An extended view of the Pisces Overdensity from the SCUSS survey

TL;DR

This paper uses the SCUSS u-band survey to identify blue horizontal branch stars and map the Pisces overdensity, revealing it as a 25-degree stream at 80 kpc with a significant BHB population, providing new insights into its structure and origin.

Contribution

First detailed mapping of the Pisces overdensity as a stellar stream using BHB stars from the SCUSS survey, extending knowledge of its spatial extent and structure.

Findings

The Pisces overdensity is a ~25 degree stream at ~80 kpc.

It contains 20-30 BHB stars with 6 sigma significance.

The stream's structure suggests a disrupted dwarf galaxy or a still-undiscovered progenitor.

Abstract

SCUSS is a u-band photometric survey covering about 4000 square degree of the South Galactic Cap, reaching depths of up to 23 mag. By extending around 1.5 mag deeper than SDSS single-epoch u data, SCUSS is able to probe much a larger volume of the outer halo, i.e. with SCUSS data blue horizontal branch (BHB) stars can trace the outer halo of the Milky Way as far as 100-150 kpc. Utilizing this advantage we combine SCUSS u band with SDSS DR9 gri photometric bands to identify BHB stars and explore halo substructures. We confirm the existence of the Pisces overdensity, which is a structure in the outer halo (at around 80 kpc) that was discovered using RR Lyrae stars. For the first time we are able to determine its spatial extent, finding that it appears to be part of a stream with a clear distance gradient. The stream, which is ~5 degrees wide and stretches along ~25 degrees, consists of 20-30 BHBs with a total significance of around 6sigma over the background. Assuming we have detected the entire stream and that the progenitor has fully disrupted, then the number of BHBs suggests the original system was similar to smaller classical or a larger ultra-faint dwarf galaxy. On the other hand, if the progenitor still exists, it can be hunted for by reconstructing its orbit from the distance gradient of the stream. This new picture of the Pisces overdensity sheds new light on the origin of this intriguing system.