Orbits of Globular Clusters in the Outer Galaxy: NGC 7006

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 [1.14)^(0.35, 0.40) mas yr~1. The total space96, velocity of NGC 7006 in a Galactocentric rest frame is 279 km s~1, placing the cluster on one of the most energetic orbits kpc) known to date for clusters within 40 kpc from the Galactic center. (R a \ 102 We compare the orbitsÈas determined from full space velocitiesÈof 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 ÏÏ 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 di †erence 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.

Absolute Proper Motion of the Fornax Dwarf Spheroidal Galaxy from Photographic andHubble Space TelescopeWFPC2 Data

“…These clusters show a strong second-parameter effect as recently confirmed by Mackey & Gilmore (2003b), whereas this is not the case for the known Sagittarius clusters (see Table 4 in Dinescu et al 2001). Also, the Fornax clusters are all metal-poor (À2:0 ½Fe=H À1:5) and have rather similar masses, while those of Sgr span a large range in metallicity (À2:0 ½Fe=H À0:6) and mass (see Strader et al 2003; Table 7 in Mackey & Gilmore 2003a; Table 4 in Dinescu et al 2001). Fornax itself has a wide metallicity range (À2:0 ½Fe=H À0:4) with a mean of À0.9 (e.g., Pont et al 2004), a characteristic shared only by Sgr among the Milky Way dSph satellites.…”

“…The reduction procedure is similar to that used in Dinescu et al (2000Dinescu et al ( , 2001; hereafter D00, D01). Since details are described thoroughly there, here we only briefly mention the necessary steps involved in the reduction.…”

“…The integration time is chosen such that a few orbits are completed, and the orbital elements are averaged over the number of orbits. The uncertainties in the orbital elements are determined from 300 integrations that have initial conditions determined randomly from the uncertainties in the proper motions, radial velocity, and distance (see Dinescu et al 1999Dinescu et al , 2000.…”

“…Of the well-known Milky Way satellites, Sagittarius and Fornax are the only dSph's that have their own system of globular clusters 5 (Hodge 1961;Da Costa & Armandroff 1995). Sgr has at least five globular clusters associated with it (Pal 12 besides the four clusters in the main body of Sgr; Dinescu et al 2000;Martínez-Delgado et al 2002;Cohen 2004) and arguably even more scattered along its orbit (Bellazzini et al 2003;Majewski et al 2004), as the dwarf is in the process of tidal disruption. Fornax is known to have five globular clusters.…”

Search for extratidal features around 17 globular clusters in the Sloan Digital Sky Survey

“…The cluster is currently moving toward the Galactic plane and the Galactic center (Dinescu et al 2001). It has not been included in any study on tidal tails searches so far.…”