arXiv subjects

S. W. Allen

Publications and source records attributed to S. W. Allen.

At least 127 records · Page 7Linked to original sources

arXiv2003

X-rays from Clusters of Galaxies

The X-ray emission from clusters of galaxies enables them to be used as good cosmological probes and as an example for massive galaxy formation. The gas mass fraction in clusters should be a universal standard which by means of Chandra observations enables Omega_m to be determined to better than 15 per cent accuracy. Future observations of its apparent variation with redshift will enable Omega_Lambda to be measured. The interplay of radiative cooling and heating in cluster cores may reveal the dominant processes acting during the formation of the baryonic part of massive galaxies.

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arXiv2003

Cosmological constraints from the local X-ray luminosity function of the most X-ray luminous galaxy clusters

We present precise constraints on the normalization of the power spectrum of mass fluctuations in the nearby universe, sigma_8, as a function of the mean local matter density, Omega_m. Using the observed local X-ray luminosity function of galaxy clusters from the extended BCS and REFLEX studies, a mass-luminosity relation determined from Chandra and ROSAT X-ray data and weak gravitational lensing observations, and the mass function predicted by the Hubble Volume simulations of Evrard et al., we obtain sigma_8 = (0.508\pm0.019) Omega_m^-(0.253\pm0.024), with Omega_m < 0.34 at 68 per cent confidence. The degeneracy between sigma_8 and Omega_m can be broken using Chandra measurements of the X-ray gas mass fractions in dynamically relaxed clusters. Using this information and including Gaussian priors on the mean baryon density of the universe and the Hubble constant, we obtain sigma_8=0.695\pm0.042 and Omega_m=0.287\pm0.036, for an assumed flat LambdaCDM cosmology (marginalized 68 per cent confidence limits). Our results are in good agreement with some recent studies based on the local X-ray temperature function of clusters, the redshift evolution of the X-ray luminosity and temperature functions of clusters, early results from the Sloan Digitized Sky Survey, the most recent results from studies of cosmic shear, and combined analyses of the 2dF galaxy redshift survey and cosmic microwave background anisotropies.

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arXiv2003

Chandra observations of the galaxy cluster A478: the interaction of hot gas and radio plasma in the core, and an improved determination of the Compton y-parameter

We present the results from a 42 ks \chandra ACIS observation of the galaxy cluster A478. This cluster is generally considered to be highly relaxed. The \chandra image reveals, for the first time, X-ray cavities in the hot gas within the central 15 kpc radius of A478. Two weak and small ($\sim$ 4 kpc) radio lobes that extend from the central nucleus, are detected in a 1.4 GHz VLA observation. The radio lobes are roughly along the direction of the X-ray cavities, but are much smaller than the X-ray cavities. We propose a ``donut'' configuration for the hot gas within the central 15 kpc, created by the interaction of the gas with the radio plasma that originated from the nucleus. The current radio activity of the central radio source is weak ($\sim$ 0.2% of Hydra A) and the total radio power is at least 10 times smaller than the minimum power needed to create the cavities. We compare A478 with other galaxy clusters where similar X-ray cavities were found. A478 and A4059 host much weaker central radio sources than do others with similar size X-ray cavities. On larger scales, deprojected temperature and density profiles are obtained for A478. We used these to derive the Compton y-parameter, for the first time, through direct integration. The result has a much smaller statistical uncertainty than previous ones. This serves as an example of how high quality X-ray data can help constrain H$_{0}$. The corresponding H$_{0}$ also was derived combining the available Sunyaev-Zeldovich effect (SZE) measurement.

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arXiv2002

The peculiar cooling flow cluster RX J0820.9+0752

This poster is a summary of a paper on the subject (Bayer-Kim et al. 2002) that has been submitted for publication in MNRAS. We present X-ray, H alpha, radio, and optical data, both imaging and spectroscopy, of the cluster RX J0820.9+0752 and its peculiar central cluster galaxy (CCG). We announce the discovery of several isolated off-nuclear patches of blue light with strong line-emssion (total L(H alpha) ~ 10^{42}{ergs}^{-1}) and distinct ionizational and kinematic properties, embedded in a region of H alpha/X-ray emission. We propose and investigate a scenario in which a secondary galaxy also featured in our observations has moved through a cooling wake produced by the CCG, thereby producing the observed clumpy morphology and helping to trigger star-formation within the clumps.

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arXiv2002

A survey of molecular hydrogen in the central galaxies of cooling flows

We present a large sample of H- and K-band spectra of 32 optically line luminous central cluster galaxies. We find significant rovibrational H_2 emission in 23 of these galaxies as well as H recombination and/or [FeII] emission in another 5. This represents a fourfold increase in the number of molecular line detections known. A number of the detections are of extended emission (5-20 kpc). In several objects we find significant [SiVI] emission that appears to correlate with the strength of high ionization lines in the optical (e.g. [OIII]). This comprehensive sample builds on previous work and confirms that warm (1000-2500K) molecular hydrogen is present wherever there is ionized material in the cores of cooling flows and in most cases it also coincides with CO molecular line emission.

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arXiv2002

Exciting molecular hydrogen in the central galaxies of cooling flows

The origin of rovibrational H_2 emission in the central galaxies of cooling flow clusters is poorly understood. Here we address this issue using data from our near-infrared spectroscopic survey of 32 of the most line luminous such systems, presented in the companion paper by Edge et al. (2002). We consider excitation by X-rays from the surrounding intracluster medium, UV radiation from young stars, and shocks. The v=1-0 K-band lines with upper levels within 10^4 K of the ground state appear to be mostly thermalised (implying gas densities above ~10^5 cm^-3), with excitation temperatures typically exceeding 2000K, as found earlier by Jaffe, Bremer and van der Werf (2001). Together with the lack of strong v=2-0 lines in the H-band, this rules out UV fluorescence. Using the CLOUDY photoionisation code, we deduce that the H_2 lines can originate in a population of dense clouds, exposed to the same hot (T~50000K) stellar continuum as the lower density gas which produces the bulk of the forbidden optical line emission in the H-alpha luminous systems. This dense gas may be in the form of self-gravitating clouds deposited directly by the cooling flow, or may instead be produced in the high-pressure zones behind strong shocks. Furthermore, the shocked gas is likely to be gravitationally unstable, so collisions between the larger clouds may lead to the formation of globular clusters.

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arXiv2002

Conduction and cooling flows

Chandra and XMM-Newton observations have confirmed the presence of large temperature gradients within the cores of many relaxed clusters of galaxies. Here we investigate whether thermal conduction operating over those gradients can supply sufficient heat to offset radiative cooling. Narayan & Medvedev (2001) and Gruzinov (2002) have noted, using published results on cluster temperatures, that conduction within a factor of a few of the Spitzer rate is sufficient to balance bremsstrahlung cooling. From a detailed study of the temperature and emission measure profiles of Abell 2199 and Abell 1835, we find that the heat flux required by conduction is consistent with or below the rate predicted by Spitzer in the outer regions of the core. Conduction may therefore explain the lack of observational evidence for large mass cooling rates inferred from arguments based simply on radiative cooling, provided that conductivity is suppressed by no more than a factor of three below the full Spitzer rate. To stem cooling in the cluster centre, however, would necessitate conductivity values at least a factor of two larger than the Spitzer values, which we consider implausible. This may provide an explanation for the observed star formation and optical nebulosities in cluster cores. The solution is likely to be time dependent. We briefly discuss the possible origin of the cooler gas and the implications for massive galaxies.

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arXiv2002

Chandra observations of Abell 2199

We present results from an analysis of two Chandra observations of the rich, nearby galaxy cluster Abell 2199. We find evidence (having corrected for projection effects) for radial gradients in temperature and metallicity in the X-ray emitting gas: the temperature drops from kT~4.2 keV at R=200 kpc to 1.6 keV within R=5 kpc of the centre. The metallicity rises from ~0.3 solar at R=200 kpc to ~0.7 solar at R=30 kpc before dropping to 0.3 solar within the central 5 kpc. We find evidence for structure in the surface brightness distribution associated with the central radio source 3C338. No evidence is found for the gas having a large spread in temperature at any particular location despite the cooling time being short (<10**9yr) within the central ~15 kpc. Heating and mass cooling rates are calculated for various assumptions about the state of the gas.

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arXiv2002

Cosmological constraints from the X-ray gas mass fraction in relaxed lensing clusters observed with Chandra

We present precise measurements of the X-ray gas mass fraction for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra Observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Parameterizing the total (luminous plus dark matter) mass profiles using the model of Navarro, Frenk & White (1997), we show that the X-ray gas mass fractions in the clusters asymptote towards an approximately constant value at a radius r_2500, where the mean interior density is 2500 times the critical density of the Universe at the redshifts of the clusters. Combining the Chandra results on the X-ray gas mass fraction and its apparent redshift dependence with recent measurements of the mean baryonic matter density in the Universe and the Hubble Constant determined from the Hubble Key Project, we obtain a tight constraint on the mean total matter density of the Universe, Omega_m = 0.30^{+0.04}_{-0.03}, and measure a positive cosmological constant, Omega_Lambda = 0.95^{+0.48}_{-0.72}. Our results are in good agreement with recent, independent findings based on analyses of anisotropies in the cosmic microwave background radiation, the properties of distant supernovae, and the large-scale distribution of galaxies.

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arXiv2002

Chandra observations of RXJ1347.5-1145: the distribution of mass in the most X-ray luminous galaxy cluster known

We present Chandra observations of RXJ1347.5-1145, the most X-ray luminous cluster of galaxies known. We report the discovery of a region of relatively hot, bright X-ray emission, located approximately 20 arcsec to the southeast of the main X-ray peak, at a position consistent with the region of enhanced Sunyaev-Zeldovich effect reported by Komatsu et al. (2001). We suggest that this region contains shocked gas resulting from a recent subcluster merger event. Excluding the data for the southeast quadrant, the cluster appears relatively relaxed. The X-ray gas temperature rises from kT~6 keV within the central 25 h_50^{-1} kpc radius to a mean value of ~16 keV between 0.1-0.5 h_50^{-1} Mpc. The mass profile for the relaxed regions of the cluster, determined under the assumption of hydrostatic equilibrium, can be parameterized by a Navarro, Frenk & White (1997) model with a scale radius r_s~0.4 h_50^{-1} Mpc and a concentration parameter c~6. The best-fit Chandra mass model is in good agreement with independent measurements from weak gravitational lensing studies. Strong lensing data for the central regions of the cluster can be also explained by the introduction of an additional mass clump centred on the second brightest galaxy. We argue that this galaxy is likely to have been the dominant galaxy of the recently merged subcluster.

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arXiv2001

Deep inside the core of Abell 1795: the Chandra view

We present X-ray spatial and spectral analysis of the Chandra data from the central 400 kpc (H0 = 50 km/s/Mpc) of the cluster of galaxies Abell 1795. The plasma temperature rises outwards by a factor of 3, whereas the iron abundance decreases by a factor of 4. The spatial distribution of Oxygen, Neon, Sulphur, Silicon and Iron shows that supernovae Type Ia dominate the metal enrichment process of the cluster plasma within the inner 150 kpc. Resolving both the gas density and temperature in 9 radial bins, we recover the gravitational mass density profile and show that it flattens within 100 kpc as rho_DM \propto r^{-0.6} with a power law index flatter than -1 at >3 sigma level. The observed motion of the central galaxy and the presence of excesses and deficits along the North-South direction in the brightness distribution indicate that the central cluster region is not relaxed. In the absence of any non-gravitational heating source, the data from the inner ~200 kpc indicate the presence of a cooling flow with an integrated mass deposition rate of about 100 Msun/yr. Over the same cluster region, the observed rate of 74 Msun/yr is consistent with the recent XMM Reflection Grating Spectrometer limit of 150 Msun/yr.

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arXiv2001

The X-ray virial relations for relaxed lensing clusters observed with Chandra

We examine the relations linking mass, X-ray temperature and bolometric luminosity for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra Observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Within radii corresponding to a fixed overdensity Delta = 2500 with respect to the critical density at the redshifts of the clusters, the observed temperature profiles, scaled in units of T_2500 and r_2500, exhibit an approximately universal form which rises within r~0.3 r_2500 and then remains approximately constant out to r_2500. We obtain best-fit slopes for the mass-temperature and temperature-luminosity relations consistent with the predictions from simple scaling arguments i.e. M_2500 \propto T_2500^{3/2} and L_2500 \propto T_2500^{2}, respectively. We confirm the presence of a systematic offset of ~40 per cent between the normalizations of the observed and predicted mass-temperature relations for both SCDM and LambdaCDM cosmologies.

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arXiv2001

The intracluster iron distribution around 4C+55.16

We report on the metal distribution in the intracluster medium around the radio galaxy 4C+55.16 (z=0.24) observed with the Chandra X-ray Observatory. The radial metallicity profile shows a dramatic change at 10 arcsec (~50 kpc) in radius from half solar to twice solar at inner radii. Also found was a plume-like feature located at ~3 arcsec to the south-west of the centre of the galaxy, which is mostly accounted for by a strong enhancement of iron L emission. The X-ray spectrum of the plume is characterized by the metal abundance pattern of Type Ia supernovae (SNeIa), i.e., large ratios of Fe to alpha-elements, with the iron metallicity being unusually high at 7.9(+24/-5.0) solar (90 per cent error). How the plume has been formed is not entirely clear. The inhomogeneous iron distribution suggested in this cluster provides important clues to understanding the metal enrichment process of the cluster medium.

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arXiv2001

Magnetic fields in the Centaurus cluster

We present multi-frequency VLA observations of the radio galaxy PKS 1246-410 at the center of the Centaurus Cluster, and compare these results to recent Chandra observations. The unusual radio morphology of PKS 1246-410 probably results from interactions with the hot, X-ray emitting gas. This gas, along with cluster magnetic fields, also produces substantial Faraday Rotation Measures towards PKS 1246-410. We discuss these observations in the context of a sample of 14 luminous X-ray clusters with embedded radio galaxies and possible cooling flows. A correlation is found between the cooling flow rate and the maximum Faraday Rotation Measures. Magnetic fields of strength 10-40 uG are found to be common to the centers of clusters with strong cooling flows, and somewhat lower field strengths of 2-10 uG are found in the non cooling-flow clusters.

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arXiv2001

Chandra observations of the galaxy cluster Abell 1835

We present the analysis of 30 ksec of Chandra observations of the galaxy cluster Abell 1835. Overall, the X-ray image shows a relaxed morphology, although we detect substructure in in the inner 30 kpc radius. Spectral analysis shows a steep drop in the X-ray gas temperature from ~12 keV in the outer regions of the cluster to ~4 keV in the core. The Chandra data provide tight constraints on the gravitational potential of the cluster which can be parameterized by a Navarro, Frenk & White (1997) model. The X-ray data allow us to measure the X-ray gas mass fraction as a function of radius, leading to a determination of the cosmic matter density of Ω_m=0.40+-0.09 h_50^-0.5. The projected mass within a radius of ~150 kpc implied by the presence of gravitationally lensed arcs in the cluster is in good agreement with the mass models preferred by the Chandra data. We find a radiative cooling time of the X-ray gas in the centre of Abell 1835 of about 3x10^8 yr. Cooling flow model fits to the Chandra spectrum and a deprojection analysis of the Chandra image both indicate the presence of a young cooling flow (~6x10^8 yr) with an integrated mass deposition rate of 230^+80_-50 M_o yr^-1 within a radius of 30 kpc. We discuss the implications of our results in the light of recent RGS observations of Abell 1835 with XMM-Newton.

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arXiv2001

BeppoSAX observations of three distant, highly luminous clusters of galaxies: RXJ1347-1145, Z3146 and A2390

We present an analysis of BeppoSAX observations of three clusters of galaxies which are amongst the most luminous in the Universe: RXJ1347-1145, Zwicky 3146 and Abell 2390. Using data from both the Low Energy (LECS) and Medium Energy (MECS) Concentrator Spectrometers, and a joint analysis with the Phoswich Detection System (PDS) data above 10 keV, we constrain, with a relative uncertainty of between 7 and 42 per cent (90 per cent confidence level), the mean gas temperature in the three clusters. These measurements are checked against any possible non-thermal contribution to the plasma emission and are shown to be robust. We confirm that RXJ1347-1145 has a gas temperature that lies in the range between 13.2 and 22.3 keV at the 90 per cent confidence level, and is larger than 12.1 keV at 3 sigma level. The existence of such a hot galaxy cluster at redshift of about 0.45 implies an upper limit on the mean mass density in the Universe, Omega_m, of 0.5. Combining the BeppoSAX estimates for gas temperature and luminosity of the three clusters presented in this work with ASCA measurements available in the literature, we obtain a slope of 2.7 in the L-T relation once the physical properties are corrected from the contamination from the central cooling flows.

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arXiv2001

Chandra X-ray observations of the 3C295 cluster core

We examine the properties of the X-ray gas in the central regions of the distant (z=0.46), X-ray luminous cluster of galaxies surrounding the powerful radio source 3C 295, using observations made with the Chandra Observatory. Between radii of 50-500 kpc, the cluster gas is approximately isothermal with an emission-weighted temperature, kT ~5 keV. Within the central 50 kpc radius this value drops to kT ~3.7 keV. The spectral and imaging Chandra data indicate the presence of a cooling flow within the central 50 kpc radius of the cluster, with a mass deposition rate of approximately 280 solar masses per year. We estimate an age for the cooling flow of 1-2 Gyr, which is approximately one thousand times older than the central radio source. We find no evidence in the X-ray spectra or images for significant heating of the X-ray gas by the radio source. We report the detection of an edge-like absorption feature in the spectrum for the central 50 kpc region, which may be due to oxygen-enriched dust grains. The implied mass in metals seen in absorption could have been accumulated by the cooling flow over its lifetime. Combining the results on the X-ray gas density profile with radio measurements of the Faraday rotation measure in 3C295, we estimate the magnetic field strength in the region of the cluster core to be B ~12 \muG.

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arXiv2001

Chandra measurements of the distribution of mass in the luminous lensing cluster Abell 2390

We present spatially-resolved X-ray spectroscopy of the luminous lensing cluster Abell 2390, using observations made with the Chandra observatory. The temperature of the X-ray gas is shown to rise with increasing radius within the central ~200 kpc of the cluster, and then remain approximately isothermal, with kT =11.5^{+1.5}_{-1.6} keV, out to the limits of the observations at r~1.0 Mpc. The total mass profile determined from the Chandra data has a form in good agreement with the predictions from numerical simulations. Using the parameterization of Navarro, Frenk & White (1997), we measure a scale radius r_s~0.9 Mpc and a concentration parameter c~4. The best-fit X-ray mass model is in good agreement with independent gravitational lensing results and optical measurements of the galaxy velocity dispersion in the cluster. The X-ray gas-to-total-mass ratio rises with increasing radius with f_gas = 21 \pm 10 per cent at r=0.9 Mpc. The azimuthally-averaged 0.3-7.0 keV surface brightness profile exhibits a small core radius and a clear `break' at r~500 kpc, where the slope changes from S_X \approxpropto r^{-1.5} to S_X \approxpropto r^{-3.6}. The data for the central region of the cluster indicate the presence of a cooling flow with a mass deposition rate of 200-300 solar masses per year and an age of 2-3 Gyr.

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