An international team of astronomers, reporting in the Astrophysical Journal (arXiv.org), used the magnification power of the massive cluster of galaxies Abell 1689 to discover a substantial population of 58 diminutive and faint early galaxies.
This image, taken with the Hubble Space Telescope, shows the giant galaxy cluster Abell 1689 and the newly discovered galaxies, circled. Image credit: NASA / ESA / B. Siana and A. Alavi, University of California, Riverside.
The newly discovered galaxies are the smallest, faintest, and most numerous galaxies ever seen in the remote Universe. They existed more than 10 billion years ago, when the Universe was roughly 3.4 billion years old.
These galaxies would normally be too faint for astronomers to see. But the team was able to detect them by teaming NASA’s Hubble Space Telescope with a natural zoom lens in space, produced by the gravity of the foreground galaxy cluster Abell 1689.
Abell 1689 is located about 2.5 billion light-years from Earth. It is so massive that it magnifies galaxies behind the cluster, making them appear far brighter than they would if the foreground cluster of galaxies were not there. The 58 galaxies are magnified by factors of 3-100 and are fainter than any galaxies seen at this distance before.
“There’s always been a concern that we’ve only found the brightest of the distant galaxies. The bright galaxies, however, represent the tip of the iceberg. We believe most of the stars forming in the early Universe are occurring in galaxies we normally can’t see at all. Now we have found those unseen galaxies, and we’re really confident that we’re seeing the rest of the iceberg,” said co-author Dr Brian Siana of the University of California at Riverside.
The scientists said that if this sample of galaxies is representative of the entire population at 3.4 billion years ago, then the majority of new stars formed in these small galaxies.
The newly discovered galaxies are very compact and very blue, meaning they have recently formed many new stars. Image credit: NASA / ESA / B. Siana and A. Alavi, University of California, Riverside.
“Though these galaxies are very faint, their increased numbers means that they account for the majority of star formation during this epoch,” said lead author Anahita Alavi, a PhD student at the University of California at Riverside.
“Uncovering these galaxies also helps bolster claims that hot stars in small galaxies pumped out enough radiation to ionize hydrogen by stripping off electrons. This process, called reionization, occurred about 13 billion years ago, within the first billion years after the Big Bang. Reionization made the Universe transparent to light, allowing astronomers to look far back into time,” Dr Siana added.
“Although the galaxies in our sample existed a few billion years after reionization, it’s presumed that galaxies like these, or possibly some of these galaxies, did play a big role in reionization.”
These galaxies do not look like the spiral and elliptical galaxies seen in our galactic neighborhood.
“The gravitational lensing stretches out the apparent shape of the distant galaxies, resolving them. Without the lensing, some of the galaxies would be just point sources to Hubble. We now have an idea about their sizes that previously were impossible to measure because the galaxies were unresolved,” Alavi explained.
The study shows these galaxies are small, irregularly shaped objects measuring just a few thousand light-years across.
Even when fully mature, they will be about one-tenth to one one-hundredth the mass the Milky Way. Because they are undergoing a firestorm of star birth, their light is dominated by the ultraviolet glow of fledgling stars.
The astronomers said that these extremely distant galaxies will be prime targets for the James Webb Space Telescope, an infrared observatory scheduled to launch in 2018. Through spectroscopy, this telescope will divide the light from each galaxy into its constituent colors, yielding information on the star birth and chemical content of each galaxy.
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Anahita Alavi et al. 2014. Ultra-faint Ultraviolet Galaxies at z ~ 2 behind the Lensing Cluster A1689: The Luminosity Function, Dust Extinction, and Star Formation Rate Density. ApJ 780, 143; doi: 10.1088/0004-637X/780/2/143
