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 is a more useful 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 paper identifies two major caustics—the splashback radius and a secondary caustic—as key markers for defining the outer boundary of Milky Way-sized dark matter halos.
• The paper reveals that stellar density and velocity profiles align with the secondary caustic, offering a potential observational marker for galaxy edges.
• The paper demonstrates distinct alignments of subhaloes and luminous dwarfs with these caustics, refining our understanding of galaxy boundaries in complex environments.

Overview of "The Edge of the Galaxy" by Deason et al.

This paper investigates the identification of a boundary for Milky Way-sized dark matter haloes using cosmological simulations. The study utilizes three simulation suites: ELVIS, APOSTLE, and Auriga, encompassing both isolated and Local Group-like environments. The authors employ these simulations to analyze the outer density profiles of dark matter, stars, and subhaloes, aiming to ascertain a definitive edge or boundary for these structures.

Key Findings

1. Dark Matter Caustics: The authors identify two main caustics within the outer regions of dark matter haloes, labeled as the splashback radius and a secondary caustic. The splashback radius is located at approximately 1.4 times the radius where the mean density is 200 times the mean cosmic density ($r_{\rm 200m}$), representing the boundary where accreted dark matter reaches its first orbital apocenter post-turnaround. The secondary caustic, typically located around 0.6 $r_{\rm 200m}$ or $r_{\rm 200c}$, corresponds to regions where material has completed at least two pericentric passages.
2. Stellar Caustics: The study observes a pronounced caustic in the distribution of stars, aligning with the second caustic of the dark matter. This stellar caustic represents a potential observational marker to define the edge of a galaxy like the Milky Way, observable in both density and velocity profiles.
3. Subhalo and Dwarf Galaxy Caustics: Analysis of subhaloes and luminous dwarf galaxies reveals that subhaloes align more with the dark matter's splashback radius while luminous dwarfs align with the second caustic, suggesting distinctions between luminous and dark substructures.
4. Implications for the Milky Way and Observations: The second dark matter caustic provides a more stable definition for the edge of the Milky Way, especially within the Local Group's complex dynamical environment. Notably, the researchers propose an edge for the Milky Way at about 292 kpc, based on the distribution of known Local Group dwarf galaxies.

Implications and Future Directions

The study's results have significant implications for how the edges of dark matter halos are defined, challenging the simplicity of traditional models by incorporating a dynamical understanding related to radial profiles. The introduction of dark matter halo edges discernible through stellar distributions provides an intriguing observational target for future wide-field surveys. Such observations could potentially validate this simulated understanding of galaxy halo boundaries and apply it to galaxies beyond the Milky Way.

Future work may involve the examination of halo edges at different mass scales or within alternative cosmological models. There is also scope for developing more sophisticated methods to trace these features within observational datasets, particularly as next-generation facilities like the Vera C. Rubin Observatory and WFIRST come online.

By considering both dark and luminous matter, this paper facilitates a bridge between theoretical predictions and measurable galactic properties, potentially refining our understanding of galaxy structures and their boundaries.