Galaxy size measurements
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Methods and Definitions for Galaxy Size Measurements
Galaxy size measurements are crucial for understanding galaxy formation and evolution. Traditionally, the effective radius (r_e), which encloses half the total light of a galaxy, has been widely used as a standard size metric. Other common measures include isophotal diameters and radii enclosing specific percentages of stellar mass (e.g., r20, r50, r80) or light. Recent work has also proposed physically motivated definitions, such as the radius at which the stellar mass density drops to a threshold associated with star formation, which can reduce scatter in size–mass relations compared to traditional measures 47.
Size–Mass and Size–Luminosity Relations in Galaxies
Large surveys have shown that galaxy sizes correlate with both stellar mass and luminosity. For example, studies using the Sloan Digital Sky Survey (SDSS) and Hyper Suprime-Cam have found that, at a given stellar mass, star-forming galaxies are generally larger than quiescent galaxies. The size–mass relation often follows a broken power law, with a pivot mass where the slope changes, reflecting different growth mechanisms for low- and high-mass galaxies 89. The scatter in these relations can be minimized by using physically motivated size definitions based on stellar mass density thresholds 47.
Evolution of Galaxy Sizes Across Cosmic Time
Galaxy sizes evolve with redshift. Observations from Hubble and JWST show that, at fixed mass, galaxies were more compact in the past. For massive star-forming galaxies, the outer regions (r80) grow over time, indicating inside-out growth, while quiescent galaxies show even stronger size evolution, especially in their inner regions 13. The median size of galaxies increases as the universe ages, and the rate of this growth depends on galaxy type and mass 39.
Impact of Wavelength and Dust on Size Measurements
The wavelength at which galaxy sizes are measured can significantly affect the results. Sizes measured in the near-infrared (tracing stellar mass) are typically smaller than those measured in the optical, especially for massive galaxies. This is because optical light can be more affected by young stars and dust, making galaxies appear larger in these bands. JWST observations have shown that galaxy half-light radii at 4.4 μm are about 9% smaller than at 1.5 μm, with the difference reaching up to 30% for the most massive galaxies . Dust also plays a role, as it can flatten the observed light profile and increase the measured effective radius, especially in the ultraviolet .
Measuring Sizes in High-Redshift and Low-Luminosity Galaxies
Gravitational lensing and deep imaging have enabled size measurements of very faint and distant galaxies. At high redshift (z ~ 4–8), star-forming galaxies show a steep size–luminosity relation, with lower-luminosity galaxies being much smaller than their brighter counterparts. Sizes can range from less than 50 parsecs to about 500 parsecs, and the relation between size and luminosity becomes steeper at lower luminosities . Non-parametric methods, which measure the total area above a surface brightness threshold, can provide more accurate sizes for irregular or clumpy galaxies, especially at high redshift, compared to traditional profile fitting .
Conclusion
Galaxy size measurements are complex and depend on the chosen definition, wavelength, and galaxy properties. Recent advances in imaging and analysis methods have improved the accuracy and physical relevance of these measurements. The evolution of galaxy sizes with mass, type, and redshift provides key insights into the processes driving galaxy growth and transformation across cosmic time 1234+6 MORE.
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