Astronomers have known that roughly 12 billion years ago, the gas in deep space was much more opaque than it is now in some regions, although the opacity varied widely from place to place. But they weren’t sure about what caused those variations. To learn why the differences occurred, University of California, Riverside astronomer George Becker and co-authors used the Subaru Telescope on the summit of Mauna Kea in Hawaii to search for galaxies of young stars in an exceptionally large region of space. Their results appear in the Astrophysical Journal.
Computer simulation of a region of the Universe wherein a low-density ‘void’ (dark blue region at top center) is surrounded by denser structures containing numerous galaxies (orange/white). Image credit: TNG Collaboration.
It has long been known that the Universe is filled with a web-like network of dark matter and gas. This cosmic web accounts for most of the matter in the Universe, whereas galaxies like our own Milky Way make up only a small fraction.
Today, the gas between galaxies is almost totally transparent because it is kept ionized — electrons detached from their atoms — by an energetic bath of ultraviolet (UV) radiation.
Over a decade ago, astronomers noticed that in the very distant past, about 12.5 billion years ago, the gas in deep space was not only highly opaque to UV light, but its transparency varied widely from place to place, obscuring much of the light emitted by distant galaxies.
Then a few years ago, Dr. Becker and colleagues found that these differences in opacity were so large that either the amount of gas itself, or more likely the radiation in which it is immersed, must vary substantially from place to place.
“Today, we live in a fairly homogeneous Universe. If you look in any direction you find, on average, roughly the same number of galaxies and similar properties for the gas between galaxies, the so-called intergalactic gas,” Dr. Becker said.
“At that early time, however, the gas in deep space looked very different from one region of the Universe to another.”
To find out what created these differences, the astronomers turned to the Subaru Telescope, one of the largest telescopes in the world.
Using its powerful camera, they looked for galaxies in a vast region, roughly 300 million light-years in size, where they knew the intergalactic gas was extremely opaque.
For the cosmic web more opacity normally means more gas, and hence more galaxies. But the team found that region contains far fewer galaxies than expected — clear evidence that starlight could not get through. The paucity of galaxies could be the reason this region is so opaque.
“It is not that the opacity is a cause of the lack of galaxies. Instead, it’s the other way around,” said University of California, Los Angeles Professor Steven Furlanetto, co-author of the study.
The researchers concluded that because the gas in deep space is kept transparent by UV light from galaxies, fewer nearby galaxies might make it murkier.
“Normally it doesn’t matter how many galaxies are nearby; UV light that keeps the gas in deep space transparent often comes from galaxies that are extremely far away. That’s true for most of cosmic history, anyway,” Dr. Becker said.
“At this very early time, it looks like UV light can’t travel very far, and so a patch of the Universe with few galaxies in it will look much darker than one with plenty of galaxies around.”
This discovery may eventually shed light on another phase in cosmic history.
In the first billion years after the Big Bang, UV light from the first galaxies filled the Universe and permanently transformed the gas in deep space.
Astronomers believe that this occurred earlier in regions with more galaxies, meaning the large fluctuations in intergalactic radiation inferred by Dr. Becker’s team may be a relic of this patchy process, and could offer clues to how and when it occurred.
“There is still a lot we don’t know about when the first galaxies formed and how they altered their surroundings,” Dr. Becker said.
“By studying both galaxies and the gas in deep space, we hope to get closer to understanding how this intergalactic ecosystem took shape in the early Universe.”
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George D. Becker et al. 2018. Evidence for Large-scale Fluctuations in the Metagalactic Ionizing Background near Redshift Six. ApJ 863, 92; doi: 10.3847/1538-4357/aacc73
