arXiv subjects

H. Boehringer

Publications and source records attributed to H. Boehringer.

At least 109 records · Page 6Linked to original sources

arXiv2002

The ROSAT All-Sky Survey: a Catalog of Clusters of Galaxies in a Region of 1 Ster around the South Galactic Pole

A field of 1.013 ster in the ROSAT all-sky survey (RASS), centered on the south galactic pole (SGP), has been searched in a systematic, objective manner for clusters of galaxies. The procedure relied on a correlation of the X-ray positions and properties of ROSAT sources with the distribution of galaxies in the COSMOS digitised data base, which was obtained by scanning the plates of the UK Schmidt IIIa-J optical southern sky survey. The study used the second ROSAT survey data base (RASS-2) and included several optical observing campaigns to measure redshifts. The search, a precursor to the larger REFLEX survey of the whole southern sky, reached the detection limits of the RASS and the COSMOS data, and yielded a catalog of 186 clusters in which the lowest flux is 1.5e-12 erg/cm2/s in the 0.1-2.4 keV band. Of these 157 have measured redshifts. Using a limit of 3.0e-12 erg/cm2/s a complete subset of 112 clusters was obtained, of which 110 have measured redshifts. The spatial distribution of the X-ray clusters out to z = 0.15 shows an extension of the Local Supercluster to the Pisces-Cetus supercluster (z<~0.07), and a more distant orthogonal structure at 0.07<z<0.15.

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arXiv2001

The REFLEX Cluster Survey: Probing the Mass Distribution in the Universe

We summarize some of the major results obtained so far from the REFLEX survey of X-ray clusters of galaxies, concentrating on the latest measurements of the cluster X-ray luminosity function and two-point correlation function. The REFLEX luminosity function provides the most homogeneous census of the distribution function of masses in the local Universe, representing a unique zero-redshift reference quantity for evolutionary studies. On the other hand, the observed clustering of REFLEX clusters is very well described both in amplitude and shape by the correlation function computed for a low-Omega_M CDM model. The bidimensional correlation map xi(r_p,pi) shows no stretching along the line of sight, indicating negligible spurious effects in the sample, with at the same time a clear compression of the contours as expected in the presence of coherent large-scale motions.

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arXiv2001

The New Emerging Model for the Structure of Cooling Cores in Clusters of Galaxies

New X-ray observations with XMM-Newton show a lack of spectral evidence for large amounts of cooling and condensing gas in the centers of galaxy clusters believed to harbour strong cooling flows. Here, we explore these diagnostics of the temperature structure of cooling cores with XMM-spectroscopy. We further find no evidence of intrinsic absorption in the center of the cooling flows of M87 and the Perseus cluster. To explain these findings we consider the heating of the core regions of clusters by jets from a central AGN. We find that the power of the AGN jets as estimated by their interaction effects with the intracluster medium in several examples is more then sufficient to heat the cooling flows. We explore which requirements such a heating model has to fulfill and find a very promising scenario of self-regulated Bondi accretion of the central black hole. In summary it is argued that most observational evidence points towards much lower mass deposition rates than previously inferred for cooling flow clusters.

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arXiv2001

The New Emerging Model for the Structure of Cooling Cores in Clusters of Galaxies

New X-ray observations with XMM-Newton show a lack of spectral evidence for large amounts of cooling and condensing gas in the centers of galaxy clusters believed to harbour strong cooling flows. The paper reexplores the cooling flow scenario in the light of the new observations. We explore the diagnostics of the temperature structure of cooling cores with XMM-spectroscopy, tests for intracluster X-ray absorption towards central AGN, the effect of metal abundance inhomogeneities, and the implications of high resolution images in the centers of clusters. We find no evidence of intrinsic absorption in the center of the cooling flows of M87 and the Perseus cluster. We further consider the effect of cluster rotation in cooling flow regions in the frame of cosmic structure evolution models. Also the heating of the core regions of clusters by jets from a central AGN is reconsidered. We find that the power of the AGN jets as estimated by their interaction effects with the intracluster medium in several examples is more then sufficient to heat the cooling flows and to reduce the mass deposition rates. We explore in more detail which requirements such a heating model has to fulfill to be consistent with all observations, point out the way such a model could be constructed, and argue that such model building seems to be successful. In summary it is argued that most observational evidence points towards much lower mass deposition rates than previously inferred in the central region of clusters thought to contain strong cooling flows.

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arXiv2001

X-ray Evidence for Spectroscopic Diversity of Type Ia Supernovae: XMM observation of the elemental abundance pattern in M87

We present the results of a detailed element abundance study of hot gas in M87, observed by XMM-Newton. We choose two radial bins, 1'-3' and 8'-16' (8'-14' for EMOS; hereafter the central and the outer zones), where the temperature is almost constant, to carry out the detailed abundance measurements of O, Ne, Mg, Si, S, Ar, Ca, Fe and Ni using EPIC-PN (EPN) and -MOS (EMOS) data. First, we find that the element abundance pattern in the central compared to the outer zone in M87 is characterized by SN Ia enrichment of a high (roughly solar) ratio of Si-group elements (Si, S, Ar, Ca) to Fe, implying that Si burning in SN Ia is highly incomplete. In nucleosynthesis modeling this is associated with either a lower density of the deflagration-detonation transition and/or lower C/O and/or lower central ignition density and observationally detected as optically subluminous SNe Ia in early-type galaxies. Second, we find that SN Ia enrichment has a systematically lower ratio of the Si-group elements to Fe by 0.2 dex in the outer zone associated with the ICM of the Virgo cluster. We find that such a ratio and even lower values by another 0.1 dex are a characteristic of the ICM in many clusters using observed Si:S:Fe ratios as found with ASCA. Third, the Ni/Fe ratio in the central zone of M87 is 1.5+/-0.3 solar (meteoritic), while values around 3 times solar are reported for other clusters. In modeling of SN Ia, this implies a reduced influence of fast deflagration SN Ia models in the chemical enrichment of M87's ISM. Thus, to describe the SN Ia metal enrichment in clusters, both deflagration as well as delayed detonation scenarios are required, supporting a similar conclusion, derived from optical studies on SNe Ia. Abridged.

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arXiv2001

A Systematic Study of X-Ray Substructure of Galaxy Clusters Detected in the ROSAT All-Sky Survey

Results of a systematic study of substructure in X-ray surface brightness distributions of a combined sample of 470 REFLEX+BCS clusters of galaxies are presented. The fully automized morphology analysis is based on data of the 3rd processing of the ROSAT All-Sky survey (RASS-3). After correction for several systematic effects, $52\pm 7$ percent of the REFLEX+BCS clusters are found to be substructured in metric apertures of 1 Mpc radius ($H_0= 50 {\rm km} {\rm s}^{-1} {\rm Mpc}^{-1}$). Future simulations will show statistically which mass spectrum of major and minor mergers contributes to this number. Another important result is the discovery of a substructure-density relation, analogous to the morphology-density relation for galaxies. Here, clusters with asymmetric or multi-modal X-ray surface brightness distributions are located preferentially in regions with higher cluster number densities. The substructure analyses techniques are used to compare the X-ray morphology of 53 clusters with radio halos and relics, and 22 cooling flow clusters with the REFLEX+BCS reference sample. After careful equalization of the different `sensitivities' of the subsamples to substructure detection it is found that the halo and relic sample tends to show more often multi-modal and elongated X-ray surface brightness distributions compared to the REFLEX+BCS reference sample. The cooling flow clusters show more often circular symmetric and unimodal distributions compared to the REFLEX+BCS and the halo/relic reference samples. Both findings further support the idea that radio halos and relics are triggered by merger events, and that pre-existing cooling flows might be disrupted by recent major mergers.

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arXiv2001

The REFLEX Galaxy Cluster Survey IV: The X-ray Luminosity Function

The X-ray galaxy cluster sample from the REFLEX Cluster Survey, which covers the X-ray brightest galaxy clusters detected in the ROSAT All-Sky Survey in the southern sky, is used to construct the X-ray luminosity function of clusters in the local Universe. With 452 clusters detected above an X-ray flux-limit of 3 10^(-12) erg s^(-1) cm^(-2) in 4.24 sr of the sky, this sample is the most comprehensive X-ray cluster sample with a well documented selection function, providing the best current census of the local X-ray galaxy cluster population. In this paper we discuss the construction of the luminosity function, the effects of flux measurement errors and of variations with sample region and we compare the results to those from previous surveys.

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arXiv2001

Cluster Evolution in the ROSAT North Ecliptic Pole Survey

The deepest region of the ROSAT All-Sky Survey, at the North Ecliptic Pole, has been studied to produce a complete and unbiased X-ray selected sample of clusters of galaxies. This sample is used to investigate the nature of cluster evolution and explore potential implications for large-scale structure models. The survey is 99.6% optically identified. Spectroscopic redshifts have been measured for all the extragalactic identifications. In this Letter, first results on cluster evolution are presented based on a comparison between the number of the observed clusters in the North Ecliptic Pole survey and the number of expected clusters assuming no-evolution models. At z>0.3 there is a deficit of clusters with respect to the local universe which is significant at > 4.7sigma. The evolution appears to commence at L_{0.5-2.0} > 1.8x10^{44} erg s^{-1} in our data. The negative evolution goes in the same direction as the original EMSS result, the results from the 160 deg^{2} survey by Vikhlinin et al. (1998) and the recent results from the RDCS (Rosati et al. 2000). At lower redshifts there is no evidence for evolution, a result in agreement with these and other cluster surveys.

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arXiv2001

The North Ecliptic Pole Supercluster

We have used the ROSAT All-Sky Survey to detect a known supercluster at z=0.087 in the North Ecliptic Pole region. The X-ray data greatly improve our understanding of this supercluster's characteristics, approximately doubling our knowledge of the structure's spatial extent and tripling the cluster/group membership compared to the optical discovery data. The supercluster is a rich structure consisting of at least 21 galaxy clusters and groups, 12 AGN, 61 IRAS galaxies, and various other objects. A majority of these components were discovered with the X-ray data, but the supercluster is also robustly detected in optical, IR, and UV wavebands. Extending 129 x 102 x 67 (1/h50 Mpc)^3, the North Ecliptic Pole Supercluster has a flattened shape oriented nearly edge-on to our line-of-sight. Owing to the softness of the ROSAT X-ray passband and the deep exposure over a large solid angle, we have detected for the first time a significant population of X-ray emitting galaxy groups in a supercluster. These results demonstrate the effectiveness of X-ray observations with contiguous coverage for studying structure in the Universe.

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arXiv2001

Cosmic Mass Functions from Gaussian Stochastic Diffusion Processes

Gaussian stochastic diffusion processes are used to derive cosmic mass functions. To get analytic relations previous studies exploited the sharp $k$-space filter assumption yielding zero drift terms in the corresponding Fokker-Planck (Kolmogorov's forward) equation and thus simplifying analytic treatments significantly (excursion set formalism). In the present paper methods are described to derive for given diffusion processes and Gaussian random fields the corresponding mass and filter functions by solving the Kolmogorov's forward and backward equations including nonzero drift terms. This formalism can also be used in cases with non-sharp $k$-space filters and for diffusion processes exhibiting correlations between different mass scales.

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arXiv2001

Details of the mass--temperature relation for clusters of galaxies

We present results on the total mass and temperature determination using two samples of clusters of galaxies. One sample is constructed with emphasis on the completeness of the sample, while the advantage of the other is the use of the temperature profiles, derived with ASCA. We obtain remarkably similar fits to the M-T relation for both samples, with the normalization and the slope significantly different from both prediction of self-similar collapse and hydrodynamical simulations. We discuss the origin of these discrepancies and also combine the X-ray mass with velocity dispersion measurements to provide a comparison with high-resolution dark matter simulations. Finally, we discuss the importance of a cluster formation epoch in the observed M-T relation.

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arXiv2000

The ROSAT-ESO Flux Limited X-Ray (REFLEX) Galaxy Cluster Survey I: The Construction of the Cluster Sample

We describe the construction of an X-ray flux-limited sample of galaxy clusters, the REFLEX survey catalogue, to be used for cosmological studies. The survey is based on the ROSAT All-Sky Survey and on extensive follow-up observations conducted in the frame of an ESO key program. In the paper we document the process by which the primary cluster candidate catalogue is constructed prior to the optical follow-up observations. We describe the reanalysis of the RASS source catalogue which enables us to impose a proper flux limit cut to the X-ray source list without introducing a severe bias against extended sources. We discuss the correlation of the X-ray and optical (COSMOS) data to find galaxy density enhancements at the RASS X-ray source positions and the further evaluation of the nature of these cluster candidates. Based also on the results of the follow-up observations we provide a statistical analysis of the completeness and contamination of the final cluster sample and show results on the cluster number counts. The final sample of identified X-ray clusters reaches a flux limit of 3e-12 erg s-1 cm-2 in the 0.1 - 2.4 keV band and comprises 452 clusters. The results imply a completeness of the REFLEX cluster sample well in excess of 90% and a contamination by non-cluster X-ray sources of less than 9%, an accuracy sufficient for the use of this cluster sample for cosmological tests.

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arXiv2000

Searching for X-ray Galaxy Clusters in the Zone of Avoidance

Based on the ROSAT All-Sky Survey we are constructing a sample of the X-ray brightest galaxy clusters for cosmological studies. We have already completed the compilation of a sample of about 1000 clusters in the sky excluding a 40 degree wide band around the galactic plane. We have demonstrated with the southern sample, which is the so far most complete, that we can get reliable statistical measures of the large-scale structure of the Universe to scales in excess of 400 Mpc (for Ho=100). With the experience gained in these projects we are extending the search for X-ray galaxy clusters into the Zone-of-Avoidance. We have compiled a sample of 181 good cluster candidates with a well defined completeness of about 70% down to a flux limit of 3 10e-12 erg/s/cm**2 in the zone with an absorption depth less than Av = 2.25. The candidates are definitely identified and redshifts are obtained in an ongoing optical follow-up program.

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arXiv2000

XMM-Newton observations of M87 and Its X-Ray Halo

We report performance verification observations of the giant elliptical galaxy M87 in the Virgo Cluster with the MOS, pn, and optical monitor instruments on board of XMM-Newton. With the energy sensitive imaging instruments MOS and pn we obtain the first spatially constrained X-ray spectra of the nucleus and the jet of the galaxy. The good photon statistics of the pn and MOS allow a detailed analysis of the radial temperature and abundance distribution of 6 elements. The data provide no indication of a multi-temperature structure for radii less than 2 arcmin. An apparent sharp metal abundance drop deduced for the regions inside this radius is probably due to resonant line scattering.

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arXiv2000

A New Radio - X-Ray Probe of Galaxy Cluster Magnetic Fields

Results are presented of a new VLA-ROSAT study that probes the magnetic field strength and distribution over a sample of 16 ``normal'' low redshift (z < 0.1) galaxy clusters. The clusters span two orders of magnitude in X-ray luminosity, and were selected to be free of (unusual) strong radio cluster halos, and widespread cooling flows. Consistent with these criteria, most clusters show a relaxed X-ray morphology and little or no evidence for recent merger activity. Analysis of the rotation measure (RM) data shows cluster-generated Faraday RM excess out to ~0.5 Mpc from cluster centers. The results, combined with RM imaging of cluster-embedded sources and ROSAT X-ray profiles indicates that the hot intergalactic gas within these ``normal'' clusters is permeated with a high filling factor by magnetic fields at levels of <|B|> = 5-10 (l/10 kpc)^{-1/2} microGauss, where l is the field correlation length. These results lead to a global estimate of the total magnetic energy in clusters, and give new insight into the ultimate energy origin, which is likely gravitational. These results also shed some light on the cluster evolutionary conditions that existed at the onset of cooling flows.

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arXiv2000

Temperature Map of the Virgo Cluster of Galaxies Observed with ASCA

The temperature distribution of the intracluster medium (ICM) in the Virgo cluster of galaxies has been derived from extensive mapping observations with ASCA covering an area of 19 deg^2. In the spectral analysis, the inner region within a radius of ~60' from M87 is characterized by an ICM temperature of kT \~2.5 keV with little variation. On the other hand, the outer regions indicate significant variation of the temperature with an amplitude of about 1 keV. The temperature map was produced from the hardness ratio (H.R.) values with a resolution of about 5'. Besides the previously reported hot region with kT > 4 keV between M87 and M49, several hot regions with kT = 3-4 keV are detected in the cluster outskirts. The auto-correlation function for the H.R. variation shows that the temperature variation is correlated within a size of about 300 kpc, suggesting that gas blobs falling in the Virgo cluster have a typical size of groups of galaxies. A correlation with the velocity dispersion of member galaxies shows that only the north-west region indicates an unusually large beta_spec value of 2-4. The upper limit for extended non-thermal emission in the Virgo cluster is obtained to be L_X ~9 x 10^41 ergs/s in the 2 - 10 keV band. We discuss that these features consistently indicate that the Virgo cluster is in a relatively early stage of the cluster evolution.

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arXiv2000

Optical follow-up of the REFLEX survey: a new perspective in cluster studies

After the completion of the catalogue built with ESO key program (REFLEX), we are beginning to explore the multiband optical characteristics of a subsample of those clusters selected in a statistically independent way. We have already observed in B, V, R, 9 of the about 50 clusters of the subsample, with 0.15 \le z \le 0.18. More time has been allocated by ESO for our project in March 2001. We will therefore be able to compare X-ray and optical morphologies and luminosities and, thanks to the large field of view of the WFI instrument, assess the luminosity function and the color segregation in the cluster on a robust statistical basis. In this poster we present the project, the technical problems we are facing in the reduction phase, and show the first preliminary results, which seem very encouraging.

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arXiv2000

The ROSAT-ESO Flux Limited X-ray (REFLEX) Galaxy Cluster Survey II: The Spatial Correlation Function

We report the results of the spatial two-point correlation function xi(r) for the new X-ray galaxy cluster survey REFLEX, which comprises of 452 X-ray selected clusters (449 with redshifts) detected by the ROSAT satellite during the ROSAT All-Sky-Survey. The REFLEX cluster sample is flux limited to 3x10^{-12} erg s^{-1} cm^{-2} in the ROSAT energy band (0.1-2.4 keV) and spans 3 decades in X-ray luminosity (10^{42} - 10^{45} h^{-2} erg s^{-1}), containing galaxy groups and rich clusters out to a redshift z~0.3. Covering a contiguous area of 4.24 sr REFLEX is the largest X-ray cluster sample to date for which spatial clustering has been analysed. Correlation studies using clusters selected on the basis of their X-ray emission are particularly interesting as they are largely free from the projection biases inherent to optical studies. For the entire flux-limited sample we find that the correlation length (the scale at which the correlation amplitude passes through unity) r_0~20 h^{-1}Mpc. For example, if a power-law fit is made to xi(r) over the range 4-40 h^{-1} Mpc then r_0=18.8+-0.9. An indication of the robustness of this result comes from the high degree of isotropy seen in the clustering pattern on scales close to the correlation length. On larger scales xi(r) deviates from a power-law, crossing zero at ~45h^{-1} Mpc. From an examination of 5 volume-limited cluster sub-samples we find no significant trend of r_0 with limiting X-ray luminosity. A comparison with recent model predictions for the clustering properties of X-ray flux-limited samples, indicates that Cold Dark Matter models with the matter density Omega_m=1 fail to produce sufficient clustering to account for the data, while Omega_m~0.3 models provide an excellent fit.

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