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| Milky Way halo | |
|---|---|
| Name | Milky Way halo |
| Type | Galactic component |
| Radius | ~100–300 kpc |
Milky Way halo The Milky Way halo is the extended, roughly spherical component surrounding the Milky Way disk and bulge, containing stars, globular clusters, dark matter, and hot gas. It is studied by astronomers at institutions such as the European Southern Observatory, Space Telescope Science Institute, Max Planck Society, Harvard–Smithsonian Center for Astrophysics, and California Institute of Technology using observatories like the Hubble Space Telescope, Gaia, Very Large Telescope, Sloan Digital Sky Survey, and Chandra X-ray Observatory. Research into the halo connects work by teams at Princeton University, Massachusetts Institute of Technology, University of Cambridge, University of California, Berkeley, and Institute for Advanced Study to theoretical frameworks from projects such as the Illustris project, EAGLE, and Via Lactea.
The halo extends well beyond the Galactic Center and encloses components first cataloged by observers like William Herschel and characterized in analyses by researchers including Jan Oort, Walter Baade, Fritz Zwicky, Vera Rubin, and teams associated with Royal Astronomical Society. Studies cite kinematics from missions such as Hipparcos and Gaia and leverage spectroscopy from instruments on Keck Observatory, Subaru Telescope, Gemini Observatory, and Arecibo Observatory alongside cosmological context provided by Lambda-CDM models and constraints from the Planck mission.
The halo comprises a diffuse Dark matter component inferred from rotation curves measured by observers like Rubin and modeled in simulations by groups at Princeton University and Kavli Institute for Cosmology. Embedded within are the stellar halo, satellite systems including the Magellanic Clouds, Sagittarius Dwarf Spheroidal Galaxy, Fornax Dwarf, Sculptor Dwarf Galaxy, and globular clusters cataloged since efforts by Shapley and Harlow Shapley at observatories such as Mount Wilson Observatory. Gas resides in a hot circumgalactic medium probed in absorption by teams using Hubble Space Telescope instruments like COS and observed in X-rays by Chandra X-ray Observatory and XMM-Newton. The halo also hosts stellar streams identified by surveys like the Sloan Digital Sky Survey and followed up by groups at University of Washington and University of Michigan.
The stellar halo contains ancient populations traced by metal-poor stars identified in surveys like RAVE, LAMOST, APOGEE, and GALAH. Its globular clusters include examples studied at Yerkes Observatory and cataloged through work at Carnegie Institution for Science and Smithsonian Astrophysical Observatory. Chemical abundance patterns connect to analyses by groups at Max Planck Institute for Astronomy and University of Cambridge, while kinematic substructures relate to merger events such as the putative Gaia Sausage and accretion of systems similar to the Sagittarius Dwarf Spheroidal Galaxy. Stellar ages derive from isochrone fitting used at Space Telescope Science Institute and theoretical stellar evolution produced by groups at University of Oxford and University of Geneva.
The dark matter halo dominates mass budgets inferred by rotation curves originally highlighted by Vera Rubin and mass modeling by researchers at University of California, Irvine and University of Chicago. Candidates and frameworks include cold dark matter, particles studied in contexts at CERN, Fermi National Accelerator Laboratory, and experiments like LUX-ZEPLIN and XENON1T; alternatives reference Modified Newtonian Dynamics proposed by Mordehai Milgrom. Halo profiles are fit with analytic forms such as the Navarro–Frenk–White profile developed by teams at Institute for Advanced Study and Princeton University, and constrained by gravitational lensing analyses applied in studies by European Southern Observatory collaborators and cosmological simulations like Illustris project and EAGLE.
The gaseous halo, or circumgalactic medium, is probed via quasar absorption lines studied with the Hubble Space Telescope, radio observations by Arecibo Observatory and Very Large Array, and X-ray imaging by Chandra X-ray Observatory and XMM-Newton. High-velocity clouds such as those cataloged by teams at Columbia University and University of Wisconsin–Madison connect to inflow and outflow processes studied in feedback models by groups at California Institute of Technology and Harvard University. Metal enrichment and cooling processes reference nucleosynthesis work by Alastair Cameron and chemical evolution models produced at Max Planck Institute for Astrophysics and Observatoire de Paris.
Formation scenarios derive from hierarchical assembly in Lambda-CDM cosmology framed by pioneers such as Jim Peebles and elaborated in simulations by the Millennium Simulation team and groups at Max Planck Society. Accretion histories include mergers with satellites like the Sagittarius Dwarf Spheroidal Galaxy and interactions with the Large Magellanic Cloud and Small Magellanic Cloud traced by kinematic work at Gaia and dynamical modeling at University of Cambridge. Chemical tagging approaches developed at Australian National University and Monash University aim to reconstruct progenitors, while semi-analytic models by researchers at Carnegie Mellon University and Kavli Institute for Cosmology explore feedback from supernovae studied by groups at Ohio State University and University of California, Santa Cruz.
Evidence for halo components comes from stellar kinematics measured by Gaia and radial velocities from Keck Observatory, Anglo-Australian Telescope, and Subaru Telescope; from globular cluster studies at Mount Stromlo Observatory and Royal Observatory, Greenwich; from gas absorption studies using Hubble Space Telescope instruments and quasar catalogs compiled by Sloan Digital Sky Survey teams; and from X-ray measurements by Chandra X-ray Observatory teams and XMM-Newton consortia. Mass estimates leverage methods developed at Princeton University and University of California, Berkeley including timing arguments applied historically by Kahn and Woltjer and modern modeling using Bayesian approaches employed at Stanford University and University of Tokyo.