The size of galaxies
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The Size of Galaxies: Insights from Recent Research
Size Distribution of Galaxies in the Sloan Digital Sky Survey
The Sloan Digital Sky Survey (SDSS) provides a comprehensive dataset to analyze the size distribution of galaxies. A study utilizing approximately 140,000 galaxies from SDSS reveals that the size distribution of galaxies, when categorized by luminosity or stellar mass, follows a log-normal function. This distribution is consistent across different morphological types of galaxies, offering a robust statistical framework for understanding galaxy sizes .
Evolution of Galaxy Sizes Over Cosmic Time
Rest-frame Near-infrared Sizes at Cosmic Noon
Recent advancements with the James Webb Space Telescope (JWST) have allowed for more precise measurements of galaxy sizes at high redshifts (z ≈ 1-2.5). Studies show that galaxy half-light radii are, on average, about 9% smaller in the near-infrared (4.4 μm) compared to optical wavelengths (1.5 μm). This size discrepancy is more pronounced in galaxies with higher stellar masses and redder colors, indicating that mass profiles are more compact than optical light profiles at these epochs .
Size Evolution in Star-forming Galaxies
Research on star-forming galaxies with redshifts between 2 and 4.5 indicates that their sizes remain relatively constant, with a median value of approximately 2.2 kpc. This finding contrasts with the significant decrease in effective radius (re) observed with increasing redshift, suggesting that traditional parametric fitting methods may underestimate the sizes of irregularly shaped galaxies .
Insights from the EAGLE Simulation
The EAGLE simulation provides a theoretical perspective on the size evolution of galaxies from redshift 2 to the present. It shows that galaxy sizes increase with stellar mass, but this relationship weakens at higher redshifts. Passive galaxies are generally smaller than active ones at a given stellar mass, and the simulation aligns well with observational data, particularly for redshifts below 1 .
Size-Mass Relation and Morphological Differences
COSMOS-DASH Survey Findings
The COSMOS-DASH survey, utilizing Hubble Space Telescope imaging, has expanded our understanding of the size-mass relation of galaxies. The survey finds that the median size of galaxies changes with redshift, and there is no significant size difference between the most massive star-forming and quiescent galaxies at certain redshifts. However, quiescent galaxies tend to have higher central densities, as indicated by their Sèrsic indices .
IllustrisTNG Simulation Results
The IllustrisTNG simulation supports the observed trends in galaxy sizes, showing that star-forming galaxies grow continuously in size, while quenched galaxies experience minimal size growth post-quenching. This results in a persistent size gap between star-forming and quenched galaxies .
The Galaxy-Halo Connection
Mass-dependent Size-Mass Relation
A study reassessing the size-mass relation using the radius containing 80% of stellar light (r80) finds a broken power law relationship. The pivot mass, where the slope changes, increases with redshift, suggesting a transition from dissipational to dissipationless growth. This relation aligns with galaxy-halo mass connections derived from other methods, indicating a consistent scaling factor across different masses and redshifts .
Tightness of Size Distributions
The relationship between galaxy size and dark matter halo virial radius (Rh) is remarkably tight, especially for more massive galaxies. This tightness challenges models based solely on angular momentum conservation and suggests that the stellar specific angular momenta (jstar) may mediate the Re-Rh connection .
Conclusion
The size of galaxies is a complex attribute influenced by various factors, including stellar mass, star formation activity, and cosmic time. Recent observational and simulation studies provide a nuanced understanding of how galaxy sizes evolve and relate to their dark matter halos. These insights are crucial for refining models of galaxy formation and evolution, offering a clearer picture of the universe's structural development.
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