For decades, astronomers have been trying to see as far as they can into the deep Universe. By observing the cosmos as it was shortly after the Big Bang, astrophysicists and cosmologists hope to learn all they can about the early formation of the Universe and its subsequent evolution. Thanks to instruments like the Hubble Space Telescope, astronomers have been able to see parts of the Universe that were previously inaccessible.
But even the venerable Hubble is incapable of seeing all that was taking place during the early Universe. However, using the combined power of some of the newest astronomical observatories from around the world, a team of international astronomers led by Tokyo University’s Institute of Astronomy observed 39 previously-undiscovered ancient galaxies, a find that could have major implications for astronomy and cosmology.
The team behind the discovery included members from Tokyo University’s Institute of Astronomy, the French National Center for Scientific Research (CNRS), the Anhui Normal University in China, the University of Ludwig-Maximilians in Munich, the National Astronomical Observatories of China, and the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) in Taiwan. Their research appeared in the Aug. 7th issue of Nature.
Spotting the “Invisible”
To put it simply, the earliest possible galaxies in the Universe have remained invisible until now because their light is very faint and occurs at long wavelengths that are undetectable by Hubble. The team therefore turned to the Atacama Large Millimeter/submillimeter Array(ALMA), whose telescopes are optimized for viewing this kind of light.
The discovery that resulted was not only unprecedented, but the discovery of this many galaxies of this type defies current cosmological models. As Tao Wang, a researcher from the AISAA and a co-author on the study, explained:
“This is the first time that such a large population of massive galaxies was confirmed during the first 2 billion years of the 13.7-billion-year life of the universe. These were previously invisible to us. This finding contravenes current models for that period of cosmic evolution and will help to add some details, which have been missing until now.”
These galaxies, though they were the largest in existence at the time, were still very difficult to spot. Much of the reason has to do with the extent to which their light has been stretched by the expansion of the Universe. In everyday astronomy, this phenomena is known as redshift, where the expansion of space (the Hubble Constant) causes the wavelength of light to become elongated, shifting it towards the red end of the spectrum.
This allows astronomers to not only tell how distant an object is, but what that object looked like in the past. But when looking to the very earliest epoch of the Universe (over 13 billion years ago) the immense distance stretches the wavelength of visible light to the point where it is no longer in the domain of visible light and becomes infrared.
Another reason these galaxies are difficult to spot is that larger galaxies tend to be shrouded in dust, especially when they are still in the early parts of their formation. This tends to obscure them more than their smaller galactic counterparts. For these reasons, there was some suspicion that these galaxies were not as old as the team suggested. As Wang indicated:
“It was tough to convince our peers these galaxies were as old as we suspected them to be. Our initial suspicions about their existence came from the Spitzer Space Telescope’s infrared data. But ALMA has sharp eyes and revealed details at submillimeter wavelengths, the best wavelength to peer through dust present in the early universe. Even so, it took further data from the imaginatively named Very Large Telescope in Chile to really prove we were seeing ancient massive galaxies where none had been seen before.”
What Does This Mean for Astronomy?
Since the discovery of these galaxies defies our current cosmological models, the team’s findings naturally have some significant implications for astronomers. As Kotaro Kohno, a professor with the Institute of Astronomy and a co-author on the study, explained:
“The more massive a galaxy, the more massive the supermassive black hole at its heart. So the study of these galaxies and their evolution will tell us more about the evolution of supermassive black holes, too,” added Kohno. “Massive galaxies are also intimately connected with the distribution of invisible dark matter. This plays a role in shaping the structure and distribution of galaxies. Theoretical researchers will need to update their theories now.”
Another interesting find was the ways in which these 39 ancient galaxies differ from our own. For starters, these galaxies had a higher density of stars than the Milky Way does today; which means that if our galaxy were similar, stargazers would be seeing something very different when they looked up at the night sky.
“For one thing, the night sky would appear far more majestic. The greater density of stars means there would be many more stars close by appearing larger and brighter,” said Wang. “But conversely, the large amount of dust means farther-away stars would be far less visible, so the background to these bright close stars might be a vast dark void.”
Since this is the first time that a galactic population of this kind has been discovered, astronomers are looking forward to what else they might find. As it stands, even ALMA is not sophisticated enough to investigate the chemical compositions and stellar populations of these galaxies. However, next-generation observatories will have the resolution for astrnomers to conduct these studies.
These include the James Webb Space Telescope, which is currently slated for launch in 2021. Ground-based observatories like the ESO’s Extremely Large Telescope (ELT), the Thirty Meter Telescope (TMT) and the Giant Magellan Telescope (GMT) are also likely to play a vital role.
It’s an exciting time for astronomers and cosmologists. Ever so slowly, they are peeling back another layer of the Universe to see what secrets lurk beneath!