their spectral and radiative properties. Combining our previous work with the present results, we either constrain or place very good limits on the active and quiescent phases of a small sample of episodic FR\,II radio galaxies. The duration of the quiescent phase can be as small as the hotspot fading time of the previous episode, and as high as a few tens of Myr; however, for none of the sources in our sample is it close to the duration of the active phase of the previous episode. We also find that for many episodic radio galaxies, the nucleus is variable at radio wavelengths. For our small sample of 7 episodic radio galaxies, we find 4 to have strongly variable cores, a much larger proportion than is generally found in samples of normal FR\,II radio galaxies. Studies with larger samples will be required to establish a statistical association between core variability and episodic radio activity.

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arXiv2013

Particle acceleration and dynamics of double-double radio galaxies: theory vs. observations

In this paper we show that a small sample of radio galaxies with evidence for multiple epochs of jet activity (so-called `double-double' radio galaxies) have the same electron injection spectral index in the two activity episodes, a result which might be considered surprising given the very different lobe dynamics expected in the first and second episode. We construct models for the dynamics of radio galaxies, with an emphasis on their episodic behaviour, and show that hotspot formation and confinement of lobes for the inner double of double-double radio galaxies are possible even without any thermal matter in the outer cocoon. We argue that (i) the observed similar injection spectral indices are due to similar jet powers in the two episodes, (ii) the `spectral index--radio power' correlation of a flux limited sample of radio galaxies is the primary one, and not the `spectral index--redshift correlation', (iii) jets are made of pair plasma and not electron-proton, (iv) and the Lorentz factor of the spine of the jet should be $\gapp 10$ to explain the observations. Furthermore, we argue that the observations show that higher power radio galaxies do not have a higher jet bulk Lorentz factors, but instead simply have a higher number density of particles in the jet rest frame. A consequence of our models is that aligned double-double radio galaxies with very old ($\gapp10^8$ yr) outer doubles, or misaligned double-double radio galaxies, are statistically more likely to have dissimilar injection indices in two different episodes, as they will probably have different jet powers.

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arXiv2013

A 325-MHz GMRT survey of the Herschel-ATLAS/GAMA fields

We describe a 325-MHz survey, undertaken with the Giant Metrewave Radio Telescope (GMRT), which covers a large part of the three equatorial fields at 9, 12 and 14.5 h of right ascension from the Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS) in the area also covered by the Galaxy And Mass Assembly survey (GAMA). The full dataset, after some observed pointings were removed during the data reduction process, comprises 212 GMRT pointings covering ~90 deg^2 of sky. We have imaged and catalogued the data using a pipeline that automates the process of flagging, calibration, self-calibration and source detection for each of the survey pointings. The resulting images have resolutions of between 14 and 24 arcsec and minimum rms noise (away from bright sources) of ~1 mJy/beam, and the catalogue contains 5263 sources brighter than 5 sigma. We investigate the spectral indices of GMRT sources which are also detected at 1.4 GHz and find them to agree broadly with previously published results; there is no evidence for any flattening of the radio spectral index below S_1.4=10 mJy. This work adds to the large amount of available optical and infrared data in the H-ATLAS equatorial fields and will facilitate further study of the low-frequency radio properties of star formation and AGN activity in galaxies out to z~1.

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arXiv2013

The Hot and Energetic Universe: AGN feedback in galaxy clusters and groups

Mechanical feedback via Active Galactic Nuclei (AGN) jets in the centres of galaxy groups and clusters is a crucial ingredient in current models of galaxy formation and cluster evolution. Jet feedback is believed to regulate gas cooling and thus star formation in the most massive galaxies, but a robust physical understanding of this feedback mode is currently lacking. The large collecting area, excellent spectral resolution and high spatial resolution of Athena+ will provide the breakthrough diagnostic ability necessary to develop this understanding, via: (1) the first kinematic measurements on relevant spatial scales of the hot gas in galaxy, group and cluster haloes as it absorbs the impact of AGN jets, and (2) vastly improved ability to map thermodynamic conditions on scales well-matched to the jets, lobes and gas disturbances produced by them. Athena+ will therefore determine for the first time how jet energy is dissipated and distributed in group and cluster gas, and how a feedback loop operates in group/cluster cores to regulate gas cooling and AGN fuelling. Athena+ will also establish firmly the cumulative impact of powerful radio galaxies on the evolution of baryons from the epoch of group/cluster formation to the present day.

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arXiv2013

Synchrotron and inverse-Compton emission from radio galaxies with non-uniform magnetic field and electron distributions

I investigate the effect of non-uniform magnetic fields in the extended structures of radio galaxies on the observed synchrotron and inverse-Compton emission. On the assumption of an isotropic field, with a given power spectrum and a Gaussian distribution of the Cartesian components of the magnetic field strength, I derive a simple integral that can be used numerically to calculate the synchrotron emissivity from any electron population. In the case of power-law spectra, I show that it is possible to estimate the difference between the synchrotron emissivity from a region with such a field and that from the commonly assumed arrangement where $B$ is constant everywhere, though fully tangled, and that this difference is small, though it increases if the electron energy density scales with the field. An aged electron spectrum in such a field produces a characteristic curved synchrotron spectrum which differs significantly from the classical Jaffe-Perola spectrum, and I discuss some effects that this might have on standard spectral age fitting. Finally, I show that inverse-Compton scattering of the cosmic microwave background is only moderately affected by such a field structure, with the effects becoming more important if the electrons follow the field. Magnetic-field estimates in the literature from combined synchrotron and inverse-Compton modelling will give reasonable estimates of the mean magnetic field energy density if the field is non-uniform but isotropic.

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arXiv2013

Radio-loud AGN: is there a link between luminosity and cluster environment?

We present here the first results from the Chandra ERA (Environments of Radio-loud AGN) Large Project, characterizing the cluster environments of a sample of 26 radio-loud AGN at z ~ 0.5 that covers three decades of radio luminosity. This is the first systematic X-ray environmental study at a single epoch, and has allowed us to examine the relationship between radio luminosity and cluster environment without the problems of Malmquist bias. We have found a weak correlation between radio luminosity and host cluster X-ray luminosity, as well as tentative evidence that this correlation is driven by the subpopulation of low-excitation radio galaxies, with high-excitation radio galaxies showing no significant correlation. The considerable scatter in the environments may be indicative of complex relationships not currently included in feedback models.

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arXiv2013

{\em Herschel}-ATLAS/GAMA: The Environmental Density of Far-Infrared Bright Galaxies at $z \leq 0.5$

We compare the environmental and star formation properties of far-infrared detected and non--far-infrared detected galaxies out to $z \sim0.5$. Using optical spectroscopy and photometry from the Galaxy And Mass Assembly (GAMA) and Sloan Digital Sky Survey (SDSS), with far-infrared observations from the {\em Herschel}-ATLAS Science Demonstration Phase (SDP), we apply the technique of Voronoi Tessellations to analyse the environmental densities of individual galaxies. Applying statistical analyses to colour, $r-$band magnitude and redshift-matched samples, we show there to be a significant difference at the 3.5$σ$ level between the normalized environmental densities of these two populations. This is such that infrared emission (a tracer of star formation activity) favours underdense regions compared to those inhabited by exclusively optically observed galaxies selected to be of the same $r-$band magnitude, colour and redshift. Thus more highly star-forming galaxies are found to reside in the most underdense environments, confirming previous studies that have proposed such a correlation. However, the degeneracy between redshift and far-infrared luminosity in our flux-density limited sample means that we are unable to make a stronger statement in this respect. We then apply our method to synthetic light cones generated from semi-analytic models, finding that over the whole redshift distribution the same correlations between star-formation rate and environmental density are found.

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