Structural parameters from ground-based observations of newly discovered globular clusters in NGC 5128
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abstract
We have investigated a number of globular cluster candidates from a recent
wide-field study by Harris et al. (2004a) of the giant elliptical galaxy NGC
5128. We used the Magellan I telescope + MagIC camera under excellent seeing
(0.3"-0.6") and obtained very high resolution images for a sample of 44
candidates. Our images allow us to study the light profiles of the likely
clusters. This is the first ground-based study of structural parameters for
globular clusters outside the Local Group. We compare the psf-deconvolved
profiles with King models and derive structural parameters, ellipticities and
surface brightnesses. In general, our clusters are similar in size,
ellipticity, core radius and central surface brightness to their counterparts
in other galaxies, in particular those in NGC 5128 observed with HST by Harris
et al. (2002). However, they extend to higher ellipticities and larger
half-light radii than their Galactic counterparts. Combining our results with
those of Harris et al. fills in the gaps previously existing in $r_h - M_V$
parameter space and indicates that any substantial difference between presumed
distinct cluster types in this diagram, including for example the Faint Fuzzies
of Larsen & Brodie (2000) and the `extended, luminous' M31 clusters of Huxor et
al. (2005) is now removed and that clusters form a continuum in this diagram.
Indeed, this continuum now extends to the realm of the Ultra Compact Dwarfs. We
have carried out additional analysis to quantify the contamination by
background galaxies. This shows that, although galaxies cannot be easily told
apart from clusters in some of the structural diagrams, the combination of
excellent image quality and Washington photometry should limit the
contamination to roughly 10% of the population of cluster candidates.
