Size of our galaxy
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Understanding the Size of Our Galaxy: Insights from Recent Research
Galaxy Size Measurements and Evolution
Rest-frame Near-infrared Sizes of Galaxies
Recent advancements in telescope technology, particularly the James Webb Space Telescope (JWST), have provided new insights into the sizes of galaxies. Studies leveraging JWST's capabilities have shown that galaxy half-light radii are, on average, about 9% smaller when measured at 4.4 µm compared to 1.5 µm. This size difference is more pronounced in galaxies with higher stellar masses and redder colors, indicating that mass profiles are more compact than optical light profiles at cosmic noon .
Size Distribution in the Sloan Digital Sky Survey
The Sloan Digital Sky Survey (SDSS) has provided a comprehensive dataset to study the size distribution of galaxies. Analysis of approximately 140,000 galaxies reveals that the size distribution at a given luminosity or stellar mass follows a log-normal function. This extensive dataset allows for accurate statistical analysis of galaxy sizes across different types and masses .
Size Evolution of Star-forming Galaxies
Research on the size evolution of star-forming galaxies, particularly those with redshifts between 2 and 4.5, indicates that galaxy sizes remain relatively constant on average, with a median value of about 2.2 kpc. However, parametric fitting methods often underestimate the sizes of complex, asymmetric galaxies. Larger galaxies at any redshift tend to be more massive and have higher star formation rates .
COSMOS-DASH Survey Findings
The COSMOS-DASH survey, utilizing the Hubble Space Telescope, has measured the sizes of galaxies across a wide redshift range. The median size of galaxies changes with redshift, and there is no significant difference in the sizes of the most massive star-forming and quiescent galaxies at certain redshifts. However, quiescent galaxies tend to have higher central densities .
EAGLE Simulation Insights
The EAGLE simulation has provided valuable insights into the evolution of galaxy sizes from redshift 2 to 0. It shows that passive galaxies are typically smaller than active ones at a fixed stellar mass. The simulation also reveals that high-redshift compact galaxies grow in size over time due to star migration and mergers .
Mass-dependent Slope of the Size-Mass Relation
Studies have shown that the size-mass relation of galaxies follows a broken power law, with a pivot mass that increases with redshift. This relation suggests a direct link between galaxy size and halo mass, with a consistent scaling factor across different masses and redshifts. The pivot mass also coincides with the transition from star-forming to quiescent galaxies .
Strong Size Evolution of Massive Galaxies
Massive galaxies, particularly those with high light concentration, have undergone significant size evolution since redshift 2. These galaxies were much smaller in the past, indicating that they have merged with other galaxies over time to form the large galaxies observed today .
Quasar Hosts and the Size-Mass Relation
Quasar host galaxies at redshifts between 1.2 and 1.7 have sizes ranging from 1 to 6 kpc, placing them between star-forming and quiescent galaxies of similar mass. This finding suggests that AGNs may occur in systems with concentrated gas reservoirs or be involved in processes that build bulges .
Tightness of the Size Distributions
The relationship between galaxy size and halo virial radius is remarkably tight, especially for more massive galaxies. This tightness challenges models based solely on angular momentum conservation and suggests a strong link between galaxy size and specific angular momentum .
Ultra-diffuse Galaxies vs. Milky Way-sized Galaxies
Ultra-diffuse galaxies (UDGs) have sizes within the range of dwarf galaxies and are significantly smaller than Milky Way-like galaxies. Using size estimators based on gas density thresholds provides a more accurate representation of galaxy boundaries compared to traditional effective radius measurements .
Conclusion
The size of our galaxy and its evolution over cosmic time is a complex topic influenced by various factors, including stellar mass, star formation activity, and mergers. Recent studies using advanced telescopes and simulations have provided deeper insights into these processes, revealing the intricate relationship between galaxy size, mass, and evolution. Understanding these dynamics is crucial for unraveling the history and future of galaxies in our universe.
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