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Register a new userInternal colour gradients for E/S0 galaxies in Abell 2218
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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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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.
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 performed B and R band surface photometry for E/S0 galaxies in a nearby rich cluster ABELL 2199 to investigate their B-R color gradients (d(B-R)/dlogr). Our aims are to study statistical properties of the color gradients and, by comparing them with those in less dense environments, to examine environmental dependence of color gradients in elliptical galaxies. We studied the distribution of the B-R color gradients in the cluster ellipticals and found that the mean value of the color gradients is -0.09 +- 0.04 mag/dex, which can be converted to a metallicity gradient (dlogZ/dlogr) of ~ -0.3 +- 0.1 assuming an old stellar population. We further studied the relations between the B-R color gradients and global properties of the galaxies. Our data suggest that for the galaxies brighter than L*, more luminous and larger galaxies tend to have steeper color gradients. The typical value of the color gradients seems to be consistent with a recent monolithic collapse model and the correlation could also appear if elliptical galaxies formed through the monolithic collapse. On the contrary, it is found based on data from the literature that any such trend is clearly weaker for ellipticals in less dense environments, while the distribution of the color gradients is quite similar to that found in the rich cluster. Based on the results from our data and the published data, we discuss formation process of elliptical galaxy and its environmental dependence.
We present results from a new and unique integral-field spectrograph, SAURON. It has a large field of view and high throughput and is primarily built for the study of stellar & gaseous kinematics and stellar populations in galaxies. Its aim is to carry out a systematic survey of the velocity fields, velocity dispersions, and line-strength distributions of nearby ellipticals, lenticular galaxies and spiral bulges. Its wide field is especially useful for the study of complicated velocity structures. Together with other spectroscopic data, images, and dynamical modelling, SAURON will help to constrain the intrinsic shapes, mass-to-light ratios, and stellar populations of early-type galaxies and spiral bulges.
The recent discovery of extended ultraviolet (XUV) disks around a large fraction of late-type galaxies provides evidence for unexpectedly large-scale disk building at recent epochs. Combining GALEX UV observations with deep optical and Spitzer IR imaging, we search for XUV disks in a sample of nearby low-to-intermediate mass E/S0 galaxies to explore evidence for disk rebuilding after mergers. Preliminary visual classification yields ten XUV-disk candidates from the full sample of 30, intriguingly similar to the ~30% frequency for late-type galaxies. These XUV candidates occur at a wide range of masses and on both the red and blue sequences in color vs. stellar mass, indicating a possible association with processes like gas accretion and/or galaxy interactions that would affect the galaxy population broadly. We go on to apply the quantitative Type 1 and Type 2 XUV-disk definitions to a nine-galaxy subsample analyzed in detail. For this subsample, six of the nine are Type 1 XUVs, i.e., galaxies with UV structure beyond the expected star formation threshold. The other three come close to satisfying the Type 2 definition, but that definition proves problematic to apply to this sample: the NUV-derived star formation threshold radii for our E/S0s often lie inside the 80% Ks-band light (K80) radii, violating an implicit assumption of the Type 2 definition, or lie outside but not as far as the definition requires. Nonetheless, the three otherwise Type 2-like galaxies (modified Type 2 XUVs) have higher star formation rates and bluer FUV - NUV colors than the Type 1 XUVs in the sample. We propose that Type 1 XUVs may reflect early or inefficient stages of star formation, while modified Type 2 XUVs perhaps reflect inside-out disk regrowth.
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