Halo Velocity Groups in the Pisces Overdensity

TL;DR

This study identifies and characterizes two distinct kinematic groups of RR Lyrae stars in the distant Pisces overdensity, revealing their likely origin as debris from disrupted galaxies in the Galactic halo.

Contribution

First spectroscopic confirmation of two separate velocity groups in the Pisces overdensity at 80 kpc, suggesting a complex tidal debris origin.

Findings

Two kinematic groups with distinct velocities identified.

Both groups are spatially extended and likely unbound.

Metallicity centered at [Fe/H]=-1.5 dex with a 0.3 dex spread.

Abstract

We report spectroscopic observations with the Gemini South Telescope of 5 faint V~20 RR Lyrae stars associated with the Pisces overdensity. At a heliocentric and galactocentric distance of ~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 8 stars and has <V_{gsr}> = 50 km/s, while the second group contains four stars at a velocity of <V_{gsr}> = -52 km/s, 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.