arXiv subjects

J. H. Croston

Publications and source records attributed to J. H. Croston.

At least 73 records · Page 4Linked to original sources

arXiv2012

X-ray observations of the merging cluster CIZA J2242.8+5301

Multiwavelength studies of radio relics at merger shocks set powerful constraints on the relics origin and formation mechanism. However, for X-ray observations, a main difficulty is represented by the low X-ray surface brightness far out in the cluster outskirts, where relics are typically found. Here, we present XMM-Newton results from a 130-ks observation of CIZA J2242.8+5301, a cluster at z=0.19 that hosts a double radio relic. We focus on the well-defined northern relic. There is a difference of ~55% between the temperature we measure behind the relic, and the temperature measured with Suzaku. We analyse the reasons for this large discrepancy, and discuss the possibility of reliably measuring the temperature beyond the northern relic.

astro-ph.CO↗

arXiv2012

The magnetized medium around the radio galaxy B2 0755+37: an interaction with the intra-group gas

We explore the magneto-ionic environment of the isolated radio galaxy B2 0755+37 using detailed imaging of the distributions of Faraday rotation and depolarization over the radio source from Very Large Array observations at 1385,1465 and 4860 MHz and new X-ray data from XMM-Newton. The Rotation Measure (RM) distribution is complex, with evidence for anisotropic fluctuations in two regions. The approaching lobe shows low and uniform RM in an unusual `stripe' along an extension of the jet axis and a linear gradient transverse to this axis over its Northern half. The leading edge of the receding lobe shows arc-like RM structures with sign reversals. Elsewhere, the RM structures are reasonably isotropic. The RM power spectra are well described by cut-off power laws with slopes ranging from 2.1 to 3.2 in different sub-regions. The corresponding magnetic-field autocorrelation lengths, where well-determined, range from 0.25 to 1.4 kpc. It is likely that the fluctuations are mostly produced by compressed gas and field around the leading edges of the lobes. We identify areas of high depolarization around the jets and inner lobes. These could be produced by dense gas immediately surrounding the radio emission containing a magnetic field which is tangled on small scales. We also identify four ways in which the well known depolarization (Faraday depth) asymmetry between jetted and counter-jetted lobes of extended radio sources can be modified by interactions with the surrounding medium.

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arXiv2011

Discovery of the correspondence between intra-cluster radio emission and a high pressure region detected through the Sunyaev-Zel'dovich effect

We analyzed new 237 MHz and 614 MHz GMRT data of the most X-ray luminous galaxy cluster, RX J1347-1145. Our radio results are compared with the MUSTANG 90 GHz Sunyaev-Zel'dovich effect map and with re-processed Chandra and XMM-Newton archival data of this cluster. We point out for the first time in an unambiguous way the correspondence between a radio excess in a diffuse intra-cluster radio source and a hot region detected through both Sunyaev-Zel'dovich effect and X-ray observations. Our result indicates that electron re-acceleration in the excess emission of the radio mini-halo at the center of RX J1347-1145 is most likely related to a shock front propagating into the intra-cluster medium.

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arXiv2011

The Suzaku View of the Disk-Jet Connection in the Low Excitation Radio Galaxy NGC 6251

We present results from an 87-ks Suzaku observation of the canonical low-excitation radio galaxy (LERG) NGC 6251. We have previously suggested that LERGs violate conventional AGN unification schemes: they may lack an obscuring torus and are likely to accrete in a radiatively inefficient manner, with almost all of the energy released by the accretion process being channeled into powerful jets. We model the 0.5-20 keV Suzaku spectrum with a single power law of photon index $Γ=1.82^{+0.04}_{-0.05}$, together with two collisionally ionized plasma models whose parameters are consistent with the known galaxy- and group-scale thermal emission. Our observations confirm that there are no signatures of obscured, accretion-related X-ray emission in NGC 6251, and we show that the luminosity of any such component must be substantially sub-Eddington in nature.

astro-ph.HE↗

arXiv2011

The dynamics and environmental impact of 3C452

We present a detailed analysis of a new XMM-Newton observation of the FRII radio galaxy 3C452 and its environment. We detect X-ray emission from the hot intragroup medium and measure its temperature as well as obtaining the surface brightness and pressure profiles. We provide evidence that 3C452 is currently heating its environment, measuring a temperature of $1.18\pm0.11$ keV for the immediate environment of the radio source compared to $0.86^{+0.13}_{-0.05}$ keV for the outer environment. We also present evidence that the outer regions of the lobes are overpressured (internal pressure of $2.6\times10^{-13}$ Pa and external pressure of $1.11\pm{0.11}\times10^{-13}$ Pa at the edge of the lobes) and therefore are driving a shock at the lobe edges (with a temperature which we constrain to be $1.7^{+0.9}_{-0.5}$ keV), while the inner regions of the lobes are underpressured and contracting. Taking into account the very large amount of energy stored in the lobes, we show that this relatively low-powered FRII radio galaxy will have an extremely significant impact on its group environment.

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arXiv2011

The Entrainment-Limited Evolution of FR II Sources: Maximum Sizes and A Possible Connection to FR Is

We construct a simple theoretical model to investigate how entrainment gradually erodes high-speed FR II jets. This process is described by embedding a mixing-layer model developed originally to describe FR I objects in a self-similar model for the lobe structure of classical FR II sources. Following the classical FR II models, we assume that the lobe is dominated by the particles injected from the central jet. The entrainment produces a boundary shear layer which acts at the interface between the dense central jet and the less denser surrounding lobe, and the associated erosion of the jet places interesting limits on the maximum size of FR II sources. The model shows that this limit depends mainly on the initial bulk velocity of the relativistic jet triggered. The bulk velocities of FR IIs suggested by our model are in good agreement with that obtained from direct pc-scale observations on ordinary radio galaxies and quasars. Finally, we discuss how FR IIs may evolve into FR Is upon reaching their maximum, entrainment-limited sizes.

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arXiv2011

Discovery of a spiral-host episodic radio-galaxy

We report the discovery of a unique radio galaxy at z=0.137, which could possibly be the second spiral-host large radio galaxy and also the second triple-double episodic radio galaxy. The host galaxy shows signs of recent star formation in the UV but is optically red and is the brightest galaxy of a possible cluster. The outer relic radio lobes of this galaxy, separated by ~1 Mpc, show evidence of spectral flattening and a high fraction of linear polarisation. We interpret that these relic lobes have experienced re-acceleration of particles and compression of the magnetic field due to shocks in the cluster outskirts. From the morphology of the relics and galaxy distribution, we argue that re-acceleration is unlikely to be due to a cluster-cluster merger and suggest the possibility of accretion shocks. The source was identified from SDSS, GALEX, NVSS and FIRST survey data but we also present follow up optical observations with the Lulin telescope and 325 MHz low frequency radio observations with the GMRT. We briefly discuss the scientific potential of this example in understanding the evolution of galaxies and clusters by accretion, mergers, star formation, and AGN feedback.

astro-ph.CO↗

arXiv2011

Modelling TeV gamma-ray emission from the kiloparsec-scale jets of Centaurus A and M87

The widespread detection of synchrotron X-ray emission from the jets of low-power, nearby radio galaxies implies the presence of electrons at and above TeV energies. In this paper we explore the possibility that the TeV gamma-rays detected from the radio galaxies Cen A and M87, which both have bright, well-studied X-ray jets, are produced at least in part by inverse-Compton scattering of various photon fields by the high-energy electrons responsible for the synchrotron X-rays on kiloparsec scales. We describe a new numerical code that we have developed to carry out inverse-Compton calculations taking account of all the relevant physics and using detailed models of the jets and the photon fields in which they are embedded, and show that existing constraints on the very high-energy (VHE) gamma-ray fluxes of these two objects already place significant constraints on the magnetic field strengths in the jet in Cen A. Finally, we discuss the prospects for constraints on radio galaxy jet physics that may be obtained from observations with the Cerenkov Telescope Array (CTA).

astro-ph.HE↗

arXiv2011

Evidence for a merger-revived radio phoenix in MaxBCG J217.95869+13.53470

We use XMM-Newton observations of the galaxy cluster MaxBCG J217.95869+13.53470 to analyze its physical properties and dynamical state. MaxBCG J217.95869+13.53470 is found at a redshift of 0.16, has a mass of ~1x10^14 Msun, and a luminosity of 7.9x10^43 erg/s. The temperature map shows the presence of hot regions towards the north and west of the brightest cluster galaxy (BCG). From the entropy distribution, regions of high entropy match the location of the hot regions; more high entropy regions are found to the west, and ~165 kpc to the southwest of the central AGN. A second X-ray bright galaxy is visible ~90 kpc to the northeast of the BCG, at a redshift of 0.162. This galaxy is likely to be the BCG of a smaller, infalling galaxy cluster. The mass of the smaller cluster is ~10 percent the mass of MaxBCG J217.95869+13.53470, yielding an impact parameter of ~30-100 kpc. We compare the results of our X-ray observations with GMRT observations of the radio source VLSS J1431.8+1331, located at the center of the cluster. Two sources are visible in the radio: a central elongated source that bends at its northern and southern ends, and a southwestern source that coincides with a region of high entropy. The radio sources are connected by a bridge of faint radio emission. We speculate that the southwestern radio source is a radio relic produced by compression of old radio plasma by a merger shock.

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arXiv2010

A large-scale shock surrounding a powerful radio galaxy

We report the Chandra detection of a large-scale shock, on scales of 200 kpc, in the cluster surrounding the powerful radio galaxy 3C 444 (PKS 2211-17). Our 20-ks Chandra observation allows us to identify a clear surface brightness drop around the outer edge of the radio galaxy, which is likely to correspond to a spheroidal shock propagating into the intracluster medium. We measure a temperature jump across the surface brightness drop of a factor ~1.7, which corresponds to a Mach number of ~1.7. This is likely to be an underestimate due to the need to average over a fairly large region when measuring the temperature of the post-shock gas. We also detect clear cavities corresponding to the positions of the radio lobes, which is only the second such detection associated with an FRII radio galaxy. We estimate that the total energy transferred to the environment is at least 8.2 x 10^60 ergs, corresponding to a jet power of >2.2 x 10^45 ergs s^-1 (assuming a timescale based on the measured shock speed). We also compare the external pressure acting on the lobes with the internal pressure under various assumptions, and conclude that a significant contribution from protons is required.

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arXiv2010

Parsec-scale Imaging of the Radio-bubble Seyfert galaxy NGC 6764

We have observed the composite active galactic nucleus (AGN)-starburst galaxy NGC 6764 with the Very Long Baseline Array at 1.6 and 4.9 GHz. These observations have detected a "core-jet" structure and a possible weak counterjet component at 1.6 GHz. The upper limits to the core and jet (1.6-4.9 GHz) spectral index are 0.6 and 0.3, respectively. Taken together with the high brightness temperature of ~10^7 K for the core region, the radio emission appears to be coming from a synchrotron jet. At a position angle of 25 degrees, the parsec-scale jet seems to be pointing closely toward the western edge of the southern kpc-scale bubble in NGC 6764. A real connection between the parsec- and sub-kpc-scale emission would not only suggest the presence of a curved jet, but also a close link between the AGN jet and the radio bubbles in NGC 6764. We demonstrate that a precessing jet model can explain the radio morphology from parsec- to sub-kpc scales, and the model best-fit parameters of jet speed and orientation are fully consistent with the observed jet-to-counterjet surface brightness ratio. The jet however appears to be disrupted on scales of hundreds of parsecs, possibly due to interaction with and entrainment of the interstellar medium gas, which subsequently leads to the formation of bubbles. The jet energetics in NGC 6764 suggest that it would take 12-21 Myr to inflate the (southern) bubble. This timescale corresponds roughly to the starburst episode that took place in NGC 6764 about 15-50 Myr ago, and could be indicative of a close connection between jet formation and the starburst activity in this galaxy.

astro-ph.CO↗

arXiv2010

Chandra Observation of 3C288 - Reheating the Cool Core of a 3 keV Cluster from a Nuclear Outburst at z = 0.246

We present results from a 42 ks Chandra/ACIS-S observation of the transitional FRI/FRII radio galaxy 3C288 at z = 0.246. We detect $\sim$3 keV gas extending to a radius of $\sim$0.5 Mpc with a 0.5-2.0 keV luminosity of 6.6 $\times$ 10$^{43}$ ergs s$^{-1}$, implying that 3C288 lies at the center of a poor cluster. We find multiple surface brightness discontinuities in the gas indicative of either a shock driven by the inflation of the radio lobes or a recent merger event. The temperature across the discontinuities is roughly constant with no signature of a cool core, thus disfavoring either the merger cold-front or sloshing scenarios. We argue therefore that the discontinuities are shocks due to the supersonic inflation of the radio lobes. If they are shocks, the energy of the outburst is $\sim$10^{60} ergs, or roughly 30% of the thermal energy of the gas within the radius of the shock, assuming that the shocks are part of a front produced by a single outburst. The cooling time of the gas is $\sim$10^8 yrs, so that the energy deposited by the nuclear outburst could have reheated and efficiently disrupted a cool core.

astro-ph.CO↗

arXiv2010

Quantifying the Importance of Ram Pressure Stripping in a Galaxy Group at 100 Mpc

We examine two members of the NGC 4065 group of galaxies: a bent-double (a.k.a. wide angle tail) radio source and an HI deficient spiral galaxy. Models of the X-ray emitting intragroup gas and the bent-double radio source, NGC 4061, are used to probe the density of intergalactic gas in this group. HI observations reveal an asymmetric, truncated distribution of neutral gas in spiral galaxy, UGC 07049, and the accompanying radio continuum emission reveals strong star formation. We examine the effectiveness of ram pressure stripping as a gas removal mechanism and find that it alone cannot account for the HI deficiency that is observed in UGC 07049 unless this galaxy has passed through the core of the group with a velocity of ~800 km/s. A combination of tidal and ram pressure stripping are necessary to produce the HI deficiency and asymmetry in this galaxy.

astro-ph.CO↗

arXiv2010

Searching for the inverse-Compton emission from bright cluster-centre radio galaxies

We use deep archival Chandra and XMM-Newton observations of three of the brightest cluster-centre radio galaxies in the sky, Cygnus A, Hercules A and Hydra A, to search for inverse-Compton emission from the population of electrons responsible for the low-frequency radio emission. Using simulated observations, we derive robust estimates for the uncertainties on the normalization of an inverse-Compton component in the presence of the variations in background thermal temperature actually seen in our target objects. Using these, together with the pressures external to the lobes, we are able to place interesting upper limits on the fraction of the energy density in the lobes of Hydra A and Her A that can be provided by a population of relativistic electrons with standard properties, assuming that the magnetic field is not dominant; these limits are consistent with the long-standing idea that the energy density in these lobes is dominated by a non-radiating particle population. In Cygnus A, we find evidence in the spectra for an additional hard component over and above the expected thermal emission, which is plausibly a detection of inverse-Compton emission; even in this case, though, some additional non-radiating particles and/or a departure from our standard assumptions on the electron spectrum are necessary to allow pressure balance at the mid-point of the lobes. As this is not the case in other Fanaroff-Riley class II radio galaxies, we suggest that the rich environment of Cygnus A may have some effect on its lobe particle content.

astro-ph.CO↗

arXiv2010

Brightest Cluster Galaxies and Core Gas Density in REXCESS Clusters

We investigate the relationship between brightest cluster galaxies (BCGs) and their host clusters using a sample of nearby galaxy clusters from the Representative XMM Cluster Structure Survey (REXCESS). The sample was imaged with the Southern Observatory for Astrophysical Research (SOAR) in R band to investigate the mass of the old stellar population. Using a metric radius of 12h^-1 kpc, we found that the BCG luminosity depends weakly on overall cluster mass as L_BCG \propto M_cl^0.18+-0.07, consistent with previous work. We found that 90% of the BCGs are located within 0.035 r_500 of the peak of the X-ray emission, including all of the cool core (CC) clusters. We also found an unexpected correlation between the BCG metric luminosity and the core gas density for non-cool core (non-CC) clusters, following a power law of n_e \propto L_BCG^2.7+-0.4 (where n_e is measured at 0.008 r_500). The correlation is not easily explained by star formation (which is weak in non-CC clusters) or overall cluster mass (which is not correlated with core gas density). The trend persists even when the BCG is not located near the peak of the X-ray emission, so proximity is not necessary. We suggest that, for non-CC clusters, this correlation implies that the same process that sets the central entropy of the cluster gas also determines the central stellar density of the BCG, and that this underlying physical process is likely to be mergers.

astro-ph.CO↗

arXiv2010

Probing Evolutionary Mechanisms in Galaxy Clusters: Neutral Atomic Hydrogen in Abell 1367

We present VLA H I imaging data for a field in the NW of the galaxy cluster Abell 1367 (z = 0.02) in an attempt to probe the effect environment has on the interstellar medium of late-type spiral galaxies. Several galaxies show pronounced tails and asymmetries, and 7 out of 10 show significant, several kpc offsets between the HI centroid and the optical. We compare our results against a sample of optically bright, late-type galaxies (spirals) across the central 1.5 Mpc of the cluster taken from the Arecibo Galaxy Environment Survey (AGES). We calculate the H I deficiency and find that the expected global trend for the H I deficiency of these spirals to increase with projected proximity to the cluster core, seen in clusters like Coma and Virgo, is not observed. We classified the spirals into four evolutionary states, with the galaxies in each state sharing a similar degree of H I deficiency and optical colour. The common characteristics of the spirals in each evolutionary state suggests they have been subject to similar environmental processes. Many of the spirals in the most common evolutionary state (moderate H I deficiency and blue colour) have an H I intensity maximum which is displaced relative to its optical counterpart. The orientation of these offsets and magnitude of their H I deficiencies together with data from other wavelengths provide observational evidence in support of varying degrees of ram pressure stripping and tidal interaction. In general, our results indicate that the H I disks of bright late-type galaxies in the central part of the cluster are subject to both gas loss and morphological disturbance as a result of their interaction with the cluster environment. This provides further observational evidence of a more complex environment in Abell 1367 as compared to Virgo and Coma.

astro-ph.CO↗

arXiv2009

Substructure of the galaxy clusters in the REXCESS sample: observed statistics and comparison to numerical simulations

We study the substructure statistics of a representative sample of galaxy clusters by means of two currently popular substructure characterisation methods, power ratios and centroid shifts. We use the 31 clusters from the REXCESS sample, compiled from the southern ROSAT All-Sky cluster survey REFLEX with a morphologically unbiased selection in X-ray luminosity and redshift, all of which have been reobserved with XMM-Newton. We investigate the uncertainties of the substructure parameters and examine the dependence of the results on projection effects, finding that the uncertainties of the parameters can be quite substantial. Thus while the quantification of the dynamical state of individual clusters with these parameters should be treated with extreme caution, these substructure measures provide powerful statistical tools to characterise trends of properties in large cluster samples. The centre shift parameter, w, is found to be more sensitive in general. For the REXCESS sample neither the occurence of substructure nor the presence of cool cores depends on cluster mass. There is a significant anti-correlation between the existence of substantial substructure and cool cores. The simulated clusters show on average larger substructure parameters than the observed clusters, a trend that is traced to the fact that cool regions are more pronounced in the simulated clusters, leading to stronger substructure measures in merging clusters and clusters with offset cores. Moreover, the frequency of cool regions is higher in the simulations than in the observations, implying that the description of the physical processes shaping cluster formation in the simulations requires further improvement.

astro-ph.CO↗

arXiv2009

A Multiwavelength Study of a Young, Z-shaped, FR I Radio Galaxy NGC 3801

We present preliminary results from a multi-wavelength study of a merger candidate, NGC3801, hosting a young FR I radio galaxy, with a Z-shaped structure. Analysing archival data from the VLA, we find two HI emission blobs on either side of the host galaxy, suggesting a 30 kpc sized rotating gas disk aligned with stellar rotation, but rotating significantly faster than the stars. Broad, faint, blue-shifted absorption wing and an HI absorption clump associated with the shocked shell around the eastern lobe are also seen, possibly due to an jet-driven outflow. While 8.0 um dust and PAH emission, from Spitzer and near and far UV emission from GALEX is seen on a large scale in an S-shape, partially coinciding with the HI emission blobs, it reveals a ~2 kpc radius ring-like, dusty, starforming structure in the nuclear region, orthogonal to the radio jet axis. Its similarities with Kinematically Decoupled Core galaxies and other evidences have been argued for a merger origin of this young, bent jet radio galaxy.

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