PERIGALACTIC DISTANCES OF GLOBULAR CLUSTERS

Abstract

Luminosities and tidal radii of globular clusters have been used to estimate the perigalactic distances P of well-observed Galactic globulars that do not have collapsed cores. It is found that the cluster metallicity [Fe/H] correlates somewhat more strongly with P than it does with the present Galactocentric distances R of globular clusters. It is also found that both P and R correlate more strongly with the cluster half-light radii rh than they do with [Fe/H]. This suggests that cluster radii are mainly determined by global parameters of the (proto)Galaxy, whereas [Fe/H] may have been more strongly affected by local evolutionary effects. The strong correlation (P) = 0.70 +/- 0.05 between the perigalactic distances and half-light radii of globular clusters is particularly striking. The half-light radii rh of globular clusters in the outer halo of the Galaxy are found to correlate with R, and even more strongly with P. It is not yet clear how this result is to be reconciled with the Searle-Zinn scenario for the formation of the outer halo of the Galaxy. The tidal radii of clusters with collapsed cores appear to have been systematically over-estimated, resulting in perigalactic distances that are too large. Halo globulars with P > 15 kpc are (with the exception of NGC 2419) typically an order of magnitude less luminous than the clusters with P < 15 kpc that are associated with the main body of the Galaxy. The population of nuclear globular clusters with P < 1 kpc appears to be deficient in objects fainter than MV ~ -7. Clusters with well-determined ages having P > 15 kpc have ages in the range 9 - 16 Gyr, whereas globulars with P < 15 kpc all appear to have ages that are restricted to the narrower range 13 < T < 16 Gyr.