arXiv subjects

Ewan O'Sullivan

Publications and source records attributed to Ewan O'Sullivan.

At least 73 records · Page 4Linked to original sources

arXiv2013

The impact of sloshing on the intra-group medium and old radio lobe of NGC 5044

We present temperature and abundance maps of the central 125 kpc of the NGC 5044 galaxy group, based an a deep XMM-Newton observation. The abundance map reveals an asymmetrical abundance structure, with the centroid of the highest abundance gas offset ~22 kpc northwest of the galaxy centre, and moderate abundances extending almost twice as far to the southeast than in any other direction. The abundance distribution is closely correlated with two previously-identified cold fronts and an arc--shaped region of surface brightness excess, and it appears that sloshing, induced by a previous tidal encounter, has produced both the abundance and surface brightness features. Sloshing dominates the uplift of heavy elements from the group core on large scales, and we estimate that the southeast extension (the tail of the sloshing spiral) contains at least 1.2x10^5 solar masses more iron than would be expected of gas at its radius. Placing limits on the age of the encounter we find that if, as previously suggested, the disturbed spiral galaxy NGC 5054 was the perturber, it must have been moving supersonically when it transited the group core. We also examine the spectral properties of emission from the old, detached radio lobe southeast of NGC 5044, and find that they are consistent with a purely thermal origin, ruling out this structure as a significant source of spectrally hard inverse-Compton emission.

astro-ph.CO↗

arXiv2012

The baryon budget on the galaxy group/cluster boundary

We present a study of the hot gas and stellar content of 5 optically-selected poor galaxy clusters, including a full accounting of the contribution from intracluster light (ICL) and a combined hot gas and hydrostatic X-ray mass analysis with XMM observations. We find weighted mean stellar (including ICL), gas and total baryon mass fractions within r500 of 0.026+/-0.003, 0.070+/-0.005 and 0.096+/-0.006, respectively, at a corresponding weighted mean M500 of (1.08_{-0.18}^{+0.21}) x 10^14 Msun. Even when accounting for the intracluster stars, 4 out of 5 clusters show evidence for a substantial baryon deficit within r500, with baryon fractions (f_bary) between 50+/-6 to 59+/-8 per cent of the Universal mean level (i.e. Omega_b / Omega_m); the remaining cluster having f_bary = 75+/-11 per cent. For the 3 clusters where we can trace the hot halo to r500 we find no evidence for a steepening of the gas density profile in the outskirts with respect to a power law, as seen in more massive clusters. We find that in all cases, the X-ray mass measurements are larger than those originally published on the basis of the galaxy velocity dispersion (sigma) and an assumed sigma-M500 relation, by a factor of 1.7-5.7. Despite these increased masses, the stellar fractions (in the range 0.016-0.034, within r500) remain consistent with the trend with mass published by Gonzalez, Zaritsky & Zabludoff (2007), from which our sample is drawn.

astro-ph.CO↗

arXiv2012

Recurrent radio outbursts at the center of the NGC1407 galaxy group

We present deep Giant Metrewave Radio Telescope (GMRT) radio observations at 240, 330 and 610 MHz of the complex radio source at the center of the NGC1407 galaxy group. Previous GMRT observations at 240 MHz revealed faint, diffuse emission enclosing the central twin-jet radio galaxy. This has been interpreted as an indication of two possible radio outbursts occurring at different times. Both the inner double and diffuse component are detected in the new GMRT images at high levels of significance. Combining the GMRT observations with archival Very Large Array data at 1.4 and 4.9 GHz, we derive the total spectrum of both components. The inner double has a spectral index α=0.7, typical for active, extended radio galaxies, whereas the spectrum of the large-scale emission is very steep, with α=1.8 between 240 MHz and 1.4 GHz. The radiative age of the large-scale component is very long, ~300 Myr, compared to ~30 Myr estimated for the central double, confirming that the diffuse component was generated during a former cycle of activity of the central galaxy. The current activity have so far released an energy which is nearly one order of magnitude lower than that associated with the former outburst. The group X-ray emission in the Chandra and XMM-Newton images and extended radio emission show a similar swept-back morphology. We speculate that the two structures are both affected by the motion of the group core, perhaps due to the core sloshing in response to a recent encounter with the nearby elliptical galaxy NGC1400.

astro-ph.CO↗

arXiv2012

The ELIXR Galaxy Survey. II: Baryons and Dark Matter in an Isolated Elliptical Galaxy

The Elliptical Isolated X-ray (ElIXr) Galaxy Survey is a volume-limited (<110Mpc) study of optically selected, isolated, Lstar elliptical galaxies, to provide an X-ray census of galaxy-scale (virial mass, Mvir < 1e13 Msun) objects, and identify candidates for detailed hydrostatic mass modelling. In this paper, we present a Chandra and XMM study of one such candidate, NGC1521, and constrain its distribution of dark and baryonic matter. We find a morphologically relaxed hot gas halo, extending almost to R500, that is well described by hydrostatic models similar to the benchmark, baryonically closed, Milky Way-mass elliptical galaxy NGC720. We obtain good constraints on the enclosed gravitating mass (M500=3.8e12+/-1e12 Msun, slightly higher than NGC\thin 720), and baryon fraction (fb500=0.13+/-0.03). We confirm at 8.2-sigma the presence of a dark matter (DM) halo consistent with LCDM. Assuming a Navarro-Frenk-White DM profile, our self-consistent, physical model enables meaningful constraints beyond R500, revealing that most of the baryons are in the hot gas. Within the virial radius, fb is consistent with the Cosmic mean, suggesting that the predicted massive, quasi-hydrostatic gas halos may be more common than previously thought. We confirm that the DM and stars conspire to produce an approximately powerlaw total mass profile (rho \propto r^-alpha) that follows the recently discovered scaling relation between alpha and optical effective radius. Our conclusions are insensitive to modest, observationally motivated, deviations from hydrostatic equilibrium. Finally, after correcting for the enclosed gas fraction, the entropy profile is close to the self-similar prediction of gravitational structure formation simulations, as observed in massive galaxy clusters.

astro-ph.CO↗

arXiv2012

A Giant Metrewave Radio Telescope/Chandra view of IRAS 09104+4109: A type 2 QSO in a cooling flow

IRAS 09104+4109 is a rare example of a dust enshrouded type 2 QSO in the centre of a cool-core galaxy cluster. Previous observations of this z=0.44 system showed that as well as powering the hyper-luminous infrared emission of the cluster-central galaxy, the QSO is associated with a double-lobed radio source. However, the steep radio spectral index and misalignment between the jets and ionised optical emission suggested that the orientation of the QSO had recently changed. We use a combination of new, multi-band Giant Metrewave Radio Telescope observations and archival radio data to confirm that the jets are no longer powered by the QSO, and estimate their age to be 120-160 Myr. This is in agreement with the ~70-200 Myr age previously estimated for star-formation in the galaxy. Previously unpublished Very Long Baseline Array data reveal a 200 pc scale double radio source in the galaxy core which is more closely aligned with the current QSO axis and may represent a more recent period of jet activity. These results suggest that the realignment of the QSO, the cessation of jet activity, and the onset of rapid star-formation may have been caused by a gas-rich galaxy merger. A Chandra X-ray observation confirms the presence of cavities associated with the radio jets, and we estimate the energy required to inflate them to be ~7.7x10^60 erg. The mechanical power of the jets is sufficient to balance radiative cooling in the cluster, provided they are efficiently coupled to the intra-cluster medium (ICM). We find no evidence of direct radiative heating and conclude that the QSO either lacks the radiative luminosity to heat the ICM, or that it requires longer than 100-200 Myr to significantly impact its environment. [Abridged]

astro-ph.CO↗

arXiv2012

The Baryonic Halos of Elliptical Galaxies: Radial Distribution of Globular Clusters and Diffuse Hot Gas

For a sample of 9 well-studied giant ellipticals we compare the projected radial distribution of their red and blue globular cluster (GC) subpopulations with their host galaxy stellar and X-ray surface brightness profiles. We support previous findings that the surface density distribution of red (metal-rich) GCs follows that of the host galaxy starlight. We find good agreement between the outer slope of the blue GC surface density and that of the galaxy X-ray emission. This coincidence of projected radial profiles is likely due to the fact that both blue GCs and X-ray emitting hot gas share the same gravitational potential in equilibrium. When deprojected the X-ray emitting hot gas has a radial density dependence that is the square root of that for the GC density. We further show that the energy per unit mass for blue GCs is roughly half that of the hot gas.

astro-ph.CO↗

arXiv2011

Interaction between the intergalactic medium and central radio source in the NGC 4261 group of galaxies

Using observations from the Chandra and XMM-Newton X-ray observatories, we examine the interaction between the intra-group medium and central radio source in the nearby NGC 4261 galaxy group. We confirm the presence of cavities associated with the radio lobes and estimate their enthalpy to be ~2.4x10^58 erg. The mechanical power output of the jets is >=10^43 erg/s, at least a factor of 60 greater than the cooling luminosity in the region the lobes inhabit. We identify rims of compressed gas enclosing the lobes, but find no statistically significant temperature difference between them and their surroundings, suggesting that the lobe expansion velocity is approximately sonic (Mach<=1.05). The apparent pressure of the radio lobes, based on the synchrotron minimum energy density argument, is a factor of 5 lower than that of the intra-group medium. Pressure balance could be achieved if entrainment of thermal gas provided additional non-radiating particles in the lobe plasma, but the energy required to heat these particles would be ~20 per cent. of the mechanical energy output of the radio source. NGC 4261 has a relatively compact cool core, which should probably be categorised as a galactic corona. The corona is capable of fuelling the active nucleus for considerably longer than the inferred source lifetime, but can be only inefficiently heated by the AGN or conduction. The expansion of the radio lobes has affected the structure of the gas in the galaxy, compressing and moving the material of the corona without causing significant shock heating, and expelling gas from the immediate neighbourhood of the jets. We discuss the possible implications of this environment for the duration of the AGN outburst, and consider mechanisms which might lead to the cessation of nuclear activity.

astro-ph.CO↗

arXiv2011

Heating the hot atmospheres of galaxy groups and clusters with cavities: the relationship between jet power and low-frequency radio emission

We present scaling relations between jet power and radio power measured using the Giant Metrewave Radio Telescope (GMRT), Chandra and XMM-Newton, for a sample of 9 galaxy groups combined with the Birzan et al. sample of clusters. Cavity power is used as a proxy for mechanical jet power. Radio power is measured at 235 MHz and 1.4 GHz, and the integrated 10 MHz-10 GHz radio luminosity is estimated from the GMRT 610-235 MHz spectral index. The use of consistently analysed, high resolution low-frequency radio data from a single observatory makes the radio powers for the groups more reliable than those used by previous studies, and the combined sample covers 6-7 decades in radio power and 5 decades in cavity power. We find a relation of the form Pjet proportional to Lradio^~0.7 for integrated radio luminosity, with a total scatter of sigma_Lrad=0.63 and an intrinsic scatter of sigma_i,Lrad=0.59. A similar relation is found for 235 MHz power, but a slightly flatter relation with greater scatter is found for 1.4 GHz power, suggesting that low-frequency or broad band radio measurements are superior jet power indicators. We find our low-frequency relations to be in good agreement with previous observational results. Comparison with jet models shows reasonable agreement, which may be improved if radio sources have a significant low-energy electron population. We consider possible factors which could bias our results or render them more uncertain, and find that correcting for such factors in those groups we are able to study in detail leads to a flattening of the Pjet:Lradio relation.

astro-ph.CO↗

arXiv2011

A combined low-radio frequency/X-ray study of galaxy groups I. Giant Metrewave Radio Telescope observations at 235 MHz and 610 MHz

We present new Giant Metrewave Radio Telescope observations at 235 MHz and 610 MHz of 18 X-ray bright galaxy groups. These observations are part of an extended project, presented here and in future papers, which combines low-frequency radio and X-ray data to investigate the interaction between central active galactic nuclei (AGN) and the intra-group medium (IGM). The radio images show a very diverse population of group-central radio sources, varying widely in size, power, morphology and spectral index. Comparison of the radio images with Chandra and XMM-Newton X-ray images shows that groups with significant substructure in the X-ray band and marginal radio emission at >= 1 GHz host low-frequency radio structures that correlate with substructures in IGM. Radio-filled X-ray cavities, the most evident form of AGN/IGM interaction in our sample, are found in half of the systems, and are typically associated with small, low- or mid-power double radio sources. Two systems, NGC5044 and NGC4636, possess multiple cavities, which are isotropically distributed around the group center, possibly due to group weather. In other systems the radio/X-ray correlations are less evident. However, the AGN/IGM interaction can manifest itself through the effects of the high-pressure medium on the morphology, spectral properties and evolution of the radio-emitting plasma. In particular, the IGM can confine fading radio lobes in old/dying radio galaxies and prevent them from dissipating quickly. Evidence for radio emission produced by former outbursts that coexist with current activity is found in six groups of the sample.

astro-ph.CO↗

arXiv2010

AGN Driven Weather and Multiphase Gas in the Core of the NGC 5044 Galaxy Group

A deep Chandra observation of the X-ray bright group, NGC 5044, shows that the central region of this group has been strongly perturbed by repeated AGN outbursts. These recent AGN outbursts have produced many small X-ray cavities, cool filaments and cold fronts. We find a correlation between the coolest X-ray emitting gas and the morphology of the Ha filaments. The Ha filaments are oriented in the direction of the X-ray cavities, suggesting that the warm gas responsible for the Halpha emission originated near the center of NGC 5044 and was dredged up behind the buoyant, AGN-inflated X-ray cavities. A detailed spectroscopic analysis shows that the central region of NGC 5044 contains spatially varying amounts of multiphase gas. The regions with the most inhomogeneous gas temperature distribution tend to correlate with the extended 235 MHz and 610 MHz radio emission detected by the GMRT. This may result from gas entrainment within the radio emitting plasma or mixing of different temperature gas in the regions surrounding the radio emitting plasma by AGN induced turbulence. Accounting for the effects of multiphase gas, we find that the abundance of heavy elements is fairly uniform within the central 100 kpc, with abundances of 60-80% solar for all elements except oxygen, which has a significantly sub-solar abundance. In the absence of continued AGN outbursts, the gas in the center of NGC 5044 should attain a more homogeneous distribution of gas temperature through the dissipation of turbulent kinetic energy and heat conduction in approximately 10e8 yr. The presence of multiphase gas in NGC 5044 indicates that the time between recent AGN outbursts has been less than approximately 10e8 yr.

astro-ph.CO↗

arXiv2010

A deep Chandra observation of the poor cluster AWM4 - II. The role of the radio jets in enriching the intra-cluster medium

We use a Chandra observation of the poor cluster AWM4 to map the temperature and abundance of the intra-cluster medium, so as to examine the influence of the central radio galaxy on its environment. While the cluster core is generally enriched to near-solar abundances, we find evidence of super-solar abundances correlated with the radio jets, extending ~35 kpc from the core of the central dominant galaxy NGC 6051 along its minor axis. We conclude that the enriched gas has been transported out of the central galaxy through the action of the radio source. We estimate the excess mass of iron in the entrained gas to be ~1.4x10^6 Msol, and find that this can be produced in the core of NGC 6051 within the timescale of the AGN outburst. The energy required to transport this gas to its current location is ~4.5x10^57 erg, a significant fraction of the estimated total mechanical energy output of the AGN, though this estimate is dependent on the degree of enrichment of the uplifted gas. The larger near-solar abundance region is also compatible with enrichment by metals mixed outward from NGC 6051 over a much longer timescale.

astro-ph.CO↗

arXiv2010

A deep Chandra observation of the poor cluster AWM 4 - I. Properties of the central radio galaxy and its effects on the intracluster medium

Using observations from the Chandra X-ray Observatory and Giant Metrewave Radio Telescope, we examine the interaction between the intracluster medium and central radio source in the poor cluster AWM 4. In the Chandra observation a small cool core or galactic corona is resolved coincident with the radio core. This corona is capable of fuelling the active nucleus, but must be inefficiently heated by jet interactions or conduction, possibly precluding a feedback relationship between the radio source and cluster. A lack of clearly detected X-ray cavities suggests that the radio lobes are only partially filled by relativistic plasma. We estimate a filling factor of phi=0.21 (3 sigma upper limit phi<0.42) for the better constrained east lobe. We consider the particle population in the jets and lobes, and find that the standard equipartition assumptions predict pressures and ages which agree poorly with X-ray estimates. Including an electron population extending to low Lorentz factors either reduces (gamma_min=100) or removes (gamma_min=10) the pressure imbalance between the lobes and their environment. Pressure balance can also be achieved by entrainment of thermal gas, probably in the first few kiloparsecs of the radio jets. We estimate the mechanical power output of the radio galaxy, and find it to be marginally capable of balancing radiative cooling.

astro-ph.CO↗

arXiv2010

Cavities and shocks in the galaxy group HCG 62 as revealed by Chandra, XMM and GMRT data

We report on the results of an analysis of Chandra, XMM-Newton and new GMRT data of the X-ray bright compact group of galaxies HCG 62, which is one of the few groups known to possess clear, small X-ray cavities in the inner regions. This is part of an ongoing X-ray/low-frequency radio study of 18 groups, initially chosen for the availability of good-quality X-ray data and evidence for AGN/hot gas interaction. At higher frequency (1.4 GHz), the HCG 62 cavity system shows minimal if any radio emission, but the new GMRT observations at 235 MHz and 610 MHz clearly detect extended low-frequency emission from radio lobes corresponding to the cavities. By means of the synergy of X-ray and low-frequency radio observations, we compare and discuss the morphology, luminosity and pressure of the gas and of the radio source. We find that the radio source is radiatively inefficient, with a ratio of radio luminosity to mechanical cavity power of $\sim 10^{-4}$, and that the radio pressure of the lobes is about one order of magnitude lower than the X-ray pressure of the surrounding thermal gas. Thanks to the high spatial resolution of the Chandra surface brightness and temperature profiles, we also identify a shock front located at 36 kpc to the south-west of the group center, close to the southern radio lobe, with a Mach number $\sim 1.5$ and a total power which is about one order of magnitude higher than the cavity power. Such a shock may have heated the gas in the southern region, as indicated by the temperature map. The shock may also explain the arc-like region of enriched gas seen in the iron abundance map, as this may be produced by a non-Maxwellian electron distribution near its front.

astro-ph.CO↗

arXiv2009

Comparing X-ray and Dynamical Mass Profiles in the Early-Type Galaxy NGC 4636

We present the results of an X-ray mass analysis of the early-type galaxy NGC 4636, using Chandra data. We have compared the X-ray mass density profile with that derived from a dynamical analysis of the system's globular clusters (GCs). Given the observed interaction between the central active galactic nucleus and the X-ray emitting gas in NGC 4636, we would expect to see a discrepancy in the masses recovered by the two methods. Such a discrepancy exists within the central ~10kpc, which we interpret as the result of non-thermal pressure support or a local inflow. However, over the radial range ~10-30kpc, the mass profiles agree within the 1-sigma errors, indicating that even in this highly disturbed system, agreement can be sought at an acceptable level of significance over intermediate radii, with both methods also indicating the need for a dark matter halo. However, at radii larger than 30kpc, the X-ray mass exceeds the dynamical mass, by a factor of 4-5 at the largest disagreement. A Fully Bayesian Significance Test finds no statistical reason to reject our assumption of velocity isotropy, and an analysis of X-ray mass profiles in different directions from the galaxy centre suggests that local disturbances at large radius are not the cause of the discrepancy. We instead attribute the discrepancy to the paucity of GC kinematics at large radius, coupled with not knowing the overall state of the gas at the radius where we are reaching the group regime (>30kpc), or a combination of the two.

astro-ph.CO↗

arXiv2009

A statistically-selected Chandra sample of 20 galaxy clusters -- II. Gas properties and cool-core/non-cool core bimodality

We investigate the thermodynamic and chemical structure of the intracluster medium (ICM) across a statistical sample of 20 galaxy clusters analysed with the Chandra X-ray satellite. In particular, we focus on the scaling properties of the gas density, metallicity and entropy and the comparison between clusters with and without cool cores (CCs). We find marked differences between the two categories except for the gas metallicity, which declines strongly with radius for all clusters (Z ~ r^{-0.31}), outside ~0.02 r500. The scaling of gas entropy is non-self-similar and we find clear evidence of bimodality in the distribution of logarithmic slopes of the entropy profiles. With only one exception, the steeper sloped entropy profiles are found in CC clusters whereas the flatter slope population are all non-CC clusters. We explore the role of thermal conduction in stabilizing the ICM and conclude that this mechanism alone is sufficient to balance cooling in non-CC clusters. However, CC clusters appear to form a distinct population in which heating from feedback is required in addition to conduction. Under the assumption that non-CC clusters are thermally stabilized by conduction alone, we find the distribution of Spitzer conduction suppression factors, f_c, to be log-normal, with a log (base 10) mean of -1.50+/-0.03 (i.e. f_c=0.032) and log standard deviation 0.39+/-0.02.

astro-ph.CO↗

arXiv2007

The nature of the ghost cavity in the NGC 741 group

We discuss the effects of energy injection into the intra-group medium of the group of galaxies associated with NGC 741. The X-ray emission reveals a large bubble, which in the absence of a currently bright central radio source, may have been inflated by a previous cycle of nuclear activity . If the bubble is filled with a light, relativistic fluid we calculate that in expanding, it has done more than sufficient work to counteract the energy lost from the intra-group medium via radiative cooling; the bubble can provide this energy as it expands and rises. Using upper limits on the flux density of the plasma filling the bubble at 330 MHz and 1.4 GHz, we derive constraints on its electron energy distribution and magnetic field strength. We show that the data require the high-energy cut-off of the electron spectrum to be very low compared to the cut-offs seen in more typical radio sources if the fluid filling the bubble is a conventional relativistic plasma. This suggests that the fluid filling the bubble may not have evolved by expansion or synchrotron losses consistent with a dead radio source, leaving a puzzle as to what the origin of the bubble may be.

astro-ph↗

arXiv2006

A statistically-selected Chandra sample of 20 galaxy clusters -- I. Temperature and cooling time profiles

We present an analysis of 20 galaxy clusters observed with the Chandra X-ray satellite, focussing on the temperature structure of the intracluster medium and the cooling time of the gas. Our sample is drawn from a flux-limited catalogue but excludes the Fornax, Coma and Centaurus clusters, owing to their large angular size compared to the Chandra field-of-view. We describe a quantitative measure of the impact of central cooling, and find that the sample comprises 9 clusters possessing cool cores and 11 without. The properties of these two types differ markedly, but there is a high degree of uniformity amongst the cool core clusters, which obey a nearly universal radial scaling in temperature of the form T \propto r^~0.4, within the core. This uniformity persists in the gas cooling time, which varies more strongly with radius in cool core clusters (t_cool \propto r^~1.3), reaching t_cool <1Gyr in all cases, although surprisingly low central cooling times (<5Gyr) are found in many of the non-cool core systems. The scatter between the cooling time profiles of all the clusters is found to be remarkably small, implying a universal form for the cooling time of gas at a given physical radius in virialized systems, in agreement with recent previous work. Our results favour cluster merging as the primary factor in preventing the formation of cool cores.

astro-ph↗

arXiv2005

Correlations of near-infrared, optical and X-ray luminosity for early-type galaxies

The relation between X-ray luminosity and near-infrared luminosity for early-type galaxies has been examined. Near-infrared (NIR) luminosities should provide a superior measure of stellar mass compared to optical luminosities used in previous studies, especially if there is significant star-formation or dust present in the galaxies. However, we show that the X-ray-NIR relations are remarkably consistent with the X-ray-optical relations. This indicates that the large scatter of the relations is dominated by scatter in the X-ray properties of early-type galaxies, and is consistent with early-types consisting of old, quiescent stellar populations. We have investigated scatter in terms of environment, surface brightness profile, Mg2, H_beta, H_gamma line strength indices, spectroscopic age, and nuclear H_alpha emission. We found that galaxies with high Mg2 index, low H_beta and H_gamma indices or a `core' profile have a large scatter in Lx, whereas galaxies with low Mg2, high H_beta and H_gamma indices or `power-law' profiles, generally have Lx<10^41 erg/s. There is no clear trend in the scatter with environment or nuclear H_alpha emission.

astro-ph↗