How big is the galaxy
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Understanding the Size of Galaxies
Galaxy Size Evolution Over Cosmic Time
The size of galaxies and their evolution over cosmic time have been extensively studied to understand galaxy formation and growth. Recent advancements, particularly with the James Webb Space Telescope (JWST), have provided new insights into the sizes of galaxies at different epochs. For instance, measurements of galaxy sizes at 4.4 µm (rest-frame near-infrared) show that galaxies are, on average, about 9% smaller than when measured at 1.5 µm (rest-frame optical)1. This difference is more pronounced in massive galaxies, indicating that their mass profiles are more compact than their optical light profiles, especially at cosmic noon (z ≈ 1-2.5)1.
Size Distribution and Measurement Techniques
The distribution of galaxy sizes varies significantly with redshift and galaxy type. Studies using the COSMOS and ECDFS fields have shown that the average size of star-forming galaxies remains roughly constant at around 2.2 kpc for redshifts between 2 and 4.52. However, traditional parametric fitting methods often underestimate the sizes of complex, asymmetric galaxies, suggesting that non-parametric approaches might provide more accurate measurements2.
Maximum Size and Mass of Spiral Galaxies
Spiral galaxies, which form more quiescently compared to giant ellipticals, have a maximum stellar mass of about 10^11.8 solar masses. This mass limit is due to their efficiency in converting gas into stars, diverging from the scaling laws that describe lower-mass spiral galaxies3. This implies that while spiral galaxies can be massive, they do not reach the extreme sizes and masses of giant ellipticals formed through major mergers.
Size-Mass Relation and Evolution
The size-mass relation of galaxies has been a key focus in understanding their growth. Data from the COSMOS-DASH survey, combined with archival data, show that the median size of galaxies changes with redshift, with massive star-forming and quiescent galaxies having similar sizes at z ≈ 1.5-3.04. However, quiescent galaxies tend to have higher central densities compared to star-forming ones, indicating different evolutionary paths4.
Large-Scale Distribution and Clustering
On a larger scale, galaxies are not randomly distributed but are clustered, with significant structures extending up to 2,000 h^-1 Mpc. This clustering is more pronounced at scales of 128 h^-1 Mpc, suggesting an excess correlation that challenges conventional cosmological theories5. Such large-scale structures are crucial for understanding the overall distribution and evolution of galaxies in the universe.
Simulation Insights on Galaxy Size Evolution
Simulations like the EAGLE project have provided valuable insights into the size evolution of galaxies. These simulations show that galaxy sizes increase with stellar mass, but the relation weakens with increasing redshift. Passive galaxies are typically smaller than active ones at a fixed stellar mass, and high-redshift compact galaxies grow in size over time due to star migration and mergers6.
Quasar Hosts and Size-Mass Relation
Quasar host galaxies at high redshifts (z ≈ 1.2-1.7) have sizes ranging from 1 to 6 kpc, placing them between star-forming and quiescent galaxies of similar mass. This suggests that AGNs (Active Galactic Nuclei) may not significantly increase galaxy sizes but are associated with concentrated gas reservoirs or secular processes that build bulges7.
Ultra-Diffuse Galaxies and Size Misconceptions
Ultra-diffuse galaxies (UDGs) have been a topic of debate regarding their sizes. Using a physically motivated size measure based on gas density thresholds, UDGs are found to be within the size range of dwarfs and significantly smaller than Milky Way-like galaxies. This challenges previous conclusions drawn from effective radius measurements, which are sensitive to light concentration8.
Conclusion
The size of galaxies is a complex attribute influenced by various factors, including mass, star formation activity, and cosmic time. Advances in observational techniques and simulations have provided a deeper understanding of galaxy size evolution, revealing that galaxy sizes are not static but change significantly over time and across different environments. These insights are crucial for refining galaxy formation models and understanding the large-scale structure of the universe.
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Most relevant research papers on this topic
Rest-frame Near-infrared Sizes of Galaxies at Cosmic Noon: Objects in JWST's Mirror Are Smaller than They Appeared
Galaxy sizes are significantly smaller in infrared light than optical light, with stronger effects at higher stellar masses and redder rest-frame colors.
Size evolution of star-forming galaxies with 2
Star-forming galaxies with a median size of 2 : 2 kpc have a relatively constant size, with the largest at any redshift being more massive and forming more stars, while their light profiles become more concentrated with increasing redshift.
COSMOS-DASH: The Evolution of the Galaxy Size–Mass Relation since z ∼ 3 from New Wide-field WFC3 Imaging Combined with CANDELS/3D-HST
The median size of galaxies in the 1.5-z range changes with redshift, with massive quiescent galaxies having higher central densities than star-forming galaxies.
Highly CitedLarge-scale distribution of galaxies at the Galactic poles
Large-scale surveys reveal an excess correlation and apparent regularity in the galaxy distribution at the Galactic poles, with a characteristic scale of 128 h1 Mpc.
Highly CitedSize evolution of normal and compact galaxies in the EAGLE simulation
The EAGLE simulation shows that galaxy sizes increase with stellar mass, but the relation weakens with increasing redshift, and passive galaxies are typically smaller than active galaxies at a fixed stellar mass.
Highly CitedWhere Do Quasar Hosts Lie with Respect to the Size–Mass Relation of Galaxies?
Quasar hosts have intermediate sizes compared to equal-mass star-forming and quiescent galaxies, suggesting they may preferentially occur in systems with concentrated gas reservoirs or are involved in secular compaction processes.
Are ultra-diffuse galaxies Milky Way-sized?
Ultra-diffuse galaxies (UDGs) are within the size range of dwarfs, not comparable to Milky Way-like objects, and using effective radius for galaxy size can lead to misleading results.
The size distribution of galaxies in the Sloan Digital Sky Survey
The size distribution of galaxies is well-defined by a log-normal function, with median being the median size for each type of galaxy.
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