arXiv subjects

M. J. Hardcastle

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

At least 289 records · Page 16Linked to original sources

arXiv2003

The X-ray jet and central structure of the active galaxy NGC 315

We report the Chandra detection of resolved X-ray emission of luminosity 3.5 \times 10^{40} ergs s^{-1} (0.4--4.5 keV) and power-law energy spectral index alpha = 1.5 \pm 0.7 from a roughly 10 arcsec length of the north-west radio jet in NGC 315. The X-ray emission is brightest at the base of the radio-bright region about 3 arcsec from the nucleus, and is consistent with a synchrotron origin. At a projected distance of 10 arcsec from the core, the jet is in approximate pressure balance with an external medium which is also detected through its X-ray emission and which has kT \approx 0.6 \pm 0.1 keV, consistent with earlier ROSAT results. The high spatial resolution and sensitivity of Chandra separates nuclear unresolved emission from the extended thermal emission of the galaxy atmosphere with higher precision than possible with previous telescopes. We measure an X-ray luminosity of 5.3 \times 10^{41} ergs s^{-1} (0.4--4.5 keV) and a power-law energy index of alpha = 0.4 \pm 0.4 for the nuclear component.

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arXiv2003

The Properties of Galaxies at z>5

In a recent paper Lehnert & Bremer have photometrically selected a sample of galaxies at z>4.8 from a single VLT/FORS2 pointing and spectroscopically confirmed half of them to be at 4.8 28, i(AB)<26.3, i(AB)-z(AB)>0) from the HST ACS images of the Chandra Deep Field South. This selection results in a sample of 44 sources from ~150 sq. arcmin. We find that such galaxies are often barely resolved in the ACS images, having half-light radii of 0.1-0.3 arcsec (<2 kpc). They show no difference in spatial clustering from sources selected by i(AB)<26.3, i(AB)-z(AB)>0, which are generally galaxies of lower redshift. However, their distribution over the field is not uniform and their surface density varies considerably over areas comparable to a single 8m or HST pointing. The reliable determination of the surface and volume densities of such galaxies requires a sky area considerably larger than the current ACS imaging of this field. No individual z>5 candidate was detected to a 3-sigma limit of 6 x 10^-17 erg s^-1 cm^-2 at 0.5-5 keV by Chandra (a limiting luminosity of below 2 x 10^43 erg s^-1 at z~5.3). By summing over all positions, we find that the mean source must be undetected at a level at least a factor 4 times fainter than this. This rules out anything other than a weak AGN contribution to the emission from these objects and thus luminous AGN made little contribution to the final stages of re-ionization of the Universe.

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arXiv2003

Radio and X-ray observations of the jet in Centaurus A

We present new, high dynamic range VLA images of the inner jet of the closest radio galaxy, Centaurus A. Over a ten-year baseline we detect apparent sub-luminal motions (v ~ 0.5c) in the jet on scales of hundreds of pc. The inferred speeds are larger than those previously determined using VLBI on smaller scales, and provide new constraints on the angle made by the jet to the line of sight if we assume jet-counterjet symmetry. The new images also allow us to detect faint radio counterparts to a number of previously unidentified X-ray knots in the inner part of the jet and counterjet, showing conclusively that these X-ray features are genuinely associated with the outflow. However, we find that the knots with the highest X-ray to radio flux density ratios do not have detectable proper motions, suggesting that they may be related to standing shocks in the jet; we consider some possible internal obstacles that the jet may encounter. Using new, high-resolution Chandra data, we discuss the radio to X-ray spectra of the jet and the discrete features that it contains, and argue that the compact radio and X-ray knots are privileged sites for the in situ particle acceleration that must be taking place throughout the jet. We show that the offsets observed between the peaks of the radio and X-ray emission at several places in the Cen A jet are not compatible with the simplest possible models involving particle acceleration and downstream advection together with synchrotron and expansion losses.

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arXiv2003

X-ray Emission from the Hot ISM and SW Radio Lobe of the Nearby Radio Galaxy Centaurus A

We present results from two {\em Chandra}/ACIS-I observations and one {\em XMM-Newton} observation of X-ray emission from the ISM and the inner radio lobes of the nearby radio galaxy Centaurus A. The ISM has an average radial surface brightness profile that is well described by a $β$-model profile with index $β$=0.40$\pm$0.04 and a temperature of $k_BT_{ISM}\sim$0.29 keV beyond 2 kpc from the nucleus. We find that diffuse X-ray emission is coincident with the outer half of the southwest radio lobe, and a bright X-ray enhancement is detected along the edge of the lobe. On the basis of energetic and lifetime arguments, we reject a nonthermal explanation for this emission. We model this emission as a thin, hot shell or cap of X-ray emitting plasma surrounding the radio lobe that was created by the supersonic inflation of the lobe. This plasma shell is both hotter than ($k_BT_{SH}\sim$2.9 keV) and greatly overpressurized relative to the ambient ISM indicating supersonic expansion. We estimate that the lobe is expanding into the ISM at approximately Mach 8.5 or 2400 km s$^{-1}$. We are not directly observing the bow shock, but rather the cooler, denser material that is accumulating ahead of the contact discontinuity. The thermal energy in the shell is a significant fraction of the thermal energy of the hot ISM, demonstrating the possibility that the hot ISM of early galaxies can be re-energized by outflows from nuclear activity. Interestingly, no similarly bright X-ray emission is detected in or along the edge of the NE lobe, implying that there are differences in the dynamics and evolution of the kpc-scale radio components.

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arXiv2002

Probing the extended emission-line region in 3C171 with high-frequency radio polarimetry

I present high-frequency polarimetric radio observations of the radio galaxy 3C171, in which depolarization is associated with an extended emission-line region. The radio hotspots, known to be depolarized at low radio frequencies, become significantly polarized above (observer's-frame) frequencies of 15 GHz. There is some evidence that some of the Faraday rotation structure associated with the emission-line regions is being resolved at the highest resolutions (0.1 arcsec, or 400 pc); however, the majority remains unresolved. Using the new radio data and a simple model for the nature of the depolarizing screen, it is possible to place some constraints on the nature of the medium responsible for the depolarization. I argue that it is most likely that the depolarization is due not to the emission-line material itself but to a second, less dense, hot phase of the shocked ISM, and derive some limits on its density and temperature.

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arXiv2002

Unifying B2 radio galaxies with BL Lac objects

In an earlier paper we presented nuclear X-ray flux densities, measured with ROSAT, for the B2 bright sample of nearby low-luminosity radio galaxies. In this paper we construct a nuclear X-ray luminosity function for the B2 radio galaxies, and discuss the consequences of our results for models in which such radio galaxies are the parent population of BL Lac objects. Based on our observations of the B2 sample, we use Monte Carlo techniques to simulate samples of beamed radio galaxies, and use the selection criteria of existing samples of BL Lac objects to compare our simulated results to what is observed. We find that previous analytical results are not applicable since the BL Lac samples are selected on beamed flux density. A simple model in which BL Lacs are the moderately beamed (gamma ~ 3) counterparts of radio galaxies, with some random dispersion (~ 0.4 decades) in the intrinsic radio-X-ray relationship, can reproduce many of the features of the radio-selected and X-ray-selected BL Lac samples, including their radio and X-ray luminosity functions and the distributions of their radio-to-X-ray spectral indices. In contrast, models in which the X-ray and radio emission have systematically different beaming parameters cannot reproduce important features of the radio-galaxy and BL Lac populations. The redshift distribution and number counts of the X-ray-selected EMSS sample are well reproduced by our best models, supporting a picture in which these objects are beamed FRI radio galaxies with intrinsic luminosities similar to those of the B2 sample. However, we cannot match the redshift distribution of the radio-selected 1-Jy sample, and it is likely that a population of FRII radio galaxies is responsible for the high-redshift objects in this sample. [abridged]

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arXiv2002

Magnetic field strengths in the hotspots and lobes of three powerful FRII radio sources

We have made deep Chandra observations of three powerful FRII radio sources: two quasars (3C263 and 3C351) and one radio galaxy (3C330). X-ray emission from hotspots and lobes, as well as from the active nucleus, is detected in each source. We model the hotspots' synchrotron spectra using VLA, BIMA and HST data. In 3C263 and 3C330, the hotspots' X-ray emission is at a level consistent with being synchrotron self-Compton (SSC) emission, with a hotspot magnetic field close to the equipartition value. In the two hotspots of 3C351, however, an SSC origin for the X-rays would require the magnetic field strength to be an order of magnitude below the equipartition value in our models: in addition, there are offsets between the radio, optical and X-ray emission from the secondary hotspot which are hard to explain in a simple SSC model. We discuss the emission mechanisms that may be responsible for these observations. On our preferred model, the X-ray emission from the radio lobes of the three sources is due to inverse-Compton scattering of the microwave background radiation. If this is the case, the magnetic field strengths in the lobes are typically about a factor 2 below the equipartition values, assuming uniform lobe electron and magnetic field distributions. We detect extended X-ray emission, which we attribute to a cluster/group environment, around 3C263 and 3C330. This detection allows us to show that the lobes are close to pressure balance with their surroundings, as long as no non-radiating particles contribute to the internal pressure of the lobes.

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arXiv2002

X-ray Detection of the Inner Jet in the Radio Galaxy M84

During the course of an investigation of the interaction of the radio galaxy M84 and its ambient cluster gas, we found excess X-ray emission aligned with the northern radio jet. The emission extends from the X-ray core of the host galaxy as a weak bridge and then brightens to a local peak coincident with the first detectable radio knot at approx. 2.5" from the core. The second radio knot at 3.3" is brighter in both radio and X-rays. The X-ray jet terminates 3.9" from the core. Although all the evidence suggests that Doppler favoritism augments the emission of the northern jet, it is unlikely that the excess X-ray emission is produced by inverse Compton emission. We find many similarities between the M84 X-ray jet and recent jet detections from Chandra data of low luminosity radio galaxies. For most of these current detections synchrotron emission is the favored explanation for the observed X-rays.

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arXiv2002

The X-ray jet and halo of PKS 0521-365

Chandra ACIS observations of PKS 0521-365 find that the X-ray emission of this BL Lac object consists of emission from an unresolved core, a diffuse halo, and a 2 arcsec jet feature coincident with the inner radio/optical jet. A comparison with a new ATCA 8.6-GHz map also finds X-ray emission from the bright hotspot south-east of the nucleus. The jet spectrum, from radio to X-ray, is probably synchrotron emission from an electron population with a broken power-law energy distribution, and resembles the spectra seen from the jets of low-power (FR I) radio galaxies. The hotspot X-ray flux is consistent with the expectations of synchrotron self-Compton emission from a plasma close to equipartition, as seen in studies of high-power (FR II) radio galaxies. While the angular structure of the halo is similar to that found by an analysis of the ROSAT HRI image, its brightness is seen to be lower with Chandra, and the halo is best interpreted as thermal emission from an atmosphere of similar luminosity to the halos around FR I radio galaxies. The X-ray properties of PKS 0521-365 are consistent with it being a foreshortened, beamed, radio galaxy.

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arXiv2002

A Chandra observation of the X-ray environment and jet of 3C31

We have used a deep Chandra observation of the central regions of the twin-jet FRI radio galaxy 3C31 to resolve the thermal X-ray emission in the central few kiloparsecs of the host galaxy, NGC 383, where the jets are thought to be decelerating rapidly. This allows us to make high-precision measurements of the density, temperature and pressure distributions in this region, and to show that the X-ray emitting gas in the centre of the galaxy has a cooling time of only 5 times 10^7 years. In a companion paper these measurements are used to place constraints on models of the jet dynamics. A previously unknown one-sided X-ray jet in 3C31, extending up to 8 arcsec from the nucleus, is detected and resolved. Its structure and steep X-ray spectrum are similar to those of X-ray jets known in other FRI sources, and we attribute the radiation to synchrotron emission from a high-energy population of electrons. In situ particle acceleration is required in the region of the jet where bulk deceleration is taking place. We also present X-ray spectra and luminosities of the galaxies in the Arp 331 chain of which NGC 383 is a member. The spectrum and spatial properties of the nearby bright X-ray source 1E 0104+3153 are used to argue that the soft X-ray emission is mostly due to a foreground group of galaxies rather than to the background broad absorption-line quasar.

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arXiv2001

Chandra Observations of the X-ray Jet in Centaurus A

We present high angular resolution X-ray images and spectra from two {\em Chandra} ACIS-I observations of the X-ray jet in the nearby radio galaxy Centaurus A. We find that the X-ray emission from the jet is composed of a low surface brightness diffuse component extending continuously from within at least 60 pc of the active nucleus into the NE radio lobe 4 kpc from the nucleus, along with 31 discrete compact knots, most of which are extended at the resolution of our observation. We find that there are small but significant differences between the X-ray and radio morphologies of the inner jet at the arcsecond level, making it unlikely that a single, spatially coincident population of ultrarelativistic electrons is responsible for the emission in both energy regimes. We suggest that the X-ray knots of the inner jet are indeed the sites of particle acceleration and shocks, and the X-ray and radio knot offsets are caused by a combination of particle diffusion and energy loss. These offsets may be a common feature of all jets in radio galaxies, or at least jets in FR I type galaxies, and may be fundamental to the physics of such jets. The X-ray spectra of several regions of the jet are well fit by absorbed power-law models with photon indices $\sim$2.2-2.5, although the spectrum of one bright knot located $\sim$1 kpc from the nucleus (knot B) is harder (photon index = 2.0).

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arXiv2001

Chandra detection of the radio and optical double hot spot of 3C 351

In this letter we report a Chandra X-ray detection of the double northern hot spot of the radio quasar 3C 351. The hot spot has also been observed in the optical with the Hubble Space Telescope (R-band) and with the 3.5m. Telescopio Nazionale Galileo (B-band). The radio-to-optical and X-ray spectra are interpreted as the results of the synchrotron and synchrotron-self-Compton (SSC) mechanisms, respectively, with hot-spot magnetic field strengths ~3 times smaller than the equipartition values. In the framework of shock acceleration theory, we show that the requirement for such a relatively small field strength is in agreement with the fitted synchrotron spectral models and with the sizes of the hot spots. Finally, we show that the combination of a lower magnetic field strength with the high frequencies of the synchrotron cut-off in the fitted synchrotron spectra provides strong evidence for electron acceleration in the hot spots.

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arXiv2001

Chandra finds X-ray jets are common in low-power radio galaxies

We present results for the first three low-power radio galaxies from the B2 bright sample to have been observed with Chandra. Two have kpc-scale radio jets, and in both Chandra resolves jet X-ray emission, and detects soft X-ray core emission and an X-ray emitting galaxy-scale atmosphere of luminosity a few 10^41 ergs s^-1. These are the first detections of X-ray jets in low-power radio galaxies more distant than Cen A and M 87. The cooling time of the galaxy-scale gas implies mass infall rates of order one solar mass per year. The gas pressure near the jets is comparable to the minimum pressure in the jets, implying that the X-ray emitting gas may play an important role in jet dynamics. The third B2 radio galaxy has no kpc-scale radio jet, and here only soft X-ray emission from the core is detected. The ratio of X-ray to radio flux is similar for the jets and cores, and the results favour a synchrotron origin for the emission. kpc-scale radio jets are detected in the X-ray in ~7 ks exposures with Chandra more readily than in the optical via HST snapshot surveys.

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arXiv2001

A Chandra detection of the radio hotspot of 3C123

Chandra X-ray Observatory observations of the powerful, peculiar radio galaxy 3C123 have resulted in an X-ray detection of the bright eastern hotspot, with a 1-keV flux density of ~ 5 nJy. The X-ray flux and spectrum of the hotspot are consistent with the X-rays being inverse-Compton scattering of radio synchrotron photons by the population of electrons responsible for the radio emission (`synchrotron self-Compton emission') if the magnetic fields in the hotspot are close to their equipartition values. 3C123 is thus the third radio galaxy to show X-ray emission from a hotspot which is consistent with being in equipartition. Chandra also detects emission from a moderately rich cluster surrounding 3C123, with Lx (2-10 keV) = 2 x 10^44 ergs/s and kT ~ 5 keV, and absorbed emission from the active nucleus, with an inferred intrinsic column density of 1.7 x 10^22 cm^-2 and an intrinsic 2-10 keV luminosity of 10^44 ergs/s.

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arXiv2001

Chandra observations of the X-ray jet in 3C66B

Our Chandra observation of the FRI radio galaxy 3C66B has resulted in the first detection of an X-ray counterpart to the previously known radio, infra-red and optical jet. The X-ray jet is detected up to 7 arcsec from the core and has a steep X-ray spectrum, alpha ~ 1.3 +/- 0.1. The overall X-ray flux density and spectrum of the jet are consistent with a synchrotron origin for the X-ray emission. However, the inner knot in the jet has a higher ratio of X-ray to radio emission than the others. This suggests that either two distinct emission processes are present or that differences in the acceleration mechanism are required; there may be a contribution to the inner knot's emission from an inverse-Compton process or it may be the site of an early strong shock in the jet. The peak of the brightest radio and X-ray knot is significantly closer to the nucleus in the X-ray than in the radio, which may suggest that the knots are privileged sites for high-energy particle acceleration. 3C66B's jet is similar both in overall spectral shape and in structural detail to those in more nearby sources such as M87 and Centaurus A.

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arXiv2001

An optical inverse-Compton hotspot in 3C196?

Several hotspots of FRII radio sources have previously been detected in the X-ray at a flux level consistent with the X-rays being due to inverse-Compton scattering of radio synchrotron photons (`synchrotron self-Compton'), if the magnetic fields in the hotspots are close to their equipartition values. However, the number of hotspots compact and bright enough to exhibit detectable X-ray emission is small, so it is worth searching for synchrotron self-Compton emission in the optical, in spite of the obvious observational difficulties of such an approach. In this note I report on a possible detection of an optical inverse-Compton hotspot in a deep Hubble Space Telescope observation of the distant quasar 3C196, at a level which implies a hotspot magnetic field strength close to equipartition if the electrons have a low-energy cutoff around gamma ~ 500.

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arXiv2001

Chandra measurements of 3C 220.1's X-ray core and cluster

We report results of an 18 ks exposure with the ACIS instrument on Chandra of the powerful z = 0.62 radio galaxy 3C 220.1. The X-ray emission separates into cluster gas of emission-weighted kT ~ 5 keV and 0.7-12 keV luminosity (to a radius of 45 arcsec) 5.6 x 10^44 ergs s^-1, and unresolved emission (coincident with the radio core). While the extended X-ray emission is clearly thermal in nature, a straightforward cooling-flow model, even in conjunction with a point-source component, is a poor fit to the radial profile of the X-ray emission. This is despite the fact that the measured properties of the gas suggest a massive cooling flow of ~ 130 M_\odot yr^-1, and the data show weak evidence for a temperature gradient. The central unresolved X-ray emission has a power-law spectral energy index α~ 0.7 and 0.7-12 keV luminosity 10^45 ergs s^-1, and any intrinsic absorption is relatively small. The two-point spectrum of the core emission between radio and X-ray energies has α_rx = 0.75. Since this is a flatter spectrum than seen in other sources where the X-ray emission is presumed to be radio-related, regions close to the AGN in this source may dominate the central X-ray output, as is believed to be the case for lobe-dominated quasars. Simple unification models would be challenged if this were found to be the case for a large fraction of high-power radio galaxies.

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arXiv2000

85-GHz BIMA observations of the double-hotspot radio galaxy 3C20

We present 85-GHz observations of the archetypal double-hotspot radio source 3C20 made with the BIMA millimetre array. The resolution of BIMA allows us to separate the two components of the eastern hotspot. By comparing the BIMA observations with existing VLA data, we show that the spectra of the two hotspot components are very similar, despite the clear differences in their radio structure and their wide separation. We discuss the implications for models of double hotspot formation. Weak emission from the lobes of 3C20 is detected at 85 GHz, at a level consistent with the predictions of standard spectral ageing models.

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