A common mass scale for satellite galaxies of the Milky Way

TL;DR

The paper investigates whether Milky Way dwarf spheroidal galaxies share a common dynamical mass within their inner regions by applying Jeans-based mass modeling to line-of-sight velocity data. It uses flexible dark matter density profiles and radially varying velocity anisotropy, marginalizing over model uncertainties to infer ${M_{0.3}}$ for each system via a maximum-likelihood framework. The key result is that ${M_{0.3}} \approx 10^7\,M_\odot$ for 18 dwarfs, implying a central density of roughly $0.1\,M_\odot\,\mathrm{pc}^{-3}$ and strong dark matter domination within 0.3 kpc, i.e., a common mass scale across satellites with vastly different luminosities. These findings have significant implications for galaxy formation at small scales and place constraints on dark matter models, including warm dark matter scenarios. The study also demonstrates robustness against potential biases from tides, rotation, and binaries, while highlighting the need for deeper velocity data to sharpen the constraints further.

Abstract

The Milky Way has at least twenty-three known satellite galaxies that shine with luminosities ranging from about a thousand to a billion times that of the Sun. Half of these galaxies were discovered in the past few years in the Sloan Digital Sky Survey, and they are among the least luminous galaxies in the known Universe. A determination of the mass of these galaxies provides a test of galaxy formation at the smallest scales and probes the nature of the dark matter that dominates the mass density of the Universe. Here we use new measurements of the velocities of the stars in these galaxies to show that they are consistent with them having a common mass of about 10^7 M_\odot within their central 300 parsecs. This result demonstrates that the faintest of the Milky Way satellites are the most dark matter-dominated galaxies known, and could be a hint of a new scale in galaxy formation or a characteristic scale for the clustering of dark matter.

Figures (3)

• Figure 1: The integrated mass of the Milky Way dwarf satellites, in units of solar masses, within their inner 0.3 kpc as a function of their total luminosity, in units of solar luminosities. The circle (red) points on the left refer to the newly-discovered SDSS satellites, while the square (blue) points refer to the classical dwarf satellites discovered pre-SDSS. The error bars reflect the points where the likelihood function falls off to 60.6% of its peak value.
• Figure 2: The likelihood function for the integrated mass within 0.3 kpc for 18 of the Milky Way satellites. We marginalize over all parameters as described in the text.
• Figure 3: The likelihood function for the rotation amplitude for Willman 1, Coma Berenices, and Ursa Major II.