The Edge of the Galaxy

Abstract

We use cosmological simulations of isolated Milky Way-mass galaxies, as well as Local Group analogues, to define the "edge" -a caustic manifested in a drop in density or radial velocity -of Galactic-sized haloes, both in dark matter and in stars. In the dark matter, we typically identify two caustics: the outermost caustic located at ∼1.4r 200m corresponding to the "splashback" radius, and a second caustic located at ∼0.6r 200m which likely corresponds to the edge of the virialized material which has completed at least two pericentric passages. The splashback radius is ill defined in Local Group type environments where the halos of the two galaxies overlap. However, the second caustic is less affected by the presence of a companion, and seems a more natural definition for the boundary of the Milky Way halo. Curiously, the stellar distribution also has a clearly defined caustic, which, in most cases, coincides with the second caustic of the dark matter. This can be identified in both radial density and radial velocity profiles, and should be measurable in future observational programmes. Finally, we show that the second caustic can also be identified in the phase-space distribution of dwarf galaxies in the Local Group. Using the current dwarf galaxy population, we predict the edge of the Milky Way halo to be 292 ± 61 kpc.

The Nature of Dark Matter

“…What prevents this matter from collapsing to the center under the influence of gravity? For example, according to refined data from the Gaia astronomical mission [2], the diameter of the dark matter halo in our Milky Way galaxy reaches 1.9 million light-years! This means we live not in absolutely free space but inside a halo, and we are 35 times closer to its center than its edge.…”

Observing Correlations Between Dark Matter Accretion and Galaxy Growth: I. Recent Star Formation Activity in Isolated Milky Way-Mass Galaxies

“…Yet, lower-mass haloes often have larger neighbouring haloes, which contaminate the distributions of nearby galaxies and dark matter with their own orbiting material (More et al 2015) and smear out the splashback feature. Deason et al (2020) analysed cosmological simulations of Milky Way-mass haloes and found that if these haloes are isolated, there are clear splashback features. By definition, isolated haloes are the largest halo (and thus the dominant source of gravity) in their nearby environment, and so exhibit much stronger splashback features and or-Figure 1.…”

Observing Correlations Between Dark Matter Accretion and Galaxy Growth: II. Testing the Impact of Galaxy Mass, Star Formation Indicator, and Neighbour Colours

“…To reduce environmental contamination in neighbour density profiles, we specifically selected isolated Milky Way-mass galaxies. By selecting isolated galaxies that are the dominant source of gravity in their local environments, they will have stronger correlations between neighbouring galaxy orbits and dark matter accretion rates (see also Deason et al 2020), which allows us to probe lower-mass halo scales than previous work. In addition, instead of identifying a single feature in the density profiles, we analyzed the shape of the entire neighbour density distribution to increase our signal-to-noise ratio.…”