Spectroscopic Confirmation of the Pisces Overdensity

TL;DR

This paper confirms the Pisces Overdensity as a likely dwarf galaxy or disrupted system through spectroscopic analysis of RR Lyrae stars, revealing a coherent velocity group indicative of a physical association.

Contribution

First spectroscopic confirmation of the Pisces Overdensity, establishing its nature as a potentially bound or disrupted galaxy through radial velocity measurements.

Findings

Majority of RR Lyrae stars share similar radial velocities

Evidence supports the overdensity being a dwarf galaxy or disrupted system

Two stars show close velocities but are not confirmed as a second system

Abstract

We present spectroscopic confirmation of the "Pisces Overdensity", also known as "Structure J", a photometric overdensity of RR Lyrae stars discovered by the Sloan Digital Sky Survey (SDSS) at an estimated photometric distance of ~85kpc. We measure radial velocities for 8 RR Lyrae stars within Pisces. We find that 5 of the 8 stars have heliocentric radial velocities within a narrow range of -87 km/s < v < -67 km/s, suggesting that the photometric overdensity is mainly due to a physically associated system, probably a dwarf galaxy or a disrupted galaxy. Two of the remaining 3 stars differ from one another by only 9 km/s, but it would be premature to identify them as a second system.