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Hans Boehringer

Publications and source records attributed to Hans Boehringer.

At least 55 records · Page 3Linked to original sources
arXiv2009

X-ray Spectroscopy of Galaxy Clusters

Galaxy clusters, the largest clearly defined objects in our Universe, are ideal laboratories to study in detail the cosmic evolution of the intergalactic intracluster medium (ICM) and the cluster galaxy population. For the ICM, which is heated to X-ray radiating temperatures, X-ray spectroscopy is the most important tool to obtain insight into the structure and astrophysics of galaxy clusters. In this review we recall the basic principles of the interpretation of X-ray spectra from a hot, tenuous plasma and we illustrate the wide range of scientific applications of X-ray spectroscopy. The determination of galaxy cluster masses, important for cosmology, rest crucially on a precise spectroscopic ICM temperature determination. The study of the thermal structure of the ICM provides a very interesting fossil record of the energy release during galaxy formation and evolution. The temperature and pressure distribution of the ICM gives us important insight into the process of galaxy cluster merging and the dissipation of the merger energy in form of turbulent motion. Cooling cores in the centers are interesting laboratories to investigate the interplay between gas cooling, star- and black hole formation and energy feedback, which is diagnosed by means of X-ray spectroscopy. The element abundances deduced from X-ray spectra of the ICM provide a cosmic history record of the contribution of different supernovae to the nucleosynthesis of heavy elements and their spatial distribution partly reflects important transport processes in the ICM. Some discussion of plasma diagnostics for conditions out of thermal equilibrium and an outlook on the future prospects of X-ray spectroscopic cluster studies complete our review.

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arXiv2009

Multi-wavelength observations of a rich galaxy cluster at z ~ 1: the HST/ACS colour-magnitude diagram

XMMU J1229+0151 is a rich galaxy cluster with redshift z=0.975, that was serendipitously detected in X-rays within the scope of the XMM-Newton Distant Cluster Project. HST/ACS observations in the i775 and z850 passbands, as well as VLT/FORS2 spectroscopy were further obtained, in addition to follow-up Near-Infrared (NIR) imaging in J- and Ks-bands with NTT/SOFI. We investigate the photometric, structural and spectral properties of the early-type galaxies in the high-redshift cluster XMMU J1229+0151. Source detection and aperture photometry are performed in the optical and NIR imaging. Galaxy morphology is inspected visually and by means of Sersic profile fitting to the 21 spectroscopically confirmed cluster members in the ACS field of view. The i775-z850 colour-magnitude relation (CMR) is derived with a method based on galaxy magnitudes obtained by fitting the surface brightness of the galaxies with Sersic models. The i775-z850 CMR of the spectroscopic members shows a very tight red-sequence with a zero point of 0.86+-0.04 mag and intrinsic scatter equal to 0.039 mag. The CMR obtained with the galaxy models has similar parameters. Stellar masses and formation ages of the cluster galaxies are derived by fitting the observed spectral energy distributions (SED) with models based on Bruzual & Charlot 2003. We obtain a star formation weighted age of 4.3 Gyr for a median mass of 7.4e10 Msun. Instead of an unambiguous brightest cluster galaxy (BCG), we find three bright galaxies with a similar z850 magnitude, which are, in addition, the most massive cluster members, with ~ 2e11 Msun. Our results strengthen the current evidence for a lack of significant evolution of the scatter and slope of the red-sequence out to z~1.

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arXiv2008

Alignment of Galaxies and Clusters

We investigated the influence of environment on cluster galaxies by examining the alignment of the brightest cluster galaxy (BCG) position angle with respect to the host cluster X-ray position angle. The cluster position angles were measured using high spatial resolution X-ray data taken from the Chandra ACIS archive, that significantly improved the determination of the cluster shape compared to the conventional method of using optical images. Meanwhile, those of the BCGs were measured using homogeneous dataset composed of high spatial resolution optical images taken with Suprime-Cam mounted on Subaru 8m telescope. We found a strong indication of an alignment between the cluster X-ray emission and optical light from BCGs, while we see no clear direct correlation between the degree of ellipticity of X-ray and optical BCG morphologies, despite the apparent alignment of two elliptical structures. We have also investigated possible dependence of the position angle alignment on the X-ray morphology of the clusters, and no clear trends are found. The fact that no trends are evident regarding frequency or degree of the alignment with respect to X-ray morphology may be consistent with an interpretation as a lack of dependence on the dynamical status of clusters.

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arXiv2008

Searching for Cool Core Clusters at High redshift

We investigate the detection of Cool Cores (CCs) in the distant galaxy cluster population, with the purpose of measuring the CC fraction out to redshift 0.7 < z < 1.4. Using a sample of nearby clusters spanning a wide range of morphologies, we define criteria to characterize cool cores, which are applicable to the high redshift sample. We analyzed azimuthally averaged surface brightness (SB) profiles using the known scaling relations and fitted single/double beta models to the data. Additionally, we measured a surface brightness concentration, c_SB, as the ratio of the peak over the ambient SB. To verify that this is an unbiased parameter as a function of redshift, we developed a model independent "cloning" technique to simulate the nearby clusters as they would appear at the same redshifts and luminosities as those in the distant sample. A more physical parameterization of CC presence is obtained by computing the cooling time at a radius of 20 kpc from the cluster center. The distribution of the SB concentration and the stacked radial profiles of the low-z sample, combined with published information on the CC properties of these clusters, show 3 degrees of SB cuspiness: non-CC, moderate and strong CC. The same analysis applied to the high-z clusters reveals two regimes: non-CC and moderate CC. The cooling time distribution corroborates this result by showing a strong negative correlation with c_SB. Our analysis indicates a significant fraction of distant clusters harboring a moderate CC out to z=1.4, similar to those found in the local sample. The absence of strong cooling which we report is likely linked with a higher merger rate expected at redshift z > 0.7, and should also be related with the shorter age of distant clusters, implying less time to develop a cool core.

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arXiv2006

Robust quantitative measures of cluster X-ray morphology, and comparisons between cluster characteristics

To investigate the relationships between dynamical status and other important characteristics of galaxy clusters, we conducted a study of X-ray cluster morphology using a sample of 101 clusters at redshift z=0.05-1 taken from the Chandra archive. The X-ray morphology is quantitatively characterized by a series of objectively measured simple statistics of the X-ray surface brightness distribution, which are designed to be robust against variations of image quality caused by various exposure times and various cluster redshifts. We found: (1) The distorted and non-distorted clusters occupy well-defined loci in the L-T plane, demonstrating the measurements of the global luminosity and temperature for distorted clusters should be interpreted with caution, or alternatively, a rigorous morphological characterization is necessary when we use a sample of clusters with heterogeneous morphological characteristics to investigate the L-T or other related scaling relations. (2) Ellipticity and Off-center show no evolutionary effects between high and low redshift cluster subsets, while there may be a hint of weak evolutions for the Concentration and Asymmetry, in such a way that high-z clusters show more distorted morphology. (3) No correlation is found between X-ray morphology and X-ray luminosity or X-ray morphology and X-ray temperature of clusters, implying that interaction of clusters may not enhance or decrease the luminosity or temperature of clusters for extended period of time.

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arXiv2006

The abundance pattern of O, Mg, Si and Fe in the intracluster medium of the Centaurus cluster observed with XMM-Newton

The abundances of O, Mg, Si and Fe in the intracluster medium of the Centaurus cluster are derived. The Fe abundance has a negative radial gradient. In solar units, the Si abundance is close to the Fe abundance, while the O and Mg abundances are much smaller. The high Fe/O and Si/O ratio indicate that metal supply from supernovae Ia is important and supernovae Ia synthesize Si as well as Fe. Within 2$'$, the O and Mg abundances are consistent with the stellar metallicity of the cD galaxy derived from the Mg$_2$ index. This result indicates that the central gas is dominated by the gas from the cD galaxy. The observed abundance pattern of the Centaurus cluster resembles to those observed in center of other clusters and groups of galaxies. However, the central Fe abundance and the Si/Fe ratio are 40 % higher and 30% smaller than those of M 87, respectively. Since the accumulation timescale of the supernovae Ia is higher in the Centaurus cluster, these differences imply a time dependence of nucleosynthesis by supernovae Ia.

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arXiv2005

The ROSAT North Ecliptic Pole Survey: The X-ray Catalog

The sky around the North Ecliptic Pole (NEP), at $α$(2000) = 18$^h00^m00^s$, $δ$(2000) = +66\degr33\arcmin39\arcsec, has the deepest exposure of the entire {\it ROSAT} All - Sky Survey (RASS). The NEP is an undistinguished region of moderate Galactic latitude, $b=29\fdg8$, and hence suitable for compiling statistical samples of both galactic and extragalactic objects. We have made such a compilation in the 80.6 deg$^2$ region surrounding the NEP. Our sample fully exploits the properties of the RASS, since the only criteria for inclusion are source position and significance, and yields the deepest large solid angle contiguous sample of X-ray sources to date. We find 442 unique sources above a flux limit $\mathrm{\sim2\times10^{-14} ~ergs cm^{-2} ~s^{-1}}$ in the 0.5--2.0 keV band. In this paper we present the X-ray properties of these sources as determined from the RASS. These include positions, fluxes, spectral information in the form of hardness ratios, and angular sizes. Since we have performed a comprehensive optical identification program we also present the average X-ray properties of classes of objects typical of the X-ray sky at these flux levels. We discuss the use of the RASS to find clusters of galaxies based on their X-ray properties alone.

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arXiv2005

Cross Correlations of X-ray and Optically Selected Clusters With Near Infrared and Optical Galaxies

We compute the real-space cluster-galaxy cross-correlation xi_cg(r) using the ROSAT-ESO Flux Limited X-ray (REFLEX) cluster survey, a group catalogue constructed from the final version of the 2dFGRS, and galaxies extracted from 2MASS and APM surveys. This first detailed calculation of the cross-correlation for X-ray clusters and groups, is consistent with previous works and shows that xi_cg(r) can not be described by a single power law. We analyse the clustering dependence on the cluster X-ray luminosity L_X and virial mass M_vir thresholds as well as on the galaxy limiting magnitude. We also make a comparison of our results with those obtained for the halo-mass cross-correlation function in a LambdaCDM N-body simulation to infer the scale dependence of galaxy bias around clusters. Our results indicate that the distribution of galaxies shows a significant anti-bias at highly non-linear small cluster-centric distances (b_cg(r) ~ 0.7), irrespective of the group/cluster virial mass or X-ray luminosity and galaxy characteristics, which show that a generic process controls the efficiency of galaxy formation and evolution in high density regions. On larger scales b_cg(r) rises to a nearly constant value of order unity, the transition occuring at approximately 2 Mpch^(-1) for 2dF groups and 5 Mpch^(-1) for REFLEX clusters.

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arXiv2005

The C4 Clustering Algorithm: Clusters of Galaxies in the Sloan Digital Sky Survey

We present the "C4 Cluster Catalog", a new sample of 748 clusters of galaxies identified in the spectroscopic sample of the Second Data Release (DR2) of the Sloan Digital Sky Survey (SDSS). The C4 cluster--finding algorithm identifies clusters as overdensities in a seven-dimensional position and color space, thus minimizing projection effects which plagued previous optical clusters selection. The present C4 catalog covers ~2600 square degrees of sky with groups containing 10 members to massive clusters having over 200 cluster members with redshifts. We provide cluster properties like sky location, mean redshift, galaxy membership, summed r--band optical luminosity (L_r), velocity dispersion, and measures of substructure. We use new mock galaxy catalogs to investigate the sensitivity to the various algorithm parameters, as well as to quantify purity and completeness. These mock catalogs indicate that the C4 catalog is ~90% complete and 95% pure above M_200 = 1x10^14 solar masses and within 0.03 <=z <= 0.12. The C4 algorithm finds 98% of X-ray identified clusters and 90% of Abell clusters within 0.03 <= z <= 0.12. We show that the L_r of a cluster is a more robust estimator of the halo mass (M_200) than the line-of-sight velocity dispersion or the richness of the cluster. L_r. The final SDSS data will provide ~2500 C4 clusters and will represent one of the largest and most homogeneous samples of local clusters.

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arXiv2004

RASS-SDSS galaxy cluster survey. II. A unified picture of the Cluster Luminosity Function

We constructed the composite luminosity function (LF) of clusters of galaxies in the five SDSS photometric bands u,g,r,i and z from the RASS-SDSS galaxy cluster catalog. Background and foreground galaxies are subtracted using both a local and a global background correction to take in account the presence of large scale structures and variations from field to field, respectively. The composite LF clearly shows two components: a bright-end LF with a classical slope of -1.25 in each photometric band, and a faint-end LF much steeper (-2.1 <= α<= -1.6) in the dwarf galaxy region. The observed upturn of the faint galaxies has a location ranging from -16 +5log(h) in the g band to -18.5 +5log(h) in the z band. To study the universality of the cluster LF we compare the individual cluster LFs with the composite luminosity function. We notice that, in agreement with the composite LF, a single Schechter component is not a good fit for the majority of the clusters. We fit a Schechter function to the bright-end of the individual clusters LFs in the magnitude region brighter than the observed upturn of the dwarf galaxies. We observe that the distributions of the derived parameters is close to a Gaussian around the value of the composite bright-end LF parameters with a dispersion compatible with the statistical errors. We conclude that the bright-end of the galaxy clusters is universal. To study the behavior of the individual faint-end LF we define the Dwarf to Giant galaxy Ratio (DGR) of the single clusters. We notice that the distribution of DGR has a spread much larger than the statistical errors. Our conclusion is that the cluster luminosity function is not universal since the cluster faint-end, differently from the bright-end, varies from cluster to cluster.

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arXiv2004

X-ray Measurement of Dark Matter "Temperature" in Abell 1795

We present a method from an X-ray observation of a galaxy cluster to measure the radial profile of the dark matter velocity dispersion, sigma_DM, and to compare the dark matter ``temperature'' defined as mu m_p sigma_DM**2 / k_B with the gas temperature. The method is applied to the XMM-Newton observation of Abell 1795. The ratio between the specific energy of the dark matter and that of the intracluster medium (ICM), which can be defined as beta_DM in analogy with beta_spec, is found to be less than unity everywhere ranging \~0.3-0.8. In other words, the ICM temperature is higher than the dark matter ``temperature'', even in the central region where the radiative cooling time is short. A beta_DM value smaller than unity can most naturally be explained by heating of the ICM. The excess energy of ICM is estimated to be ~1-3 keV per particle.

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arXiv2004

Detection of X-ray Clusters of Galaxies by Matching RASS Photons and SDSS Galaxies within GAVO

A new method for a simultaneous search for clusters of galaxies in X-ray photon maps and optical galaxy maps is described. The merging of X-ray and optical data improves the source identification so that a large amount of telescope time for spectroscopic follow-up can be saved. The method appears thus ideally suited for the analysis of the recently proposed wide-angle X-ray missions like DUO and ROSITA. As a first application, clusters are extracted from the 3rd version of the ROSAT All-Sky Survey and the Early Date Release of the Sloan Digital Sky Survey (SDSS). The time-consuming computations are performed within the German Astrophysical Virtual Observatory (GAVO). On a test area of 140 square degrees, 75 X-ray clusters are detected down to an X-ray flux limit of $3-5\times 10^{-13} {\rm erg} {\rm s}^{-1} {\rm cm}^{-2}$ in the ROSAT energy band 0.1-2.4 keV. The clusters have redshifts $z\le 0.5$. The survey thus fills the gap between traditional large-area X-ray surveys and serendipitous X-ray cluster searches based on pointed observations, and has the potential to yield about 4,000 X-ray clusters after completion of SDSS.

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arXiv2004

Chemical and Thermal History of the Intracluster Medium

Clusters of galaxies can be seen as giant astrophysical laboratories enclosing matter in a large enough volume, so that the matter composition can be taken as representing the composition of our Universe. X-ray observations allow a very precise investigation of the physical properties of the intracluster plasma allowing us to probe probe the cluster structure, determine its total mass, and measure the baryon fraction in clusters and in the Universe as a whole. We can determine the abundance of heavy elements from O to Ni which originate from supernova explosions and draw from this important conclusions on the history of star formation in the cluster galaxy population. From the entropy structure of the intracluster medium we obtain constraints on the energy release during early star bursts. With the observational capabilities of the X-ray observatories XMM-Newton and Chandra this field of research is rapidly evolving. In particular, first detailed observations of the intracluster medium of the Virgo cluster around M87 have provided new insights. The present contribution gives an account of the current implications of the intracluster medium observations, but more importantly illustrates the prospects of this research for the coming years.

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arXiv2004

Abundance Constraints and Direct Redshift Measurement of the Diffuse X-ray Emission from a Distant Cluster of Galaxies

We report on the XMM-Newton (XMM) observation of RXJ1053.7+5735, one of the most distant X-ray selected clusters of galaxies, which also shows an unusual double-lobed X-ray morphology, indicative of a possible equal-mass cluster merger. The cluster was discovered during the ROSAT deep pointings in the direction of the Lockman Hole. All XMM Lockman Hole observations (PV, AO-1 & AO-2 phases) with the European Photon Imaging Camera (EPIC) were combined for the analysis, totaling exposure times ~ 648 ks, 738 ks, and 758 ks for pn, MOS1, and MOS2, respectively. With this 'deep' dataset, we could detect the Fe K line and obtain a strong constraint on cluster metallicity, which is difficult to achieve for clusters at z > 1. The best-fit abundance is 0.46 +0.11/-0.07 times the solar value. The Fe line emission also allows us to directly estimate the redshift of diffuse gas, with a value z = 1.14 +0.01/-0.01. This is one of the first clusters whose X-ray redshift is directly measured prior to the secure knowledge of cluster redshift by optical/NIR spectroscopy. We could also estimate the X-ray redshift separately for each of the two lobes in the double-lobed structure, and the result is consistent with the two lobes being part of one cluster system at the same redshift. Comparison with other metallicity measurements of nearby and distant clusters shows that there is little evolution in the ICM metallicity from z ~ 1 to the present.

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arXiv2002

The ROSAT-ESO Flux-Limited X-Ray (REFLEX) Galaxy Cluster Survey VI: Constraints on the cosmic matter density from the KL power spectrum

The Karhunen-Loéve (KL) eigenvectors and eigenvalues of the sample correlation matrix are used to analyse the spatial fluctuations of the REFLEX clusters of galaxies. The method avoids the disturbing effects of correlated power spectral densities which affects all previous cluster measurements on Gpc scales. Comprehensive tests use a large set of independent REFLEX-like mock cluster samples extracted from the Hubble Volume Simulation. It is found that unbiased measurements on Gpc scales are possible with the REFLEX data. The distribution of the KL eigenvalues are consistent with a Gaussian random field on the 93.4% confidence level. Assuming spatially flat cold dark matter models, the marginalization of the likelihood contours over different sample volumes, fiducial cosmologies, mass/X-ray luminosity relations and baryon densities, yields the 95.4% confidence interval for the matter density of $0.03<Ω_mh^2<0.19$. The N-body simulations show that cosmic variance, although difficult to estimate, is expected to increase the confidence intervals by about 50%.

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arXiv2001

A New Measurement of the X-ray Temperature Function of Clusters of Galaxies

We present a newly measured X-ray temperature function of galaxy clusters using a complete flux-limited sample of 61 clusters. The sample is constructed with the total survey area of 8.14 steradians and the flux limit of 1.99e-11 ergs/s/cm^2 in the 0.1-2.4keV band. X-ray temperatures and fluxes of the sample clusters were accurately measured with ASCA and ROSAT data. The derived temperature function covers an unprecedentedly wide temperature range of 1.4-11keV. By fitting these data with theoretically predicted temperature functions given by the Press-Schechter formalism together with a recent formation approximation and the CDM power spectrum, we obtained tight and individual constraints on Omega_m,0 and sigma_8. We also employed the Formation-Epoch model in which the distribution in the formation epoch of clusters as well as the temperature evolution are taken into account, showing significantly different results. Systematics caused by the uncertainty in the mass-temperature relation are studied and found to be as large as the statistical errors.

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arXiv2001

The Mass Function of an X-Ray Flux-Limited Sample of Galaxy Clusters

A new X-ray selected and X-ray flux-limited galaxy cluster sample is presented. Based on the ROSAT All-Sky Survey the 63 brightest clusters with galactic latitude |bII| >= 20 deg and flux fx(0.1-2.4 keV) >= 2 * 10^{-11} ergs/s/cm^2 have been compiled. Gravitational masses have been determined utilizing intracluster gas density profiles, derived mainly from ROSAT PSPC pointed observations, and gas temperatures, as published mainly from ASCA observations, assuming hydrostatic equilibrium. This sample and an extended sample of 106 galaxy clusters is used to establish the X-ray luminosity--gravitational mass relation. From the complete sample the galaxy cluster mass function is determined and used to constrain the mean cosmic matter density and the amplitude of mass fluctuations. Comparison to Press--Schechter type model mass functions in the framework of Cold Dark Matter cosmological models and a Harrison--Zeldovich initial density fluctuation spectrum yields the constraints OmegaM = 0.12^{+0.06}_{-0.04} and sigma8 = 0.96^{+0.15}_{-0.12} (90% c.l.). Various possible systematic uncertainties are quantified. Adding all identified systematic uncertainties to the statistical uncertainty in a worst case fashion results in an upper limit OmegaM < 0.31. For comparison to previous results a relation sigma8 = 0.43 OmegaM^{-0.38} is derived. The mass function is integrated to show that the contribution of mass bound within virialized cluster regions to the total matter density is small, i.e., OmegaCluster = 0.012^{+0.003}_{-0.004} for cluster masses larger than 6.4^{+0.7}_{-0.6} * 10^{13} h_{50}^{-1} Msun.

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