Are the Ultra-Faint Dwarf Galaxies Just Cusps?

TL;DR

This paper introduces a statistical method to distinguish between cusp caustics and gravitationally bound dwarf galaxies, applying it to Hercules and finding it is not a cusp, thus aiding in understanding the nature of ultra-faint dwarf satellites.

Contribution

The authors develop a Poisson maximum likelihood test to differentiate cusp caustics from bound systems, and demonstrate its effectiveness on the Hercules dwarf galaxy.

Findings

Hercules is better modeled as a flattened exponential system.

The cusp model is strongly disfavored for Hercules.

The method can be applied to other dwarf galaxies and stellar halos.

Abstract

We develop a technique to investigate the possibility that some of the recently discovered ultra-faint dwarf satellites of the Milky Way might be cusp caustics rather than gravitationally self-bound systems. Such cusps can form when a stream of stars folds, creating a region where the projected 2-D surface density is enhanced. In this work, we construct a Poisson maximum likelihood test to compare the cusp and exponential models of any substructure on an equal footing. We apply the test to the Hercules dwarf (d ~ 113 kpc, M_V ~ -6.2, e ~ 0.67). The flattened exponential model is strongly favored over the cusp model in the case of Hercules, ruling out at high confidence that Hercules is a cusp catastrophe. This test can be applied to any of the Milky Way dwarfs, and more generally to the entire stellar halo population, to search for the cusp catastrophes that might be expected in an accreted stellar halo.