We report the discovery of two new halo velocity groups ( Cancer groups A and B ) traced by 8 distant RR Lyrae stars and observed by the Palomar Transient Factory ( PTF ) survey at { R . A } \sim 129 \arcdeg , { Dec } \sim 20 \arcdeg ( { l } \sim 205 \arcdeg , { b } \sim 32 \arcdeg ) .
Located at 92 kpc from the Galactic center ( 86 kpc 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 5.1-m Hale telescope and W. M. Keck Observatory 10-m Keck I telescope indicate that the two groups are moving away from the Galaxy at \bar { v } ^ { A } _ { gsr } = 78.0 \pm 5.6 { km~ { } s } ^ { -1 } ( Cancer group A ) and \bar { v } ^ { B } _ { gsr } = 16.3 \pm 7.1 { km~ { } s } ^ { -1 } ( Cancer group B ) .
The groups have velocity dispersions of \sigma ^ { A } _ { v _ { gsr } } = 12.4 \pm 5.0 { km~ { } s } ^ { -1 } and \sigma ^ { B } _ { v _ { gsr } } = 14.9 \pm 6.2 { km~ { } s } ^ { -1 } , and are spatially extended ( about several kpc ) making it very unlikely that they are bound systems , and are 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 \sim 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 \sim 25 \arcdeg east , suggesting possible extension of the groups in that direction .