Structural Parameters for Globular Clusters in NGC 5128

Abstract

We present new imaging measurements of 27 individual globular clusters in the halo of the nearby elliptical galaxy NGC 5128, obtained with the Hubble Space Telescope Imaging Spectrograph and Wide Field Planetary Camera 2. We use the cluster light profiles to determine their structural parameters (core and half-light radii, central concentration, and ellipticity). Combining these with similar data for selected inner halo clusters from Holland et al., we now have a total sample of 43 NGC 5128 globular clusters with measured structural properties. We find that classic King model profiles match the clusters extremely well and that their various structural parameters (core and half-light radius, central surface brightness, and central concentration) fall in very much the same range as do the clusters in the Milky Way and M31. We find half a dozen bright clusters that show tentative evidence for extratidal light that extends beyond the nominal tidal radius, similar in nature to several such objects previously found in the Milky Way and M31; these may represent clusters being tidally stripped or possibly ones in which anisotropic velocity distributions are important. We also confirm previous indications that NGC 5128 contains relatively more clusters with large ( > 0.2) ellipticity than does the Milky Way. Instead, the -distribution of the NGC 5128 clusters strongly resembles that of the old clusters in the Large Magellanic Cloud and also in M31. Finally, calculations of the cluster binding energies Eb as defined by McLaughlin show that the NGC 5128 clusters occupy the same extremely narrow region of the parametric fundamental plane as do their Milky Way counterparts. Our data are thus strongly consistent with the claim that the globular clusters in both NGC 5128 and the Milky Way are fundamentally the same type of object: old star clusters with similar mass-to-light ratios and King model structures.