X-ray Surface Brightness Distribution Research Articles

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 automated morphology analysis is based on data of the 3rd processing of the ROSAT All-Sky survey (RASS-3). After correction for several systematic eects, 52 7 percent of the REFLEX+BCS clusters are found to be substructured in metric apertures of 1 Mpc radius (H0 =5 0 km s 1 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 analysis 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 dierent \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 ndings 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.

Temperature Map and Iron Abundance of the Ophiuchus Cluster of Galaxies

Abstract We present an X-ray surface-brightness distribution and a temperature map of the Ophiuchus cluster of galaxies observed by ASCA. The Ophiuchus cluster is a nearby rich cluster located at $ z= 0.028$ having an angular extent of $\sim 1 {{}^{\circ }}$. The observations were made by five pointings that cover the whole cluster. After deconvolution with the ASCA complex response function, we obtained the surface-brightness distribution with an angular resolution of $ {2{}^{\mathrm {\prime }}}$ and a temperature map with $ {8{}^{\mathrm {\prime }}}$ resolution. The X-ray surface-brightness distribution shows a nearly spherically symmetric distribution, except for a $ 15\sigma$ image excess at the position of $ {10{}^{\mathrm {\prime }}}$ to the north of the cluster center. On the other hand, a hot region with a temperature $ > 13 \,\mathrm{keV}$ is spread over a region of $ {20{}^{\mathrm {\prime }}}\times {30{}^{\mathrm {\prime }}}$ at $ {20{}^{\mathrm {\prime }}}$ to the west of the cluster center. Because these characteristics are similar to those of the Coma cluster, we conclude this cluster has never relaxed dynamically, and is also recently experienced a merger.

The X‐Ray Surface Brightness Distribution from Diffuse Gas

We use simulations to predict the X-ray surface brightness distribution arising from hot, cosmologically distributed diffuse gas. The distribution is computed for two bands, 0.5-2 keV and 0.1-0.4 keV, using a cosmological-constant-dominated cosmology that fits many other observations. We examine a number of numerical issues such as resolution, simulation volume, and pixel size and show that the predicted mean background is sensitive to resolution such that higher resolution systematically increases the mean predicted background. Although this means that we can compute only lower bounds to the predicted level, these bounds are already quite restrictive. Since the observed extragalactic X-ray background is mostly accounted for by compact sources, the amount of the observed background attributable to diffuse gas is tightly constrained. We show that without physical processes in addition to those included in the simulations (such as radiative cooling or nongravitational heating), both bands exceed observational limits. In order to examine the effect of nongravitational heating we explore a simple model of energy injection and show that if this is the only mechanism operating to suppress the background then substantial amounts of heating would be required (i.e., 5 keV per particle when averaged over all baryons). Finally, we also compute the distribution of surface brightness on the sky and show that it has a well-resolved characteristic shape. This shape is substantially modified by nongravitational heating and can be used as a probe of such energy injection.

On the Distribution of X-Ray Surface Brightness from Diffuse Gas

Hot intergalactic gas in clusters, groups, and filaments emanates a continuous background of 0.5-2.0 keV X-rays that ought to be detectable with the new generation of X-ray observatories. Here we present selected results from a program to simulate the surface brightness distribution of this background with an adaptive mesh cosmological hydrodynamics code. We show that the bright end of this distribution is well approximated by combining the cluster temperature function with a β-model for surface brightness and appropriate luminosity-temperature and core radius-luminosity relations. Our simulations verify that the X-ray background from hot gas vastly exceeds observational limits if nongravitational processes do not modify the intergalactic entropy distribution. An entropy floor ~100 keV cm2, which could be established by either heating or cooling, appears necessary to reconcile the simulated background with observations. Because the X-ray background distribution is so sensitive to the effects of nongravitational processes, it offers a way to constrain the thermal history of the intergalactic medium that is independent of the uncertainties associated with surveys of clusters and groups.

Robustness of the Sunyaev-Zel'dovich Effect as aMeasure of the Hubble Constant

After inversing the x-ray surface brightness of the cooling flow clusterAbell 1689 through the isothermal β model, the isothermal double βmodel and the x-ray surface brightness distribution model derived from theuniversal dark matter density profile, we calculatethe electron distribution and the Sunyaev-Zel'dovich (SZ) effect of the cluster.The resulting temperature decrements,which are inversely proportional to the square root of H0,are very different for the three models, with an uncertainty of 15%.Thus, the SZ/x-ray route cannot be taken as a precise measurementof H0 until further observations (e.g. from the satellite AXAF)constrain well the amount and distribution of cluster gas.

Emission Mechanisms in X‐Ray–faint Galaxies

Hot gas dominates the emission in X-ray-luminous early-type galaxies, but in relatively X-ray-faint systems, integrated X-ray emission from discrete stellar-like sources is thought to be considerable, although the amount of the contribution is controversial. To help resolve this issue, we examine the radial X-ray surface brightness distribution of 17 X-ray-faint galaxies observed with the ROSAT HRI and PSPC. We assume that the stellar contribution follows a de Vaucouleurs law while the hot gas component follows a King β model. For some galaxies, both models fit equally well, but for a number of systems, a dual component model yields the best fit, from which upper bounds are placed on the stellar contribution. Best-fit values for the stellar contribution are inconsistent with (lower than) that suggested by Fabbiano, Gioia, & Trinchieri and estimated from the bulge of M31 but are consistent with the Forman, Jones, & Tucker estimate of the stellar fraction in X-ray-faint elliptical and S0 galaxies. Our results indicate an upper limit to discrete sources of LX/LB = 1.6 × 1029 ergs s-1 L.

Searching for cluster substructure using APM and ROSAT data

We present a detailed study of the morphological features of 22 rich galaxy clusters. Our sample is constructed from a cross-correlation of optical (Abell + APM) data with X-ray (0.1–2.4 keV) ROSAT pointed observations. We systematically compare cluster images and morphological parameters in an attempt to reliably identify possible substructure in both optical and the X-ray images. To this end, we compute various moments of the optical and X-ray surface-brightness distribution such as the ellipticities, centre-of-mass shifts and ellipsoidal orientations. We assess the significance of our results using Monte Carlo simulations. We find significant correlations between the optical and X-ray morphological parameters, indicating that in both parts of the spectrum it is possible to identify correctly the dynamical state of a cluster. Most of our clusters (17/22) have a good one-to-one correspondence between the optical and the X-ray images, and about 10 appear to have strong indications of substructure. This corresponds to a minimum percentage of order ∼45 per cent, which is in very good accordance with other similar analyses. Finally, five out of 22 systems (∼22 per cent) seem to have distinct subclumps in the optical which are not verified in the X-ray images, and thus are suspect of being due to optical projection effects. These results will serve as a useful guide in interpreting subsequent analyses of large optical cluster catalogues.

Radio study of X-ray clusters of galaxies--II. A1300: a post-merger cluster at z=0.31 containing a halo and a relic

As part of an extensive radio—IR—optical—X-ray study of ROSAT clusters of galaxies in the Hydra region we have observed the Abell cluster A1300, one of the most distant (z≃ 0.31) and X-ray luminous (LX∼ 1.7 × 1045 erg s−1 cm−2, 0.1–2.4 keV) clusters in the sample, with the Molonglo Observatory Synthesis Telescope at 843 MHz and the Australia Telescope Compact Array at 1.34, 2.4, 4.8 and 8.6 GHz. Previous results in the X-ray and optical have suggested that A1300 is in a post-merging phase. We present further evidence of this with the discovery of two diffuse radio sources projected within the extended X-ray emission of A1300. These may be among the most distant radio halo and relic sources yet found. We argue that the halo source, which is seen in projection near the centre of the cluster and coincides with an extension in the X-ray surface brightness distribution, is the result of a cluster-wide process related to the merger.

Cluster Mass Estimate and a Cusp of the Mass‐Density Distribution in Clusters of Galaxies

We study density cusps in the center of clusters of galaxies in order to reconcile X-ray mass estimates with gravitational lensing masses. For various mass-density models with cusps, we compute X-ray surface brightness distributions and fit them to observations to measure the range of parameters in the density models. The Einstein radii estimated from these density models are compared with Einstein radii derived from the observed arcs for Abell 2163, Abell 2218, and RX J1347.5-1145. The X-ray masses and lensing masses corresponding to these Einstein radii are also compared. While steeper cusps give smaller ratios of lensing mass to X-ray mass, the X-ray surface brightnesses estimated from flatter cusps are better fits to the observations. For Abell 2163 and Abell 2218, although the isothermal sphere with a finite core cannot produce giant arc images, a density model with a central cusp can produce a finite Einstein radius that is smaller than the observed radii. We find that a total mass-density profile that declines as ~r-1.4 produces the largest radius in models that are consistent with the X-ray surface brightness profile. As a result, the extremely large ratio of the lensing mass to the X-ray mass is improved from 2.2 to 1.4 for Abell 2163 and from 3 to 2.4 for Abell 2218. For RX J1347.5-1145, which is a cooling flow cluster, we cannot reduce the mass discrepancy.

Physical properties of numerical clusters

As part of an on-going study of numerical simulations of large-scale structure formation we present an initial analysis of the physical properties, such as X-ray surface brightness distributions and weak lensing maps, of galaxy clusters. We focus on a recent high-resolution simulation of a “Virgo-sized” cluster.

The relationship between cooling flows and metallicity measurements for X-ray-luminous clusters

ABSTRA C T We explore the relationship between the metallicity of the intracluster gas in clusters of galaxies, determined by X-ray spectroscopy, and the presence of cooling flows. Using ASCA spectra and ROSAT images, we demonstrate a clear segregation between the metallicities of clusters with and without cooling flows. On average, cooling-flow clusters have an emissionweighted metallicity a factor ,1:8 times higher than that of non-cooling-flow systems. We suggest that this is caused by the presence of metallicity gradients in the cooling-flow clusters, coupled with the sharply peaked X-ray surface brightness profiles of these systems. Noncooling-flow clusters have much flatter X-ray surface brightness distributions and are thought to have undergone recent merger events, which may have mixed the central high-metallicity gas with the surrounding less metal-rich material. We find no evidence for evolution in the emission-weighted metallicities of clusters within z , 0:3.

Anatomy of a Merger: A Numerical Model of A754

A754 is a well-observed cluster of galaxies (z = 0.054) that exhibits a variety of morphological peculiarities. These include a bar of X-ray emission that is offset significantly from the galaxy distribution, an elongated X-ray surface brightness distribution extending between two distinct peaks in the galaxy distribution, and an extremely nonisothermal and asymmetric intracluster medium temperature morphology. Using these observational constraints, we present a numerical hydro/N-body model of A754 in which two clusters (2.5:1 mass ratio) have merged nearly in the plane of the sky less than 0.5 Gyr ago with an impact parameter of ~120 kpc and an impact velocity of ~2500 km s-1 (roughly the escape velocity of the primary cluster). Our models allow us to identify the origin of A754's peculiar X-ray and temperature morphologies, the underlying hydrodynamical processes that shape them, and their future evolution. We make detailed predictions for future high-resolution X-ray spectroscopic observations (e.g., Astro-E). We discuss general properties of our models that will be characteristic of off-axis mergers. In particular, we find significant nonthermal pressure support within the central region that could bias cluster mass estimates. We find significant angular momentum imparted on the gas distribution in the cluster. We find that mixing of the subcluster gas components is an inefficient process, particularly at large radii. Finally, we find that subsequent dark matter core passages result in an extended relaxation timescale.

Measures of Galactic and Intergalactic Mass in Clusters

If a galaxy cluster's X-ray gas distribution follows an isothermal polytropic β model, we may write the electron radial density distribution as; ne = ne0(1 + r2/rc2)–3/2β, rc being the core radius and ne0 the central electron density. This may be related to both an X-ray surface brightness distribution and a Sunyaev-Zel'dovich effect distribution (Sarazin 1986). Fitting to observational data then enables us to constrain the value of β. The normalisation value, ne0, to obtain a total mass estimate is calculated via the relationship between the X-ray and S-Z distribution normalisation constants, and the gas temperature and spectral emissivity parameters from fits to the X-ray spectrum. We are then in a position to evaluate ne(r) and its integral; the total electron gas mass. If we can further assume that there exists a simple ratio between the electron and proton number densities within the gas, we may straightforwardly posit a value for the total gas mass. An additional method of determining the polytropic gas index exists, with optical constraints on the galactic velocity dispersion, through the relation; β = μmHσz2/kBTe. Studies at optical, as well as X-ray and radio wavelengths are thus useful as a corroborative measure in determining the total gas mass.

Temperature Structure in Merging Coma Cluster of Galaxies

The Coma cluster has been recognized as an archetype of rich and relaxed clusters, until recent ROSAT observations reveal that the intracluster medium (ICM) has a complex distribution (Briel et al. 1992; White et al. 1993). The X-ray surface brightness distribution shows a secondary peak around the galaxy NGC 4839, at 40' SW from the cluster center.

Butcher & Oemler cluster A 2111: a head-on merger at z = 0.23

We present ROSAT PSPC and HRI observations of A2111, the richest galaxy cluster photometrically surveyed by Butcher & Oemler (1984). The cluster contains a distinct comet-shaped X-ray subcomponent, which appears hotter than the rest of the cluster. The galaxy and X-ray surface brightness distributions of the cluster show a similarly elongated morphology. These results suggest that the cluster is undergoing a head-on subcluster merger. This merger may also be responsible for the high fraction of gas-rich blue galaxies observed in the cluster. We have further detected a poor cluster along the merging axis and at a projected distance of about 5 Mpc from A2111.

Baryon Fractions for Poor Clusters of Galaxies

view Abstract Citations (33) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Baryon Fractions for Poor Clusters of Galaxies dell'Antonio, Ian P. ; Geller, Margaret J. ; Fabricant, Daniel G. Abstract We analyze x ray and optical data for six poor clusters of galaxies (MKW4, AWM4, AWM7, MKW4S, N79-299, and N67-336). For four of the systems, AWM4, AWM7, MKW4, and MKW4S, the x-ray emission is regular enough to measure the virial mass, gas mass, and temperature from the x-ray emission. We have obtained digitized plate scans for five of the poor clusters. We derive positions and photographic magnitudes for galaxies in a 30'x 30' region around each group center (1.5^deg^ x 1.5^deg^ for AWM7). From the x-ray surface brightness distribution, we construct models of the mass distribution. We use a modified V/V_max_ test [Avni & Bahcall, ApJ, 27,446(1980)] to test whether the galaxies trace the potential marked by the x-ray emitting gas. The galaxy distribution is consistent with the density distribution inferred from the x rays. The mass in galaxies is ~3h^-1^% of the total mass of the systems. The galaxies contribute significantly to the baryonic mass total. The ratio M_gas_/M_gal_ ~ 1.4 h^-1/2^, similar to the value for rich clusters. The baryon fraction in rich groups is ~0.08 (for H_0_ = 100), or about half that in rich clusters. This result has significant consequences for models of the formation of large-scale structure. Publication: The Astronomical Journal Pub Date: August 1995 DOI: 10.1086/117537 Bibcode: 1995AJ....110..502D Keywords: GALAXIES: ABUNDANCES; GALAXIES: CLUSTERING; GALAXIES: MASS FUNCTION full text sources ADS | data products NED (14) SIMBAD (9)

Resolving the beta-discrepancy for clusters of galaxies

Previous comparisons of optical and X-ray observations of clusters of galaxies have lead to the so-called ``$\beta$ - discrepancy'' that has persisted for the last decade. The standard hydrostatic-isothermal model for clusters predicts that the parameter $\beta_{spec} = \sigma_{r}^{2}/(kT/\mu m_{p})$, which describes the ratio of energy per unit mass in galaxies to that in the gas, should equal the parameter $\beta_{fit}$ (where $\rho_{gas}(r) \propto \rho_{gal}(r)^{\beta_{fit}}$) determined from the X-ray surface brightness distribution. The observations suggest an apparent discrepancy : $\beta_{spec} \sim 1.2$ (i.e., the galaxies are ``hotter'' than the gas) while $\beta_{fit} \sim 0.65$ (i.e., the gas is ``hotter'' and more extended than the galaxies). Here we show that the discrepancy is resolved when the actual observed galaxy distribution in clusters is used, $\rho_{gal}(r) \propto r^{-2.4 \pm 0.2}$, instead of the previously assumed steeper King approximation, $\rho_{gal}(r) \propto r^{-3}$. Using a large sample of clusters, we find best-fit mean values of $\beta_{spec} = 0.94 \pm 0.08$ and $\beta_{fit}^{corrected} = 1.25 \times \beta_{fit} = 0.84 \pm 0.10$. These results resolve the $\beta$ - discrepancy and provide support for the hydrostatic cluster model.

X-Ray Emission from a Simulated Cluster of Galaxies

view Abstract Citations (43) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS X-Ray Emission from a Simulated Cluster of Galaxies Tsai, John C. ; Katz, Neal ; Bertschinger, Edmund Abstract Using the 1993 cluster simulation of Katz & White, we analyze the intracluster medium and investigate the accuracy of the standard hydrostatic method for determining cluster masses. We show that the simulated cluster gas is in hydrostatic equilibrium with a subsonic flow toward the center. Inside a radius of ∼100 kpc, this flow is in a steady state. The cooling time is shorter than a Hubble time within the central 50 kpc. The flow rate is regulated by the gas sink in the middle of the cluster and the PdV work done as the gas flows in, verifying the standard cooling flow scenario. We simulate observations of the cluster using the instrument parameters of the EXOSAT ME detector and the Einstein IPC detector. Even though the intracluster gas is not isothermal, isothermal models of the cluster, excluding regions within 100 kpc of galaxies, fit the EXOSAT X-ray spectra as well as they fit real clusters. The X-ray surface brightness distribution is similar to that of real clusters, again excluding the galaxies. We simulate the procedure used to determine the masses of real clusters. We use the equation of hydrostatic equilibrium together with the temperature derived from an isothermal fit to the simulated EXOSAT spectrum and the density profile derived from a fit to the simulated IPC surface brightness profile to determine the mass. A comparison of the derived mass profile to the actual mass profile shows that errors of a factor of 2 are possible. If the actual temperature profile is used, the cluster mass is found to an accuracy of better than 25% within the virial radius. Publication: The Astrophysical Journal Pub Date: March 1994 DOI: 10.1086/173834 Bibcode: 1994ApJ...423..553T Keywords: GALAXIES: CLUSTERING; GALAXIES: INTERGALACTIC MEDIUM; GALAXIES: COOLING FLOWS; HYDRODYNAMICS; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (3)

An X-ray method for detecting substructure in galaxy clusters - Application to Perseus, A2256, Centaurus, Coma, and Sersic 40/6

We use the moments of the X-ray surface brightness distribution to constrain the dynamical state of a galaxy cluster. Using X-ray observations from the Einstein Observatory IPC, we measure the first moment FM, the ellipsoidal orientation angle, and the axial ratio at a sequence of radii in the cluster. We argue that a significant variation in the image centroid FM as a function of radius is evidence for a nonequilibrium feature in the intracluster medium (ICM) density distribution. In simple terms, centroid shifts indicate that the center of mass of the ICM varies with radius. This variation is a tracer of continuing dynamical evolution. For each cluster, we evaluate the significance of variations in the centroid of the IPC image by computing the same statistics on an ensemble of simulated cluster images. In producing these simulated images we include X-ray point source emission, telescope vignetting, Poisson noise, and characteristics of the IPC. Application of this new method to five Abell clusters reveals that the core of each one has significant substructure. In addition, we find significant variations in the orientation angle and the axial ratio for several of the clusters.

The elongation of Abell clusters. III- Rich clusters and initial conditions

view Abstract Citations (27) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Elongation of Abell Clusters. III. Rich Clusters and Initial Conditions Rhee, George ; van Haarlem, Michael ; Katgert, Peter Abstract We present the results of a study of the elongation and orientation of a sample of 107 rich clusters of galaxies. The data consist of galaxy lists derived from digitised images of the Palomar Sky Survey red plate copies. We work with three measures of the cluster orientation (1) the cluster position angle derived from the Fourier transform of the azimuthal galaxy distribution, (2) the first-ranked galaxy position angle derived from the moments of the digitized galaxy image, and (3) the cluster position angle derived from the x-ray surface brightness distribution [Einstein data]. We have examined the orientation of clusters with respect to their nearest neighbors as a function of distance to the nearest neighbor. The orientation of clusters with respect to members of the same supercluster has also been examined. We find that alignment effects are present for clusters residing in superclusters. We also find that cD galaxies are significantly aligned with the direction to the nearest neighbor cluster. These results are discussed in the context of current ideas on galaxy formation. Publication: The Astronomical Journal Pub Date: June 1992 DOI: 10.1086/116189 Bibcode: 1992AJ....103.1721R Keywords: Galactic Clusters; Galactic Evolution; Astronomical Maps; Sky Surveys (Astronomy); Astrophysics; EARLY UNIVERSE; GALAXIES: DISTANCES AND REDSHIFTS; GALAXIES: CLUSTERING full text sources ADS | data products NED (103) SIMBAD (102)