Milky way
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Understanding the Milky Way: Structure, Mass, and Dynamics
Spiral Structure of the Milky Way
Multiple-Arm Morphology
The Milky Way's spiral structure has been a subject of extensive research, yet it remains somewhat uncertain. Recent studies using precise distance measurements of very young objects suggest that our galaxy exhibits a multiple-arm morphology. This structure includes two primary arms, the Perseus and Norma Arms, in the inner regions, extending to several long, irregular arms such as the Centaurus, Sagittarius, Carina, Outer, and Local Arms in the outer regions .
Four-Arm Spiral Model
Further analysis of trigonometric parallaxes and proper motions of molecular masers associated with high-mass stars supports the notion that the Milky Way is a four-arm spiral galaxy. This model includes additional arm segments and spurs, significantly expanding our understanding of the galaxy's structure . The Local Arm, in particular, has been found to be larger than previously thought, with its pitch angle and star formation rate comparable to the major spiral arms like Sagittarius and Perseus .
Impact of Sagittarius Dwarf Galaxy
Simulations indicate that the infall of the Sagittarius dwarf galaxy has played a significant role in shaping the Milky Way's spiral arms and central bar. This interaction has also led to the formation of a flared outer disk and ring-like structures towards the Galactic anti-Centre, suggesting that the Milky Way's morphology is influenced by external forces .
Mass Distribution and Gravitational Potential
Mass Models and Observational Constraints
Mass models of the Milky Way, created to fit observational constraints and theoretical expectations, provide a detailed understanding of the galaxy's mass distribution. These models incorporate gas discs, variations in the halo density profile, and new observations of maser sources. The best-fitting model estimates the Sun's distance from the Galactic Centre at approximately 8.20 kpc, with a circular speed of 232.8 km/s. The total stellar mass of the Milky Way is estimated at 54.3 × 10^9 M☉, with a total virial mass of 1.30 × 10^12 M☉ and a local dark-matter density of 0.40 GeV cm^-3 .
Galactic Mergers and Evolution
Historical Mergers
The Milky Way's halo has been shaped by the merging of numerous progenitor galaxies. Recent studies using Gaia EDR3-based measurements have identified six major merger events, including the well-known Sagittarius, Cetus, and Gaia-Sausage/Enceladus mergers. These mergers have significantly contributed to the stellar population of the Galactic halo and provide a reference for galaxy formation models .
Dynamically Young Disk
The Milky Way's disk, containing most of the galaxy's stars, is dynamically young and has been perturbed by various phenomena. These include the bar and spiral arms inducing radial migration of stars, and external perturbations from satellite galaxies causing dynamical heating and phase wrapping signatures. Analysis of the motions of six million stars reveals substructures such as snail shells and ridges, indicating recent perturbations, likely from the Sagittarius dwarf galaxy .
Conclusion
The Milky Way is a complex and dynamic galaxy with a rich history of interactions and mergers. Its spiral structure, mass distribution, and dynamical properties are influenced by both internal processes and external forces. Continued research and advanced observational techniques are essential to further unravel the mysteries of our galaxy.
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