Modeling the Disruption of the Globular Cluster Palomar 5 by Galactic Tides

The globular cluster Palomar 5 is remarkable not only because of its extended massive tidal tails, but also for its very low mass and velocity dispersion, and its size, which is much larger than its theoretical tidal radius. In order to understand these extreme properties, we performed more than 1000 N-body simulations of clusters traversing the Milky Way on the orbit of Pal 5. Tidal shocks at disk crossings near perigalacticon dominate the evolution of extended low-concentration clusters, resulting in massive tidal tails and often in a quick destruction of the cluster. The very large size of Pal 5 can be explained as the result of an expansion following the heating induced by the last strong disk shock ~150 Myr ago. Some of the models can reproduce the low observed velocity dispersion and the relative fractions of stars in the tails and between the inner and outer parts of the tails. Our simulations illustrate to what extent the observable tidal tails trace out the orbit of the parent object. The tidal tails of Pal 5 show substantial structure not seen in our simulations. We argue that this structure is probably caused by Galactic substructure, such as giant molecular clouds, spiral arms, and dark matter clumps, which was ignored in our modeling. Clusters initially larger than their theoretical tidal limit remain so, because after being shocked, they settle into a new equilibrium near apogalacticon where they are unaffected by the perigalactic tidal field. This implies that, contrary to previous wisdom, globular clusters on eccentric orbits may well remain supertidally limited and hence vulnerable to strong disk shocks, which dominate their evolution until destruction. Our simulations unambiguously predict the destruction of Pal 5 at its next disk crossing in ~110 Myr. This corresponds to only 1% of the cluster lifetime, suggesting that many more similar systems could once have populated the inner parts of the Milky Way but have been transformed into debris streams by the Galactic tidal field.

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