HALO VELOCITY GROUPS IN THE PISCES OVERDENSITY

We report spectroscopic observations of five faint (V similar to 20) RR Lyrae stars associated with the Pisces overdensity conducted with the Gemini South Telescope. At a heliocentric and galactocentric distance of similar to 80 kpc, this is the most distant substructure in the Galactic halo known to date. We combined our observations with literature data and confirmed that the substructure is composed of two different kinematic groups. The main group contains eight stars and has < V-gsr > = 50 km s(-1), while the second group contains four stars at a velocity of < V-gsr > = -52 km s(-1), where V-gsr is the radial velocity in the galactocentric standard of rest. The metallicity distribution of RR Lyrae stars in the Pisces overdensity is centered on [Fe/H] = -1.5 dex and has a width of 0.3 dex. The new data allowed us to establish that both groups are spatially extended making it very unlikely that they are bound systems, and are more likely to be debris of a tidally disrupted galaxy or galaxies. Due to small sky coverage, it is still unclear whether these groups have the same or different progenitors.