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D. J. Burke

Publications and source records attributed to D. J. Burke.

30 records · Page 2Linked to original sources
arXiv2001

The XMM-Newton $Ω$ Project

The abundance of high-redshift galaxy clusters depends sensitively on the matter density $\OmM$ and, to a lesser extent, on the cosmological constant $Λ$. Measurements of this abundance therefore constrain these fundamental cosmological parameters, and in a manner independent and complementary to other methods, such as observations of the cosmic microwave background and distance measurements. Cluster abundance is best measured by the X-ray temperature function, as opposed to luminosity, because temperature and mass are tightly correlated, as demonstrated by numerical simulations. Taking advantage of the sensitivity of XMM-Newton, our Guaranteed Time program aims at measuring the temperature of the highest redshift (z>0.4) SHARC clusters, with the ultimate goal of constraining both $\OmM$ and $Λ$.

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arXiv2000

The Bright SHARC Survey: The Cluster Catalog

We present the Bright SHARC (Serendipitous High-Redshift Archival ROSAT Cluster) Survey, which is an objective search for serendipitously detected extended X-ray sources in 460 deep ROSAT PSPC pointings. The Bright SHARC Survey covers an area of 178.6 sq.deg and has yielded 374 extended sources. We discuss the X-ray data reduction, the candidate selection and present results from our on-going optical follow-up campaign. The optical follow-up concentrates on the brightest 94 of the 374 extended sources and is now 97% complete. We have identified thirty-seven clusters of galaxies, for which we present redshifts and luminosities. The clusters span a redshift range of 0.0696<z<0.83 and a luminosity range of 0.065<Lx<8.3e44 erg/s [0.5-2.0 keV] (assuming Ho = 50 km/s/Mpc and qo=0.5). Twelve of the clusters have redshifts greater than z=0.3, eight of which are at luminosities brighter than Lx=3e44 erg/s. Seventeen of the 37 optically confirmed Bright SHARC clusters have not been listed in any previously published catalog. We also report the discovery of three candidate ``fossil groups'' of the kind proposed by Ponman et al. (1994).

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arXiv2000

Chandra study of an overdensity of X-ray sources around two distant (z~0.5) clusters

We present results from a Chandra X-ray Observatory study of the field X-ray source populations in 4 different observations: two high-redshift (z~0.5) clusters of galaxies 3C295 and RXJ003033.2+261819; and two non-cluster fields with similar exposure time. Surprisingly, the 0.5-2 keV source surface densities (~900-1200 sources deg**-2 at a flux limit of 1.5x10**-15 erg cm**-2s**-1) measured in an ~8'x8' area surrounding each cluster exceed by a factor of ~2 the value expected on the basis of the ROSAT and Chandra logN-logS, with a significance of ~2 sigma each, or ~3.5 sigma when the 2 fields are combined (i.e. a probability to be a statistical fluctuation of <1% and <0.04%, respectively). The same analysis performed on the non-cluster fields and on the outer chips of the cluster fields does NOT show evidence of such an excess. In both cluster fields, the summed 0.5-10 keV spectrum of the detected objects is well fitted by a power-law with Gamma~1.7 similar to AGNs and shows no sign of intrinsic absorption. The few (~10 out of 35) optical identifications available to date confirm that most of them are, as expected, AGNs but the number of redshifts available is too small to allow conclusions on their nature. We discuss possible interpretations of the overdensity in terms of: a statistical variation of Cosmic Background sources; a concentration of AGNs and/or powerful starburst galaxies associated with the clusters; and g ravitational lensing of background QSO's by the galaxy clusters. All explanations are however difficult to reconcile with the large number of excess sources detected. Deeper X-ray observations and more redshifts measurements are clearly required to settle the issue.

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arXiv2000

Cluster Selection and the Evolution of Brightest Cluster Galaxies

The K-band Hubble diagram of Brightest Cluster Galaxies (BCGs) is presented for a large, X-ray selected cluster sample extending out to z = 0.8. The controversy over the degree of BCG evolution is shown to be due to sample selection, since the BCG luminosity depends upon the cluster environment. Selecting only the most X-ray luminous clusters produces a BCG sample which shows, under the assumption of an Einstein-de Sitter cosmology, significantly less mass growth than that predicted by current semi-analytic galaxy formation models, and significant evidence of any growth only if the dominant stellar population of the BCGs formed relatively recently (z <= 2.6).

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arXiv2000

Chandra Observation of Abell 2142: Survival of Dense Subcluster Cores in a Merger

We use Chandra data to map the gas temperature in the central region of the merging cluster A2142. The cluster is markedly nonisothermal; it appears that the central cooling flow has been disturbed but not destroyed by a merger. The X-ray image exhibits two sharp, bow-shaped, shock-like surface brightness edges or gas density discontinuities. However, temperature and pressure profiles across these edges indicate that these are not shock fronts. The pressure is reasonably continuous across these edges, while the entropy jumps in the opposite sense to that in a shock (i.e. the denser side of the edge has lower temperature, and hence lower entropy). Most plausibly, these edges delineate the dense subcluster cores that have survived a merger and ram pressure stripping by the surrounding shock-heated gas.

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arXiv1999

The Bright SHARC Survey: The X-ray Cluster Luminosity Function

We present here initial results on the X-ray Cluster Luminosity Function (XCLF) from the Bright SHARC sample of distant X-ray clusters of galaxies. This sample is 97% complete in its optical identifications and contains 12 X-ray luminous clusters in the range 0.3 < z < 0.83 (median z = 0.42) and 1.1 < Lx(44) < 8.5 erg/s [0.5-2.0 keV]. We present a preliminary selection function for the Bright SHARC based on Monte Carlo simulations. Using this selection function, we have computed the Bright SHARC XCLF and find it to be fully consistent with a non-evolving XCLF to Lx(44) = 5 erg/s and z = 0.7. At Lx(44) > 5 erg/s, we find evidence for a deficit of clusters compared to that expected from a non-evolving XCLF. We detect only one such cluster in the redshift range 0.3 < z < 0.7 when we would expect 4.9 clusters based on the local XCLF of De Grandi et al. (1999). The statistical significance of this deficit is 96%. To increase the statistical significance of this possible deficit, we have combined the Bright SHARC and the 160 sq. deg. survey of Vikhlinin et al (1998a). This joint survey covers 260 sq. degs. and contains only one confirmed 0.3 < z < 0.7, Lx(44) > 5 erg/s cluster, while we would expect 7.6 such clusters based on the local XCLF (De Grandi et al. 1999). The statistical significance of the deficit in this joint survey increases to 99.5%. These results remain preliminary because of incompletenesses in the optical follow-up and uncertainties in the local XCLF.

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arXiv1999

A Deficit of High-Redshift, High-Luminosity X-Ray Clusters: Evidence for a High Value of Omega_m?

From the Press-Schechter mass function and the empirical X-ray cluster luminosity-temperature (L-T) relation, we construct an X-ray cluster luminosity function that can be applied to the growing number of high-redshift, X-ray cluster luminosity catalogs to constrain cosmological parameters. In this paper, we apply this luminosity function to the Einstein Medium Sensitivity Survey (EMSS) and the ROSAT Brightest Cluster Sample (BCS) luminosity function to constrain the value of Omega_m. In the case of the EMSS, we find a factor of 4 - 5 fewer X-ray clusters at redshifts above z = 0.4 than below this redshift at luminosities above L_X = 7 x 10^44 erg s^-1 (0.3 - 3.5 keV), which suggests that the X-ray cluster luminosity function has evolved above L_*. At lower luminosities, this luminosity function evolves only minimally, if at all. Using Bayesian inference, we find that the degree of evolution at high luminosities suggests that Omega_m = 0.96(+0.36,-0.32), given the best-fit L-T relation of Reichart, Castander, & Nichol (1998). When we account for the uncertainty in how the empirical L-T relation evolves with redshift, we find that Omega_m is approximately 1.0 +/- 0.4. However, it is unclear to what degree systematic effects may affect this and similarly obtained results.

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arXiv1997

The Southern SHARC Survey: the z = 0.3 - 0.7 Cluster XLF

We present the z = 0.3 - 0.7 cluster X-ray luminosity function (XLF) determined from the Southern SHARC (Serendipitous High-redshift Archival ROSAT Cluster) survey. Over the luminosity range L ~ (0.3 - 3) x 10^44 erg/s (0.5 - 2.0 keV) the XLF is in close agreement with that of the low redshift X-ray cluster population. This result greatly strengthens our previous claim of no evolution of the cluster population, at these luminosities, at a median redshift of z =0.44.

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arXiv1997

On The Evolution of X-ray Clusters at High Redshift

We report on the first results from a redshift survey of a flux-limited sample of X-ray clusters selected serendipitously from the ROSAT PSPC data archive. We spectroscopically confirm 15 clusters in the range 0.3 < z < 0.7, to a flux limit of ~ 3.9 x 10^-14 erg s^-1 cm^-2, over a survey area of 17.2 deg^2. The surface density of clusters in our survey is 2.0 (+0.4,-0.3) deg^-2, in good agreement with the number density of cluster candidates detected using algorithms designed to search for very extended sources. The number of clusters detected between 0.3 < z < 0.7 is consistent with a prediction based on a simple extrapolation of the local X-ray cluster luminosity function, which indicates that over this redshift range no significant evolution in the cluster population has taken place. These results are in conflict with recent claims that the number density of X-ray clusters found in deep ROSAT PSPC pointings evolves rapidly beyond z = 0.3.

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arXiv1996

Evolution in the X-ray Cluster Luminosity Function Revisited

(Shortened Abstract) We present new X-ray data taken from the ROSAT PSPC pointing archive for 21 clusters in the Einstein Extended Medium Sensitivity Survey (EMSS). We have supplemented these data with new optical follow-up observations found in the literature and overall, 32 of the original 67 z>0.14 EMSS clusters now have new information. Using this improved EMSS sample, we have re-determined the EMSS X-ray Cluster Luminosity Function as a function of redshift. We have removed potential mis-classifications and included our new measurements of the clusters X-ray luminosities and redshifts. We find similar luminosity functions to those originally presented by Henry etal (1992); albeit with two important differences. First, we show that the original low redshift EMSS luminosity function is insufficiently constrained. Secondly, the power law shape of our new determination of the high redshift EMSS luminosity function (z=0.3 to 0.6) has a shallower slope than that seen by Henry etal. We have quantified the degree of evolution seen in the X-ray cluster luminosity using several statistical tests. The most restrictive analysis indicates that our low and high redshift EMSS luminosity functions are statistically different at only the 95% level. However, other tests indicate that these luminosity functions differ by as little as one sigma. These data are therefore, consistent with no evolution in the X-ray cluster luminosity function out to z=0.5.

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arXiv1995

A search for distant X-ray galaxy clusters

We report on the progress of our search for serendipitous distant X-ray galaxy clusters in the ROSAT PSPC pointed observations archive. The initial aim of our work is to measure the X-ray luminosity function of clusters down to 4 x 10^-14 erg cm^-2 s^-1 and so test the claims of cluster evolution at moderate redshifts. We have R band images and spectroscopy for 45 extended sources detected in 50 of the 100 deepest southern fields. A preliminary analysis suggests we cover a range of distances, from nearby clusters to possible distant clusters, z \simeq 1.0.

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