Mapping the Galactic Halo VIII: Quantifying substructure

TL;DR

This study quantifies the amount of kinematic substructure in the Galactic halo using a new clustering measure, revealing significant substructure linked to known tidal debris and implications for the shape of the dark matter halo.

Contribution

The paper introduces the '4distance' clustering estimator and applies it to the Spaghetti survey data, providing new insights into halo substructure and dark halo shape.

Findings

Approximately 20% of halo stars are in substructures.

The data favors a spherical or prolate dark halo shape.

Results are consistent with a halo formed from disrupted satellites.

Abstract

We have measured the amount of kinematic substructure in the Galactic halo using the final data set from the Spaghetti project, a pencil-beam high latitude sky survey. Our sample contains 101 photometrically selected and spectroscopically confirmed giants with accurate distance, radial velocity and metallicity information. We have developed a new clustering estimator: the "4distance" measure, which when applied to our data set leads to the identification of 1 group and 7 pairs of clumped stars. The group, with 6 members, can confidently be matched to tidal debris of the Sagittarius dwarf galaxy. Two pairs match the properties of known Virgo structures. Using models of the disruption of Sagittarius in Galactic potentials with different degrees of dark halo flattening, we show that this favors a spherical or prolate halo shape, as demonstrated by Newberg et al. (2007) using SDSS data. One additional pair can be linked to older Sagittarius debris. We find that 20% of the stars in the Spaghetti data set are in substructures. From comparison with random data sets we derive a very conservative lower limit of 10% to the amount of substructure in the halo. However, comparison to numerical simulations shows that our results are also consistent with a halo entirely built up from disrupted satellites, provided the dominating features are relatively broad due to early merging or relatively heavy progenitor satellites.