Researchers using the Atacama Large Millimetre/submillimetre Array (ALMA) identified 39 faint galaxies that are not seen with the Hubble Space Telescope’s deepest view of the universe 10 billion light-years away. They are 10 times more numerous than similarly massive but optically bright galaxies detected with Hubble.

The research team assumes that these faint galaxies are the ancestors of massive elliptical galaxies in the present universe, however interestingly, no major theories for the evolution of the universe have predicted such a rich population of star-forming, dark, massive galaxies. The new ALMA results throw into question our understanding of the early universe. These results are published in the latest issue of the journal Nature.

“Previous studies have found extremely active star-forming galaxies in the early universe, but their population is quite limited,” says Tao Wang, the lead author of the paper and a researcher at the University of Tokyo, French Alternative Energies and Atomic Energy Commission (CEA), and the National Astronomical Observatory of Japan. “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.”

Wang and his team targeted three ALMA windows to the deep universe opened up by the Hubble Space Telescope (HST): the CANDELS fields. The team discovered 63 extremely red objects in the infrared images taken by NASA’s Spitzer Space Telescope: they are too red to be detected with HST. However, Spitzer’s limited spatial resolution prevented astronomers from identifying their nature.

ALMA detected submillimetre-wave emission from 39 out of the 63 extremely red objects. Thanks to its high resolution and sensitivity, ALMA confirmed that they are massive, star-forming galaxies that are producing stars 100 times more efficiently than the Milky Way. These galaxies are representative of the majority of massive galaxies in the universe 10 billion years ago, most of which have so far been missed by previous studies.

“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,” says David Elbaz, an astronomer at CEA, and coauthor on the paper, “But there is a problem. They are unexpectedly abundant.”

The researchers estimated their number density to be equivalent to 530 objects in a square degree in the sky. This number density well exceeds predictions from current theoretical models and computer simulations. In addition, according to the widely accepted model of the universe with a certain type of dark matter, it is difficult to build a large number of massive objects in such an early phase of the universe. Together, the present ALMA results challenge our current understanding of the evolution of the universe.

“Like the galaxy M87, in which recently astronomers obtained the first-ever image of the black hole, massive elliptical galaxies are located in the heart of galaxy clusters, which are believed to form most of their stars in the early universe. “ explains Kotaro Kohno, a professor at the University of Tokyo and member of the research team. “However, previous searches for the progenitors of these massive galaxies have been unsuccessful because they were based solely on galaxies that are easily detectable by HST. The discovery of this large number of massive, HST-dark galaxies provides direct evidence for the early assembly of massive galaxies during the first Gyr [1 gigayear = 1 billion years] of the universe.”

More detailed follow-up observations with ALMA and NASA’s James Webb Space Telescope will be essential to provide further insights into the nature of these galaxies, enabling a complete view of galaxy formation in the early universe.