4. Structural parameters
M 31 is at a distance where the core radii of typical globular clusters are considerably smaller in angular size than the typical, ground-based seeing disk. This makes measurements of difficult, as attested to by the large range of values obtained by different authors for the same clusters. Fortunately, six of the clusters in our sample have recently been observed with the Hubble Space Telescope (HST), and accurate values of have been published (Fusi-Pecci et al. 1994 [Bo218, 384]; Grillmair et al. 1996 [Bo6, 45, 343, 358]). The values obtained span a large range but are typical for the brighter globulars in our own Galaxy. For the remaining 3 clusters in Table 1, we use the ground-based measurements of Battistini et al. (1982) and Crampton et al. (1985). For two clusters (Bo6, 45) common to all three samples, the agreement between the ground-based values and those measured by Grillmair et al. (1996) is typically quite good, with the worst cases differing by no more than 30%.
The surface density profiles of these clusters have also been difficult to characterize from the ground owing the very low surface brightness relative to the M 31 background and the inability to resolve individual stars. However with the advent of HST we now know that globular clusters in M 31 are structurally similar to those found in our own Galaxy. These clusters can generally be characterized by King models, though departures from King models in the form of collapsed cores (Bo343, Bendinelli et al. 1993; Grillmair et al. 1996) and tidal tails (Bo6, 343, 358, Grillmair et al. 1996) have also been found. In all respects, it seems that globular clusters in M 31 as a group are of the same breed as the clusters belonging to our own Galaxy.
For the clusters observed by Grillmair et al. (1996), we have determined the half-light radii (the radii within which half the total light of the clusters is contained in projection) by integrating over King models with appropriate core radii and concentration parameters (). For the 2 clusters observed by Fusi Pecci et al. (1994) we have used their core and half-light radii and integrated over a grid of King models to infer the corresponding concentration parameter. For the three clusters for which we have only ground-based measurements of , we follow Battistini et al. (1982) in assuming a uniform value of pc. Our adopted values of and are tabulated in Table 2. In all cases, the published values of have been scaled in accordance with our adopted distance to M 31 of 770kpc (Ajhar et al. 1996).
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998