Astronomers using ALMA, one of the most advanced observatories in the world, have discovered a large population of galaxies from the first 2 billion years of the universe. The finding came as a shock to the researchers who were not expecting to find such a large number of optically invisible galaxies.
As reported in Nature, these dark galaxies are both massive and forming lots of stars. They could potentially be the ancestors of the modern massive elliptical galaxies. The team's starting point was to use the Hubble Space Telescope’s CANDELS fields. These are regions from the dawn of the universe and Hubble was able to spot the brightest, most star-forming galaxies from that epoch. These objects were undergoing a starburst, producing 1,000 new stars like our Sun in just a year.
The fields were also imaged with NASA’s Spitzer, an infrared telescope. The observations showed astronomers that there were a lot more objects out there than Hubble was capable of seeing using visible light. Unfortunately, Spitzer has limited resolution so the scientists weren’t able to gain much insight into the properties of these objects. Enter, ALMA.
ALMA, or the Atacama Large Millimeter/submillimeter Array, had what it took. Using its high resolution and sensitivity, the team was able to measure the mass and star-formation rate of 39 out of 63 of these dark galaxies.
“Previous studies have found extremely active star-forming galaxies in the early universe, but their population is quite limited,” lead author Tao Wang, from the University of Tokyo, the French Alternative Energies and Atomic Energy Commission (CEA), and the National Astronomical Observatory of Japan (NAOJ), said in a statement. “Star formation in the dark galaxies we identified is less intense, but they are 100 times more abundant than the extreme starbursts. It is important to study such a major component of the history of the universe to comprehend galaxy formation.”
These galaxies are forming stars at a rate of 200 new Suns per year. This might not be as impressive as the starburst ones from the same epoch, but it is still a huge number. It’s about 100 times more stars than the Milky Way produces every year.
“By maintaining this rate of star formation, these ALMA-detected galaxies will likely transform into the first population of massive elliptical galaxies formed in the early universe,” co-author David Elbaz, also at CEA, added. “But there is a problem. They are unexpectedly abundant.”
Current models cannot explain how there are so many massive dark galaxies. On top of that, they cannot explain how they have gotten so massive in such a (relatively) short amount of time. The first 2 billion years of the universe continue to intrigue astronomers. And this research only adds to the mystery.