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Register a new userAbell 2218: X-Ray Lensing, Merger, or Both?
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Authors
Maxim Markevitch
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Comparison of the high resolution X-ray ROSAT HRI image of A2218 with the optical HST image shows several interesting correlations. The X-ray emission within a 1 radius core is resolved into several components; the central dominant galaxy does not coincide with either of them or the emission centroid. The major X-ray peak is an elongated feature which coincides with optical arcs at r=20 from the cD. We speculate that this may be lensed X-ray emission, for example, of the same object lensed in the optical. Alternatively, this feature may be a merger shock, or a gas trail of an infalling subgroup. Two other X-ray enhancements are close to the two major mass concentrations known from the lensing analysis. Both lensing and a merger are likely. Previous X-ray derivations of the A2218 mass used the data with angular resolution that blurred the features mentioned above into a broad constant core. As the HRI data show, such a core does not exist. Because of this, the hydrostatic estimate of the projected mass within the lensing radius can in principle be increased by a factor of 1.4 (and the mass within a sphere by a factor of 2.6) compared to previous analyses. However, for a merging cluster, the hydrostatic analysis is generally inapplicable. Most other lensing clusters are more distant than A2218 and obtaining adequate X-ray data for them is even more difficult. Together with the likely overestimation of mass by the lensing analysis (as in the simulations), oversimplification of the gas model resulting from inadequate resolution may account for the lensing/X-ray mass discrepancy as suggested for A2218. (ABRIDGED)
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We present results from two observations (combined exposure of ~17 ks) of galaxy cluster A2218 using the Advanced CCD Imaging Spectrometer on board the Chandra X-ray Observatory that were taken on October 19, 1999. Using a Raymond-Smith single temperature plasma model corrected for galactic absorption we find a mean cluster temperature of kT = 6.9+/-0.5 keV, metallicity of 0.20+/-0.13 (errors are 90 % CL) and rest-frame luminosity in the 2-10 keV energy band of 6.2x10^{44} erg/s in a LambdaCDM cosmology with H_0=65 km/s/Mpc. The brightness distribution within 4.2 of the cluster center is well fit by a simple spherical beta model with core radius 66.4 and beta = 0.705 . High resolution Chandra data of the inner 2 of the cluster show the x-ray brightness centroid displaced ~22 from the dominant cD galaxy and the presence of azimuthally asymmetric temperature variations along the direction of the cluster mass elongation. X-ray and weak lensing mass estimates are in good agreement for the outer parts (r > 200h^{-1}) of the cluster; however, in the core the observed temperature distribution cannot reconcile the x-ray and strong lensing mass estimates in any model in which the intracluster gas is in thermal hydrostatic equilibrium. Our x-ray data are consistent with a scenario in which recent merger activity in A2218 has produced both significant non-thermal pressure in the core and substructure along the line of sight; each of these phenomena probably contributes to the difference between lensing and x-ray core mass estimates.
We present results from a deep photometric study of the rich galaxy cluster Abell 2218 (z=0.18) based on archival HST WFPC2 F606W images. These have been used to derive the luminosity function to extremely faint limits (M_{F606W}=-13.2 mag, mu_{0}=24.7 mag arcsec^{-2}) over a wide field of view (1.3 h^{-2} Mpc^2). We find the faint-end slope of the luminosity function to vary with environment within the cluster, going from alpha=-1.23pm0.13 within the projected central core of the cluster (100 < r < 300 h^{-1} kpc) to alpha=-1.49pm 0.06 outside this radius (300 < r < 750 h^{-1} kpc). We infer that the core is dwarf depleted, and further quantify this by studying the ratio of dwarf to giant galaxies and its dependency as a function of cluster-centric radius and local galaxy density. We find that this ratio varies strongly with both quantities, and that the dwarf galaxy population in A2218 has a more extended distribution than the giant galaxy population.
Determination of cluster masses is a fundamental tool for cosmology. Comparing mass estimates obtained by different probes allows to understand possible systematic uncertainties. The cluster Abell 315 is an interesting test case, since it has been claimed to be underluminous in X-ray for its mass (determined via kinematics and weak lensing). We have undertaken new spectroscopic observations with the aim of improving the cluster mass estimate, using the distribution of galaxies in projected phase space. We identified cluster members in our new spectroscopic sample. We estimated the cluster mass from the projected phase-space distribution of cluster members using the MAMPOSSt method. In doing this estimate we took into account the presence of substructures that we were able to identify. We identify several cluster substructures. The main two have an overlapping spatial distribution, suggesting a (past or ongoing) collision along the line-of-sight. After accounting for the presence of substructures, the mass estimate of Abell 315 from kinematics is reduced by a factor 4, down to M200=0.8 (-0.4,+0.6) x 10^14 Msun. We also find evidence that the cluster mass concentration is unusually low, c200=r200/r-2 <~ 1. Using our new estimate of c200 we revise the weak lensing mass estimate down to M200=1.8 (-0.9,+1.7) x 10^14 Msun. Our new mass estimates are in agreement with that derived from the cluster X-ray luminosity via a scaling relation, M200=0.9+-0.2 x 10^14 Msun. Abell 315 no longer belongs to the class of X-ray underluminous clusters. Its mass estimate was inflated by the presence of an undetected subcluster in collision with the main cluster. Whether the presence of undetected line-of-sight structures can be a general explanation for all X-ray underluminous clusters remains to be explored using a statistically significant sample.
We report Chandra X-ray observations and optical weak-lensing measurements from Subaru/Suprime-Cam images of the double galaxy cluster Abell 2465 (z=0.245). The X-ray brightness data are fit to a beta-model to obtain the radial gas density profiles of the northeast (NE) and southwest (SW) sub-components, which are seen to differ in structure. We determine core radii, central temperatures, the gas masses within $r_{500c}$, and the total masses for the broader NE and sharper SW components assuming hydrostatic equilibrium. The central entropy of the NE clump is about two times higher than the SW. Along with its structural properties, this suggests that it has undergone merging on its own. The weak-lensing analysis gives virial masses for each substructure, which compare well with earlier dynamical results. The derived outer mass contours of the SW sub-component from weak lensing are more irregular and extended than those of the NE. Although there is a weak enhancement and small offsets between X-ray gas and mass centers from weak lensing, the lack of large amounts of gas between the two sub-clusters indicates that Abell 2465 is in a pre-merger state. A dynamical model that is consistent with the observed cluster data, based on the FLASH program and the radial infall model, is constructed, where the subclusters currently separated by ~1.2Mpc are approaching each other at ~2000km/s and will meet in ~0.4Gyr.
We determine colour gradients of $-0.15 pm 0.08$ magnitudes per decade in radius in F450W$-$F606W and $-0.07 pm 0.06$ magnitudes per decade in radius in F606W$-$F814W for a sample of 22 E/S0 galaxies in Abell 2218. These gradients are consistent with the existence of a mild ($sim -0.3$ dex per decade in radius) gradient in metal abundance, (cf. previous work at lower and higher redshift for field and cluster galaxies). The size of the observed gradients is found to be independent of luminosity over a range spanning $M^*-1$ to $M^*+1.5$ and also to be independent of morphological type. These results suggest a fundamental similarity in the distributions of stellar populations in ellipticals and the bulges of lenticular galaxies. These results are not consistent with simple models of either monolithic collapse or hierarchical mergers.
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