By looking deeper into space (and farther back in time), astronomers and cosmologists continue to push the boundaries of what is known about the Universe. Thanks to improvements in instrumentation and observation techniques, we are now at the point where astronomers are able to observe some of the earliest galaxies in the Universe – which in turn is providing vital clues about how our Universe evolved.
Using data obtained by the Kitt Peak National Observatory, a team of astronomers with the Cosmic Deep And Wide Narrowband (Cosmic DAWN) Survey were able to observe the farthest galaxy group to date. Known as EGS77, this galaxy existed when the Universe was just 680 million years old (less than 5% of the age of the Universe). Analysis of this galaxy is already revealing things about the period that followed shortly after the Big Bang.
The team responsible for the observations recently presented their findings at the 235th meeting of the American Astronomical Society, which began on Jan. 4th and will conclude tomorrow (Jan. 8th) in Honolulu. In the course of the presentation, James Rhoads (a researcher with NASA’s Goddard Space Flight Center (and the principal investigator of the DAWN survey) indiated, this latest find provides new insight into the cosmic “Dark Ages”.
Put simply, the Dark Ages began roughly 380,000 years after the Big Bang, at a time when the first neutral hydrogen atoms had formed and the Universe was filled with ionized particles. While photons were able to travel freely at this point, they could only go so far before interacting with this plasma. This had the effect of blocking out the light and making this period impossible to see with telescopes.
This period ended with what is known as reionization, where light from the first stars began to change the nature of hydrogen throughout the Universe. This effectively ended the Dark Ages 1 billion years after the Big Bang and transformed the Universe into the transparent and light-filled reality we see today. Given that it was around 680 million years after the Big Bang, EGS77 would have played a role in this transformation.
As Rhoads explained in a recent NASA press statement:
“The young universe was filled with hydrogen atoms, which so attenuate ultraviolet light that they block our view of early galaxies. EGS77 is the first galaxy group caught in the act of clearing out this cosmic fog.”
While more distant individual galaxies have been observed, EGS77 is the farthest galaxy group to shows signs of the far-ultraviolet light associated with reionization – aka. Lyman alpha light (which has a wavelength of 121.6 nanometers). When the first stars and galaxies formed, some of the Lyman light they emitted was absorbed and reemitted, creating bubbles of ionized hydrogen around them.
Vithal Tilvi, a researcher at Arizona State University, was the lead author on the study that describes their findings. As he explained:
“Intense light from galaxies can ionize the surrounding hydrogen gas, forming bubbles that allow starlight to travel freely. EGS77 has formed a large bubble that allows its light to travel to Earth without much attenuation. Eventually, bubbles like these grew around all galaxies and filled intergalactic space, reionizing the universe and clearing the way for light to travel across the cosmos.”
Because of the way the Universe is expanding, Lyman alpha light from EGS77 has been stretched (aka. cosmic redshift) to the point where it is only visible in the near-infrared wavelengths. This, Rhoades and his colleagues did with the National Optical Astronomy Observatory’s Extremely Wide-Field InfraRed Imager (NEWFIRM) on the 4-meter Mayall telescope at Kitt Peak National Observatory.
After detecting several galaxies, the researchers compared their data to images of the same region taken with the Hubble and Spitzer space telescopes. Some of these galaxies were rejected because they were observable in visible light, a telltale indication that they were being seen as they were after reionization occurred.
However, three galaxies were found to have Lyman alpha emission lines at slightly different wavelengths. This indicated a slight difference in distance between the three galaxies, which the team estimated to be around 2.3 million light-years – slightly less than the distance between the Milky Way and the Andromeda galaxy are right now.
The deployment of next-generation space telescopes is expected to reveal a great deal more in the coming years. In addition to the JWST (which is scheduled to launch in 2021) there’s also the Wide-Field Infrared Survey Telescope (WFIRST) – scheduled for launch by 2025. With their high-sensitivity to near-infrared radiation, both telescopes are expected to find additional examples of early galaxies surrounded by bubbles of ionized radiation.
As Sangeeta Malhotra, a researcher at Goddard and a co-author on the study:
“While this is the first galaxy group identified as being responsible for cosmic reionization, future NASA missions will tell us much more. The upcoming James Webb Space Telescope is sensitive to Lyman alpha emission from even fainter galaxies at these distances and may find more galaxies within EGS77.”
The study of these galaxies and other galaxies that emerged from the cosmic Dark Ages will not only tell astronomers a great deal about this transitional period in cosmic history. It will also reveal much about how the first galaxies formed and subsequently evolved. The study that describes the research team’s findings will be published in The Astrophysical Journal.