Other
Scientific paper
Aug 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt.........4s&link_type=abstract
Thesis (PHD). STANFORD UNIVERSITY , Source DAI-B 58/02, p. 751, Aug 1997, 99 pages.
Other
Abell 2218
Scientific paper
Gravitational lensing is an interesting astronomical phenomenon predicted by General Relativity. Recent observations have detected a number of lensing events in clusters of galaxies. These lensing systems give us new insight into cluster mass distributions. Abell 2218, with redshift z = 0.17, is one of the first clusters in which gravitationally lensed arcs have been found. Abell 2218 contains several arcs, and although none are as large as some of the more studied arcs (Abell 370, Cl 2244-02), they are interesting nonetheless. We present photometric and spectroscopic data of the cluster taken at the NOAO 4 m telescope, and show broadband spectra of some of the more interesting objects in the field. One arc in the cluster is redder than a typical galaxy, which is unusual since most lensed arcs have a bluer color. The spectrum of this arc reveals a redshift of 0.703. There is an almost perfectly circular blue ring surrounding one galaxy. There are several other blue arclets including two, called the twin arcs, that are on either side of two close galaxies. We model the system with a simple mass distribution consisting of the visible galaxies and a diffuse dark matter component. We are able to model the red arc and the twin arcs, but we find that the ring is most likely not a gravitationally lensed image. Combining this with the observed velocity dispersion of galaxies, we set limits on the masses of some of the galaxies and on the parameters describing the distribution of dark matter. Many lensing clusters are X-ray sources and these X-rays can be used to determine the cluster mass distribution. We compare the mass distribution from the X-ray data and lensing data for Abell 2218 as well as several other clusters, and find that the dark matter component from the lensing models is more compact than from the X-ray distribution. We investigate how modifying our assumptions of hydrostatic equilibrium, constant temperature, and spherical symmetry might reconcile this discrepancy.
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