The Absolute Proper Motion of Palomar 12: A Case for Tidal Capture from the Sagittarius Dwarf Spheroidal Galaxy

We have measured the absolute proper motion of the young globular cluster Pal12 with respect to background galaxies, using plate material spanning a 40-yeartime baseline, and measuring stars down to a magnitude V~22. The measuredabsolute proper motion has an uncertainty of 0.3 mas/yr in each coordinate. Pal 12's young age for a globular cluster led to the hypothesis that thecluster originated in the Large Magellanic Cloud (LMC) and was later capturedby the Milky Way (Lin and Richer 1992). Here we investigate this hypothesisusing the complete kinematical data. We present the orbital characteristics ofPal 12 and compare them with those of the LMC and Sagittarius dwarf galaxy(Sgr). The present kinematical data suggest that, from the two parentcandidates for Pal 12, Sgr presents a more plausible case for the host galaxythan the LMC. We explore this scenario in the context of the uncertainties in the orbitsand using two different analyses: the direct comparison of the orbits of Pal 12and Sgr as a function of time, and the analytical model of Sgr's tidaldisruption developed by Johnson (1998). We find that, within the presentuncertainties of the observables, this scenario is viable in both methods.Moreover, both methods place this event at the same point in time. Our bestestimate of the time of Pal 12's tidal capture from Sgr is ~ 1.7 Gyr ago.Comment: 37 pages, 5 tables, 5 figures, accepted for publication in AJ, Oct. 200