A NEW FORMATION SCENARIO FOR THE MILKY WAY CLUSTER NGC 2419

We present a new formation scenario for NGC 2419, which is one of the most luminous, one of the most distant, and as well one of the most extended globular clusters of the Milky Way. We propose that NGC 2419 is the remnant of a merged star cluster complex (CC), which was possibly formed during an interaction between a gas-rich galaxy and the Milky Way. To test this hypothesis, we performed numerical simulations of 27 different models of CCs moving on a highly eccentric orbit in the Galactic halo. We vary the CC mass, the CC size, and the initial distribution of star clusters in the CC to analyze the influence of these parameters on the resulting objects. In all cases, the vast majority of star clusters merged into a stable object. The derived parameters mass, absolute V-band magnitude, effective radius, velocity dispersion, and the surface brightness profile are, for a number of models, in good agreement with those observed for NGC 2419. Despite the large range of CC sizes, the effective radii of the merger objects are constrained to a relatively small interval. A turnover in the r(eff) versus M-encl space leads to degenerate states, i.e., relatively compact CCs can produce an object with the same structural parameters as a more massive and larger CC. In consequence, a range of initial conditions can form a merger object comparable to NGC 2419 preventing us from pinpointing the exact parameters of the original CC which formed NGC 2419. We conclude that NGC 2419 can be well explained by the merged CC scenario. Some of the recently discovered stellar streams in the Galactic halo might be related to the parent galaxy, which produced the CC in our scenario. Measurements of the proper motion of NGC 2419 are necessary to prove an association with one of the stellar streams.