11264(  1) - 03/30/07 11:16  - [  1]

                  HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 11264

Version: 1    Check-in Time: 30-Mar-2007 15:16:04       STScI Edit Number: 0

Title
Blue Tilts and Other Properties of Halo Globular Clusters in Nearby Galaxies -
Cosmological or Observational Bias?
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Type             Cycle     Parallel Pointing Tolerance
AR               16        <none>
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Investigators
                                                                                     Contact?
    PI: Dr. Arunav Kundu                  Michigan State University
   CoI: Dr. Stephen E. Zepf               Michigan State University                     N

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Abstract

Old metal-poor globular clusters are seen in large numbers in the halos of all
nearby galaxies. Cosmological simulations suggest that these clusters, which appear
as the blue peak in the typical bimodal color distribution of globular cluster
systems, are among the first objects to form at high redshift. Until recently these
primordial clusters were assumed to be `universal' because the properties of blue
globular clusters found in all types and sizes of galaxies appeared to be identical.
A series of new ACS-based studies, using the ACS Virgo Cluster Survey (GO-9401) data
and other samples (GO-9427, GO/DD-9714), suggest that there is a mass-metallicity
relation in the blue halo globular clusters. The brighter blue clusters trend to
redder colors in this `blue-tilt' phenomenon. This seems to suggest that larger
globular clusters have self enriched either because they were formed in large gas
clouds, or because in the past they had significant dark matter halos that have
subsequently been stripped. These ACS studies also find that the mean metallicity of
the halo clusters increases with the mean mass of the host galaxy. It has been
argued that this is evidence of self enrichment in large halos and points towards an
in situ model of galaxy formation. We suggest that both these correlations are
caused by subtle systematic observational effects that have been overlooked in
previous studies. We propose to undertake a systematic analysis of the globular
cluster systems of the 145 galaxies observed by the ACS Virgo and Fornax cluster
surveys (GO-9401, GO-10217), and the mosaic of M104 (GO/DD-9714) in order to answer
the crucial question, `Just how universal are the properties of the metal-poor halo
globular clusters?'. These ACS observations are likely to remain the largest
globular cluster datasets for the foreseeable future. Our independent analysis of
these extended datasets will provide an important check on the results found for
globular cluster systems and their implications on galaxy formation and evolution.
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