A Robust Measure of Dark Matter Halo Ellipticities

TL;DR

This paper proposes a robust method using Jeans equations to measure the ellipticity of dark matter halos, and demonstrates that upcoming telescopes can detect asphericity in dwarf galaxies, testing the LCDM model.

Contribution

It introduces a new gravitational torque-based measure for dark matter halo asphericity and shows its potential for observational detection with future telescopes.

Findings

TMT can distinguish LCDM torque values from zero with high confidence.

Two observation epochs enable a 5 sigma detection of halo asphericity.

The method provides a new test for the standard cosmological model.

Abstract

In simulations of the standard cosmological model (LCDM), dark matter halos are aspherical. However, so far the asphericity of an individual galaxy's halo has never been robustly established. We use the Jeans equations to define a quantity which robustly characterizes a deviation from rotational symmetry. This quantity is essentially the gravitational torque and it roughly provides the ellipticity projected along the line of sight. We show that the Thirty Meter Telescope (TMT), with a single epoch of observations combined with those of the Gaia space telescope, can distinguish the LCDM value of the torque from zero for each Sculptor-like dwarf galaxy with a confidence between 0 and 5 sigma, depending on the orientation of each halo. With two epochs of observations, TMT will achieve a 5 sigma discovery of torque and so asphericity for most such galaxies, and so will provide a new and powerful test of the LCDM model.