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Register a new userThe dwarf galaxy population in Abell 2218
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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.
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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 the first results of a new spectroscopic survey of the cluster Abell 85 targeting 1466 candidate cluster members within the central $sim$1 deg$^2$ of the cluster and having magnitudes $m_r < 20.5$ using VIMOS/VLT and HYDRA/WIYN. A total of 520 galaxies are confirmed as either relaxed cluster members or part of an infalling population. A significant fraction are low mass; the median stellar mass of the sample is $10^{9.6} M_{odot} $, and 25% have stellar masses below $10^9 M_{odot}$ (i.e. 133 dwarf galaxies). We also identify seven active galactic nuclei (AGN), four of which reside in dwarf host galaxies. We probe the evolution of star formation rates, based on H$alpha$ emission and continuum modeling, as a function of both mass and environment. We find that more star forming galaxies are observed at larger clustercentric distances, while infalling galaxies show evidence for recently enhanced star forming activity. Main sequence galaxies, defined by their continuum star formation rates, show different evolutionary behavior based on their mass. At the low mass end, the galaxies have had their star formation recently quenched, while more massive galaxies show no significant change. The timescales probed here favor fast quenching mechanisms, such as ram-pressure stripping. Galaxies within the green valley, defined similarly, do not show evidence of quenching. Instead, the low mass galaxies maintain their levels of star forming activity, while the more massive galaxies have experienced a recent burst.
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)
We present a study of the luminosity and color properties of galaxies selected from a sample of 57 low-redshift Abell clusters. We utilize the non-parametric dwarf-to-giant ratio (DGR) and the blue galaxy fraction (fb) to investigate the clustercentric radial-dependent changes in the cluster galaxy population. Composite cluster samples are combined by scaling the counting radius by r200 to minimize radius selection bias. The separation of galaxies into a red and blue population was achieved by selecting galaxies relative to the cluster color-magnitude relation. The DGR of the red and blue galaxies is found to be independent of cluster richness (Bgc), although the DGR is larger for the blue population at all measured radii. A decrease in the DGR for the red and red+blue galaxies is detected in the cluster core region, while the blue galaxy DGR is nearly independent of radius. The fb is found not to correlate with Bgc; however, a steady decline toward the inner-cluster region is observed for the giant galaxies. The dwarf galaxy fb is approximately constant with clustercentric radius except for the inner cluster core region where fb decreases. The clustercentric radial dependence of the DGR and the galaxy blue fraction, indicates that it is unlikely that a simple scenario based on either pure disruption or pure fading/reddening can describe the evolution of infalling dwarf galaxies; both outcomes are produced by the cluster environment.
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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