The Milky Way galaxy is a vast barred spiral system. While the bright central disk and spiral arms are its most visible features, they are enveloped by an immense, sparsely populated region known as the galactic halo. This extensive, outermost structure is a major component of the galaxy’s overall architecture. Determining the precise geometry of the halo is crucial, as its shape holds the key to understanding the Milky Way’s history and its place in the universe.
Defining the Galactic Halo
The galactic halo is an enormous, diffuse region that extends far beyond the visible disk of stars and gas. Its scale dwarfs the main body of the galaxy, reaching out to an estimated diameter of nearly two million light-years. This vast envelope provides the gravitational anchor for the entire structure and is composed of two primary components.
The most massive part is the invisible dark matter halo, which constitutes the majority of the Milky Way’s total mass. Its existence is inferred solely through its gravitational influence on the motions of visible matter, such as the flat rotation curve of the galaxy’s disk. This dark matter scaffolding provides the underlying structure into which the visible galaxy is nested.
The second component is the much smaller baryonic, or visible, halo, which is made up of ordinary matter. This includes the hot gas halo, a reservoir of gas heated to millions of degrees, and the stellar halo. The stellar halo is a sparse population of old stars, globular clusters, and stellar streams, containing only about one percent of the galaxy’s total stellar mass.
Observational Challenges in Determining Shape
Pinpointing the exact shape of the halo is difficult because it is immense and contains a low density of visible material. Unlike external galaxies, where the halo can be viewed from an outside perspective, our solar system is situated deep within the Milky Way’s disk. This makes surveying the entire surrounding structure challenging, especially since the halo lacks a clear, defined edge and simply fades into intergalactic space.
To overcome these obstacles, researchers rely on “tracers” to map the gravitational field and infer the shape of the invisible dark matter. These tracers are objects whose movements are governed by the halo’s gravity, such as globular clusters and stellar streams. Stellar streams are long, thin trails of stars ripped from cannibalized dwarf galaxies or globular clusters, and their orbits provide indirect evidence of the halo’s gravitational influence and geometry.
Leading Theories on Halo Geometry
Early models often assumed the halo was perfectly spherical, or isotropic, for simplicity, but recent observational data has challenged this view. The main geometric possibilities considered are spherical (uniformly round), oblate (flattened like a disk or lentil), and prolate (elongated like a cigar or rugby ball).
Current evidence suggests a non-spherical shape, with the consensus leaning toward a triaxial ellipsoid—a 3D shape where all three axes have different lengths. Studies using the motions of globular clusters and stellar streams have helped constrain the possibilities. Analysis of the inner dark matter halo, out to about 20 kiloparsecs, suggests a mildly non-spherical shape, ruling out both perfectly oblate and strongly prolate models.
For the stellar halo specifically, combining data from the Gaia spacecraft and surveys like H3 has revealed an oblong, tilted shape. This non-spherical structure is often described as resembling a kicked football or zeppelin. Some studies also indicate the halo’s shape may change with distance, being more flattened (oblate) in the inner regions and becoming more spherical further out.
Implications for Galaxy Formation Models
The shape of the Milky Way’s halo acts as a fossil record of the galaxy’s evolutionary history. The geometry is a direct product of the galaxy’s merger and accretion past, a key process in the Lambda-CDM cosmological model of structure formation. A highly non-spherical shape, such as the oblong and tilted stellar halo, points toward recent, significant merger events.
The discovery that the stellar halo is oblong and tilted strongly supports the theory that the dwarf galaxy Gaia-Sausage-Enceladus (GSE) collided with the Milky Way seven to ten billion years ago. This massive collision ripped apart the dwarf galaxy, scattering its stars into the current, elongated stellar halo. The fact that the stellar halo has not settled into a simple spherical shape suggests the underlying dark matter halo is also non-spherical and possibly tilted, maintaining the off-kilter structure.
The precise shape of the halo is used to test and refine cosmological simulations of galaxy growth. By comparing the observed geometry to the outcomes of different simulations, researchers can validate the models that best describe the hierarchical merging process that built the Milky Way. The non-spherical nature of the halo reveals that the visible disk is merely a small, settled feature nestled within a much larger, dynamically complex, and historically active gravitational structure.