In this paper, we present projected velocity dispersion measurements from integrated-light spectra for a sample of 9 M 31 globular clusters. By means of comprehensive numerical simulations, we show that the typical relative uncertainty of our measurements is 5%. Because of the relatively large distance of M 31 and the aperture angular size used our integrated-light observations sample a large fraction of these very bright clusters. Consequently, statistical uncertainties due to small samples of bright dominant stars which can affect integrated-light measurements of nearby Galactic clusters are completely negligible. This is confirmed by numerical simulations presented in Dubath et al. (1996a). Paradoxically, it is in some respects easier to measure the global velocity dispersion of a M 31 cluster than that of a Galactic clusters. For the brightest M 31 clusters, reliable velocity dispersions can be measured with a 3-m-class telescope using single exposures of order one hour long.
Previous velocity dispersion measurements are available from Peterson (1988) for 3 of our clusters. The agreement is good for the cluster Bo20, while we obtain significantly lower and more accurate values for Bo158 and 225.
Combining the new velocity dispersions with structural parameters, we compute the cluster ratios using relations derived from King models and the Virial theorem. These ratios appear remarkably similar to those found for Galactic clusters (see e.g., Pryor & Meylan 1993). Two clusters (Bo158 & 225) having only ground-based measurements of have ratio estimates somewhat above the range of Galactic values. It is entirely possible that these estimates of suffer from incomplete removal of the effects of seeing and are consequently too high.
Another way to investigate similarities of globular clusters located in different parent galaxies is to look at the relation between velocity dispersion, luminosity and a physical scale (Fig 5). Using additional data from previous papers, we find remarkable similarities, in terms of ratios and structures, between the globular clusters located in our Galaxy, the Magellanic clouds, the Fornax dwarf spheroidal, M 31, and Centaurus A. It is worth emphasizing that our samples include some of the brightest M 31 and Centaurus A clusters, which are both brighter and more massive than any Galactic cluster, as expected because of the larger number of clusters in both these galaxies.
Our M 31 cluster sample is not large enough to investigate possible correlations between the ratio estimates and other cluster parameters. Only a weak correlation is observed with Galactic data (Pryor & Meylan 1993), and it is possible that the variations of the current estimates from one cluster to another result to a large extent from measurement errors.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998