Resonant Trapping of the Moving Groups G18-39 and G21-22 in the Galactic Halo

Orbital resonances in the Galactic halo have been studied using the Galactic mass model of Pichardo et al. (2003, 2004), including a Galactic bar. For the two moving groups of the Galactic halo, G18-39 and G21-22 (Silva et al. 2012), the majority of stars in both groups appear trapped in two resonances over the Galactic plane, generated by the bar. We have taken the rotation speed of the bar, Ω_b, as 45-55 km s^-1 kpc^-1. So, these two moving groups are part of stellar supergroups which populate these two resonances. The position of these two groups in the Bottlinger diagram can be explained by the mean (U,V) field generated by these two resonances crossing the solar vicinity, in contrast with the alternate explanation of Silva et al. (2012), based on the simulations of Meza et al. (2005), that these two groups, seen as two peaks in the U Galactic velocity, have been created by the accretion of a dwarf galaxy by the Milky Way, such as that of Ω Centauri.