Milky way galaxy rotation and orbit
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Milky Way Galaxy Rotation Curve: Structure and Key Features
The rotation curve of the Milky Way describes how the orbital speed of stars and gas changes with distance from the Galactic center. Multiple studies using different tracers—such as high-mass star-forming regions, Cepheid variables, red clump giants, and neutral hydrogen—consistently show that the Milky Way’s rotation curve is nearly flat out to large distances, rather than declining as would be expected if only visible matter were present. This flatness is a key piece of evidence for the existence of dark matter in the Galaxy’s halo 345610.
Recent high-precision measurements using Gaia data and VLBI astrometry have further refined our understanding. The rotation speed at the Sun’s position is measured to be around 233–240 km/s, and the rotation curve remains flat out to about 20–25 kpc from the Galactic center before showing a gradual decline at even larger radii 35910. Some studies also report localized dips in the rotation curve at specific radii, which may be linked to features in the Galaxy’s mass distribution .
Non-Circular and Elliptical Motions in the Milky Way
The Milky Way’s rotation is not perfectly circular. The gravitational potential is non-axisymmetric, influenced by structures like the central bar and possibly a triaxial halo. This causes elliptical distortions in the orbits of stars and gas, with the ellipticity increasing toward the outer disk. The major axis of these elliptical orbits is found near the Sun’s position, with a displacement of about 32 degrees . Additionally, high-mass star-forming regions are observed to orbit the Galaxy about 15 km/s slower than expected for purely circular orbits, indicating the presence of non-circular motions .
Galactic Mass and Dark Matter
The flatness of the rotation curve at large distances implies the presence of a significant amount of unseen mass—dark matter—extending well beyond the visible disk. Estimates of the Milky Way’s total mass, based on rotation curve modeling, range from about 0.9 × 10¹² to 2 × 10¹² solar masses, depending on the method and data used 578. Some recent analyses using Gaia data suggest a declining rotation curve at very large radii, which could imply a lower total mass for the Galaxy than previously thought, but still much higher than the mass of visible matter alone .
Orbits of Dwarf Galaxies and Satellite Structure
The orbits of the Milky Way’s dwarf satellite galaxies, as measured by Gaia, provide additional constraints on the Galaxy’s mass and gravitational potential. These satellites show a range of orbital types, with many concentrated near their closest approach to the Galactic center (pericenter). A significant fraction of these satellites share a common orbital plane, suggesting a real structure in their distribution .
The Role of General Relativity and Alternative Models
Some recent work has explored whether general relativistic effects, rather than dark matter, could explain the observed flatness of the rotation curve. These studies find that both classical dark matter models and general relativistic models can fit the observed data with similar accuracy, but the origin of the required gravitational effects remains uncertain and is an area for further research .
Conclusion
The rotation and orbital dynamics of the Milky Way reveal a complex interplay between visible matter, dark matter, and the Galaxy’s non-axisymmetric structure. The rotation curve is flat out to large distances, supporting the presence of a massive dark matter halo. Non-circular and elliptical motions, as well as the orbits of satellite galaxies, provide further insights into the Galaxy’s mass distribution and evolution. Ongoing and future observations, especially with Gaia and VLBI, will continue to refine our understanding of the Milky Way’s rotation and orbit.
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