Orbits of Globular Clusters in the Outer Galaxy: NGC 7006

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Abstract
We present a proper-motion study of the distant globular cluster NGC 7006 based on the measurement of 25 photographic plates spanning a 40 yr interval. The absolute proper motion determined with respect to extragalactic objects is (μα cos δ, μδ) = (-0.96, -1.14) ± (0.35, 0.40) mas yr-1. The total space velocity of NGC 7006 in a Galactocentric rest frame is 279 km s-1, placing the cluster on one of the most energetic orbits (Ra = 102 kpc) known to date for clusters within 40 kpc from the Galactic center. We compare the orbits—as determined from full space velocities—of four clusters that have apocentric radii larger than 80 kpc with those of Galactic satellites with well-measured proper motions. These clusters are NGC 5466, 6934, and 7006 and Pal 13, and the satellites are the Sagittarius dwarf spheroidal galaxy (dSph), the Large Magellanic Cloud, the Ursa Minor dSph, and the Sculptor dSph. Only NGC 5466 and 6934 seem to have similar orbital parameters, indicating a possible phase-space association. NGC 7006, Pal 13, and the "pair" NGC 5466 and 6934 do not show any dynamical association with the Galactic satellites considered here. NGC 5466, 6934, and 7006 and Pal 13 have orbits that are highly eccentric and of various inclinations with respect to the Galactic plane. In contrast, the orbits of the Galactic satellites are of low to moderate eccentricity and highly inclined. Based on orbit types, chemical abundances, and cluster parameters, we discuss the properties of the hypothetical host systems of the remote globular clusters in the Searle-Zinn paradigm. It is apparent that clusters such as NGC 5466, 6934, and 7006 formed in systems that more likely resemble the Fornax dSph rather than the Sagittarius dSph. We also discuss plausible causes for the difference found so far between the orbit type of outer halo clusters and that of Galactic satellites and for the tentative, yet suggestive, phase-space scatter found among outer halo clusters.
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