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Session 25 - Galaxies, Clusters of Galaxies and their Evolution.
Oral session, Monday, June 08
Presidio,
Using images from the Hubble Space Telescope archives, I studied the surface brightness fluctuations (SBF) and globular cluster populations of a sample of elliptical and lenticular galaxies in the Virgo cluster of galaxies. After calibration of the F814W SBF magnitudes, these measurements provided the distances necessary for mapping the three dimensional structure of the Virgo cluster, in particular the membership and relative positions of the groups of galaxies near M49, M60, M86, and M87. The precision of these SBF measurements with HST is such that uncertainty in the distance calibration is the dominant source of error in the distance. Preliminary results indicate that the group of galaxies centered on M86 lies slightly behind that centered on M87, and that the group around M60 lies slightly ahead. NGC 4476, which appears close to M87 in projection, lies in the background, while the distances to its other apparent companions are consistent with that of M87. For each field, globular clusters are identified and cataloged. The distances measured from the peak of the globular cluster luminosity functions provide an independent (though lower precision) means of checking the SBF distances using the same data; these measurements are in good agreement with our SBF distances. Furthermore, examination of additional properties of the globular cluster populations yields interesting results. For example, the catalog of globular clusters associated with M87, created from a number of fields in and around this galaxy, indicates two populations of clusters with different colors and spatial distributions. Each of the several fields show bimodal color distributions, and the proportion of clusters associated with the blue peak rises as one examines fields further from the center of the the galaxy. This is in agreement with the globular cluster formation model of Ashman amp; Zepf, in which a low metallicity globular cluster population is formed in progenitor galaxies, and higher metallicity globular clusters are formed in mergers between these galaxies. One consequence of this model is a more concentrated spatial distribution for the red clusters relative to the blue ones, as the M87 globular cluster catalog indicates.
