Estimated number of galaxies in the universe
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Estimated Number of Galaxies in the Universe
Introduction to Galaxy Counting Challenges
Estimating the number of galaxies in the universe is a complex task due to biases and incompleteness in astronomical surveys. These challenges arise because it is easier to detect brighter and closer galaxies, while many faint and distant galaxies remain undetected 2. Despite these difficulties, advancements in observational techniques and surveys have provided more accurate estimates.
Historical Estimates and Recent Findings
Historically, the number of galaxies in the observable universe was estimated to be around 100 billion. However, recent studies have significantly revised this number. Using data from the Hubble Space Telescope and other deep-field observations, researchers have found that the universe contains approximately 2 trillion galaxies, which is almost ten times higher than previous estimates 5. This increase is attributed to the detection of many faint galaxies that were previously unobserved.
Evolution of Galaxy Number Density
The number density of galaxies has evolved over cosmic time. Studies have shown that the total number density of galaxies decreases as the universe ages, particularly for galaxies with stellar masses greater than (10^6 M_\odot) 5. This trend indicates that the formation of new galaxies has slowed down, and many existing galaxies have merged or evolved into larger structures.
Implications for Cosmic Star Formation
The discovery of a larger number of galaxies has significant implications for our understanding of cosmic star formation. For instance, a survey of galaxies 9 to 12 billion years ago revealed a population 1.6 to 6.2 times larger than previous estimates, with high star formation rates 1. This suggests that the cosmic star formation rate was higher in the early universe than previously thought.
Present-Day Galaxy Relics
In the present-day universe, only a small fraction of massive galaxies have remained relatively unchanged since their formation at high redshifts. Studies using semi-analytical models based on the Millennium simulation estimate that less than 2% of massive galaxies have evolved in stellar mass by less than 10% since their formation 4. This small fraction highlights the dynamic nature of galaxy evolution.
Conclusion
The estimated number of galaxies in the universe has undergone significant revisions, with current estimates suggesting around 2 trillion galaxies. This increase is due to the detection of many faint and distant galaxies that were previously unobserved. The evolution of galaxy number density and the implications for cosmic star formation provide valuable insights into the history and future of the universe. Understanding these dynamics is crucial for advancing our knowledge of galaxy formation and evolution.
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Most relevant research papers on this topic
A large population of galaxies 9 to 12 billion years back in the history of the Universe
This survey reveals a large population of galaxies at 9-12 billion years back in the history of the Universe, 1.6 to 6.2 times larger than previous estimates, with vigorous star formation rates.
The stellar masses of 25 000 galaxies at 0.2 ≤ z ≤ 1.0 estimated by the COMBO-17 survey
The total stellar mass density of the universe has roughly doubled since $z = 1$, with the growth of total mass in red sequence galaxies dominating the growth of total stellar mass density.
Highly CitedEXPECTED NUMBER OF MASSIVE GALAXY RELICS IN THE PRESENT DAY UNIVERSE
Only a small fraction of massive galaxies formed at z 2 have remained unaltered since their formation, with the observed number of relic candidates being uncertain.
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 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 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 mass of the Andromeda galaxy
The Milky Way galaxy is probably the largest member of the Local Group, with the Andromeda galaxy likely being less massive than the Milky Way.
Highly CitedThe total satellite population of the Milky Way.
There should be at least 124+4027 (68% CL, statistical error) brighter than MV = 0 within 300kpc of the Sun, a smaller population than previously reported, with half of this population likely to be detected by the Large Synoptic Survey Telescope.
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