![]() ![]() Now, Alis Deason, an astrophysicist at Durham University in England, and her colleagues have used nearby galaxies to locate the Milky Way’s edge. But because the dark halo emits no light, its diameter is hard to measure. A vast halo of dark matter, presumably full of invisible particles, engulfs both disks and stretches far beyond them ( SN: 10/25/16). Beyond this stellar disk is a disk of gas. The number could lead to a better estimate of how massive the galaxy is and how many other galaxies orbit it.Īstronomers have long known that the brightest part of the Milky Way, the pancake-shaped disk of stars that houses the sun, is some 120,000 light-years across ( SN: 8/1/19). New work finds that the Milky Way stretches nearly 2 million light-years across, more than 15 times wider than its luminous spiral disk. Using the current dwarf galaxy population, we predict the edge of the Milky Way halo to be 292 +/- 61 kpc.Our galaxy is a whole lot bigger than it looks. ![]() ![]() Finally, we show that the second caustic can also be identified in the phase-space distribution of dwarf galaxies in the Local Group. This can be identified in both radial density and radial velocity profiles, and should be measurable in future observational programmes. Curiously, the stellar distribution also has a clearly defined caustic, which, in most cases, coincides with the second caustic of the dark matter. 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. The splashback radius is ill defined in Local Group type environments where the halos of the two galaxies overlap. 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. Deason (Durham) and 7 other authors Download PDF 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. Download a PDF of the paper titled The Edge of the Galaxy, by Alis J. ![]()
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