Number of galaxies in the observable universe
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Number of Galaxies in the Observable Universe
Overview of Galaxy Count Estimates
The observable universe is vast, containing an immense number of galaxies. Current estimates suggest that there are approximately 100 billion galaxies in the observable universe, each containing an average of 100 billion stars1 2. This estimate is based on various observational evidences and cosmological models, although the exact number can vary depending on the methods and instruments used for detection.
Evolution and Density of Galaxies
The number density of galaxies has evolved over time. Studies using galaxy stellar mass functions up to a redshift of z ≈ 8 indicate that the total number of galaxies in the universe is around 2 trillion, which is significantly higher than what is observed in deep sky surveys like the Hubble Ultra-Deep Field3. This discrepancy is due to the limitations of current observational technologies, which cannot detect the faintest galaxies contributing to the total count.
Challenges in Counting Galaxies
Counting galaxies is a complex task due to biases and incompleteness in astronomical surveys. It is easier to detect brighter and closer galaxies, while many faint and distant galaxies remain undetected4. This leads to underestimations in the total number of galaxies when relying solely on direct observations.
Implications for Galaxy Formation Models
The high number of galaxies has significant implications for models of galaxy formation and evolution. For instance, the presence of numerous faint galaxies suggests that the cosmic background light in the optical and near-infrared likely arises from these unobserved galaxies3. Additionally, the difficulty in reproducing the observed number counts of certain galaxy types, such as submillimetre galaxies (SMGs), indicates potential issues in current galaxy formation models5.
Conclusion
In summary, the observable universe contains an estimated 100 billion to 2 trillion galaxies, depending on the observational methods and depth of surveys. This vast number of galaxies poses challenges for direct counting and has important implications for our understanding of galaxy formation and evolution. As observational technologies advance, our estimates of the total number of galaxies will become more precise, providing deeper insights into the structure and history of the universe.
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Most relevant research papers on this topic
Gamma-Ray Emission of Active Galaxies
The observable universe consists of billions of galaxies, many of them containing 100 billion or more stars, like our “Milky Way” for example. On a morphological basis these galaxies are grouped into three main categories: spirals, ellipticals, and irregulars.
Hydrodynamics, Vortices and Angular Momenta of Celestial Objects
Non-irrotational hydrodynamic flow in the universe can explain the formation of large numbers of galaxies and stars, indicating a primordial rotation broken up into vortex line structures.
THE EVOLUTION OF GALAXY NUMBER DENSITY AT z < 8 AND ITS IMPLICATIONS
The total number of galaxies in the universe up to z = 8 is (2 trillion), a factor of 10 higher than would be seen in an all sky survey at Hubble Ultra-Deep Field depth, and this result resolves the Olbers' paradox.
Highly CitedHow many galaxies are in the universe?
Galaxy in the Milky Way than in the Virgo cluster, and surveys are biased towards the Milky Way, making it harder to detect distant galaxies.
Submillimetre galaxies in a hierarchical universe: number counts, redshift distribution, and implications for the IMF
Our model predicts well-matched observations of high-redshift submillimetre galaxies, suggesting that the difficulty in explaining SMG counts is likely due to more general problems rather than specific to the SMG population.
Highly CitedHerMES: SPIRE galaxy number counts at 250, 350, and 500 μm
HerMES has determined galaxy number counts down to 20 mJy, providing a more precise benchmark for galaxy population evolution models and testing pre-existing models of galaxies.
Highly CitedThe 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.
Highly CitedGalaxy number counts. - I. Photographic observations to B=23.5 mag.
COSMOS machine measurements of Anglo-American Telescope photographs provide accurate galaxy number-magnitude counts and color distributions for faint galaxies up to B=23.5 mag.
Highly CitedA common mass scale for satellite galaxies of the Milky Way
The Milky Way's satellite galaxies have a common mass of about 107 within their central 300 parsecs, indicating they are the most dark-matter-dominated galaxies known.
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