THE OBSERVATIONAL AND THEORETICAL TIDAL RADII OF GLOBULAR CLUSTERS IN M87
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abstract
Globular clusters have linear sizes (tidal radii) which theory tells us are
determined by their masses and by the gravitational potential of their host
galaxy. To explore the relationship between observed and expected radii, we
utilize the globular cluster population of the Virgo giant M87. Unusually deep,
high signal-to-noise images of M87 are used to measure the effective and
limiting radii of approximately 2000 globular clusters. To compare with these
observations, we simulate a globular cluster population that has the same
characteristics as the observed M87 cluster population. Placing these simulated
clusters in the well-studied tidal field of M87, the orbit of each cluster is
solved and the theoretical tidal radius of each cluster is determined. We
compare the predicted relationship between cluster size and projected
galactocentric distance to observations. We find that for an isotropic
distribution of cluster velocities, theoretical tidal radii are approximately
equal to observed limiting radii for Rgc < 10 kpc. However, the isotropic
simulation predicts a steep increase in cluster size at larger radii, which is
not observed in large galaxies beyond the Milky Way. To minimize the
discrepancy between theory and observations, we explore the effects of orbital
anisotropy on cluster sizes, and suggest a possible orbital anisotropy profile
for M87 which yields a better match between theory and observations. Finally,
we suggest future studies which will establish a stronger link between
theoretical tidal radii and observed radii.
