Two distant halo velocity groups discovered by the Palomar transient factory

We report the discovery of two new halo velocity groups (Cancer groups A and B) traced by eight distant RR Lyrae stars and observed by the Palomar Transient Factory survey at R.A. 129°, decl. 20° (l 205°, b 32°). Located at 92kpc from the Galactic center (86kpc from the Sun), these are some of the most distant substructures in the Galactic halo known to date. Follow-up spectroscopic observations with the Palomar Observatory 5.1m Hale telescope and W. M. Keck Observatory 10m Keck I telescope indicate that the two groups are moving away from the Galaxy at km s^-1 (Cancer group A) and km s^-1 (CancergroupB). The groups have velocity dispersions of km s^-1 and km s^-1 and are spatially extended (about several kpc), making it very unlikely that they are bound systems, and more likely to be debris of tidally disrupted dwarf galaxies or globular clusters. Both groups are metal-poor (median metallicities of [Fe/H]^A = -1.6 dex and [Fe/H]^B = -2.1 dex) and have a somewhat uncertain (due to small sample size) metallicity dispersion of 0.4 dex, suggesting dwarf galaxies as progenitors. Two additional RR Lyrae stars with velocities consistent with those of the Cancer groups have been observed 25° east, suggesting possible extension of the groups in that direction.