Large number of dwarf galaxies discovered in the early universe

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A UC Riverside-lead team of astronomers has discovered a large number of dwarf galaxies in the early universe by using the gravitational lensing phenomenon, completing astronomers’ census of star-forming galaxies in that epoch.



Dwarf galaxies are the smallest and dimmest galaxies in the universe. Though diminutive, they are incredibly important for understanding the history of the universe. It is believed that dwarf galaxies played a significant role during the reionization era, in transforming the early universe from being dark, neutral and opaque to one that is bright, ionized and transparent. Despite their importance, distant dwarf galaxies remain elusive, because they are extremely faint and beyond the reach of even best telescopes. Consequently, our current picture of the early universe is not yet complete.

However, there is a way around this limitation. As predicted by Einstein’s general theory of relativity, a massive object such as a galaxy located along the line of sight to another distant object, can act as a natural lens, magnifying the light coming from that background source. This phenomenon, known as gravitational lensing, causes the background object to appear brighter and larger. Therefore, these natural telescopes can allow us to discover unseen distant dwarf galaxies. As a proof of concept, in 2014, the UC Riverside team targeted one cluster of galaxies that produce the gravitational lensing effect, and got a glimpse of what appeared to be a large population of distant dwarf galaxies.


The team used the Wide Field Camera 3 on the Hubble Space Telescope to take deep images of three clusters of galaxies.  “Our study found, for the first time, a large population of distant dwarf galaxies from when the universe was between two to six billion years old,” said Dr. Anahita Alavi, leader of the research project. This cosmic time is critical as it is the most productive time for star formation in the universe. In addition, the team took advantage of the spectroscopic data from Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) on the W.M. Keck Observatory, to confirm that the galaxies belonged to this important cosmic period.

“These dwarf galaxies are 10-100 times fainter than galaxies that have been previously observed during these epochs”, added prof. Brian Siana, an assistant professor of the department of Physics and Astronomy at UCR, and principal investigator of the observing programs. Though faint, these galaxies are far more numerous than their brighter counterparts. This study demonstrates that the number of dwarf galaxies evolves during this important epoch such that they are even more abundant at earlier times. Therefore, the team has unveiled the most numerous population of galaxies in the early universe.

Despite their faintness, these dwarf galaxies produce more than half of the ultraviolet light during this era. As ultraviolet radiation is produced by young hot stars, dwarf galaxies host a significant fraction of newly-formed stars at these cosmic times and likely played a prominent role in the reionization era. These galaxies will be the primary targets of the next generation of telescopes, particularly the James Webb Space Telescope, scheduled to launch in October 2018.


The UC Riverside-lead team results were published in The Astrophysical Journal in the paper titled “The Evolution Of The Faint End Of The UV Luminosity Function During The Peak Epoch Of Star Formation (1 < z < 3)”.


The research group is composed of Anahita Alavi (UC Riverside), Brian Siana (UC Riverside), Johan Richard (Université de Lyon), Marc Rafelski (NASA Goddard/STScI), Mathilde Jauzac (Durham University/University of KwaZulu-Natal), Marceau Limousin (Laboratoire d’Astrophysique de Marseille), William R. Freeman (UC Riverside), Claudia Scarlata (University of Minnesota), Brant Robertson (UC Santa Cruz), Daniel P. Stark (University of Arizona), Harry I. Teplitz (IPAC/Caltech) and Vandana Desai (Caltech).


Image: Massive cluster of galaxies Abell 1689 is creates a strong gravitational effect on background and older galaxies, seen as arcs of light. Image by: NASA, ESA, and B. Siana and A. Alavi (University of California, Riverside)