Astronomers have filled in a big hole in the puzzle of how the universe evolved. Using multiple telescopes to peer out into space and back in time, they have spotted a hidden population of large galaxies dating back to when the universe was less than 2 billion years old that are invisible to optical telescopes like the Hubble Space Telescope (HST). The galaxies’ surprising abundance so early in the history of the universe may challenge conventional theories of galaxy formation, the observers say.
“This paper demonstrates that we were missing 90% of the massive galaxies,” says Mauro Giavalisco, an astronomer at the University of Massachusetts in Amherst who was not involved in the new work. “I think it will spur a lot of further research.”
For astronomers, proving that an ultrafaint spot of light on the sky is a distant galaxy is no mean feat. To do so, they use a trick that depends on the spectrum of light the galaxy pumps out. Stars crank out copious light, but the hydrogen gas from which the star forms absorbs wavelengths that are shorter than a specific part of the ultraviolet (UV) wavelength, creating a distinct cutoff light’s spectrum. Before it reaches human observers, the light is redshifted: stretched to longer wavelengths by the expansion of the universe. That slides the cutoff into another part of the spectrum: visible or near-infrared wavelengths. Searching for this telltale feature, the HST has found hundreds of galaxies that were shining when the universe was less than 2 billion years old.