Stellar Over-densities in the Outer Halo of the Milky Way

TL;DR

This paper maps and characterizes new stellar over-densities in the Milky Way's outer halo using SDSS data, revealing structures likely formed by tidal disruption of dwarf galaxies, but their exact origins remain uncertain.

Contribution

It introduces three new candidate halo substructures and provides detailed distance and luminosity estimates, enhancing understanding of the Milky Way's halo complexity.

Findings

Discovery of three new halo substructures.

Recovery of Sagittarius dwarf galaxy features.

Evidence supporting tidal disruption origins.

Abstract

This study presents a tomographic survey of a subset of the outer halo (10-40 kpc) drawn from the Sloan Digital Sky Survey Data Release 6. Halo substructure on spatial scales of $>3$ degrees is revealed as an excess in the local density of sub-giant stars. With an appropriate assumption of a model stellar isochrone it is possible for us to then derive distances to the sub-giant population. We describe three new candidate halo substructures; the 160- and 180-degree over-densities (at distances of 17 and 19 kpc respectively and radii of 1.3 and 1.5 kpc respectively) and an extended feature at 28 kpc that covers at least 162 square degrees, the Virgo Equatorial Stream. In addition, we recover the Sagittarius dwarf galaxy (Sgr) leading arm material and the Virgo Over-density. The derived distances, together with the number of sub-giant stars associated with each substructure, enables us to derive the integrated luminosity for the features. The tenuous, low surface brightness of the features strongly suggests an origin from the tidal disruption of an accreted galaxy or galaxies. Given the dominance of the tidal debris of Sgr in this region of the sky we investigate if our observations can be accommodated by tidal disruption models for Sgr. The clear discordance between observations and model predictions for known Sgr features means it is difficult to tell unambiguously if the new substructures are related to Sgr or not. Radial velocities in the stellar over-densities will be critical in establishing their origins.