Galaxy hunters spot hidden giants in the early universe

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.

This technique has one big problem, however: It’s keenly sensitive to smaller, younger galaxies, but blind to bigger, older ones. In the larger, more mature galaxies of this early epoch, there were more supernovae, and dust from these stellar explosions absorbed most of the UV light. That would have obliterated the telltale spectral cutoff. In fact, to observers on Earth, early massive galaxies are simply invisible at optical wavelengths, says Tao Wang, an astrophysicist at the University of Tokyo.

Now, Wang, David Elbaz, an astrophysicist at the French Alternative Energies and Atomic Energy Commission in Saclay, and an international team have found a way to spot the missing galaxies by turning their enemy—dust—into an ally. The dust that absorbs starlight must heat up and radiate at longer infrared wavelengths, which Wang, Elbaz, and colleagues could look for.

The researchers confined their search to three tiny patches of the sky that the HST had imaged with great sensitivity. They used NASA’s Spitzer Space Telescope to scan those patches at midinfrared wavelengths. Spitzer spotted 63 candidate galaxies, but the scope lacked the spatial resolution to clinch the identifications. So the researchers then studied each candidate at longer far-infrared wavelengths using the international Atacama Large Millimeter/submillimeter Array (ALMA), an array of 66 dishes in the high desert of northern Chile. Those observations confirmed 39 of the candidate galaxies, the researchers report today in Nature . What’s more, by searching previously taken images, the team revealed that these ancient galaxies are invisible to the HST—leading them to classify the galaxies as “H-dropouts.”

Data from ALMA and other telescopes show the newly found galaxies typically weigh about 100 billion solar masses and are filled with dust gently warmed to about 35 K—just as one would expect if it were being heated by stars in a typical star-forming galaxy and not by, say, the massive black hole at the center of a quasar. If there are just as many of these ancient galaxies in the rest of the sky, they must account for half of all star production at the time in the universe, the scientists estimate. “They really seem to be quite common sources in the early universe,” says Giulia Rodighiero, an astronomer at the University of Padova in Italy who was not involved in the work. “So, they really represent the average mode of galaxy assembly” in the youthful universe.

Spotting these early galaxies should be a boon to theorists modeling galaxy formation and evolution, Elbaz says. In fact, he argues, the observations already challenge the prevailing theory. “You would not expect to make so efficiently galaxies so massive so early in the universe,” he says.

Others are less sure. Rather than clashing with the prevailing theory, the new observation will probably help firm it up, Giavalisco says. “At this stage, discoveries like this are very important because they inform the theory,” he says. Rodighiero notes that the same models already have problems re-creating dusty galaxies later in the history of the universe, so the new observations are not the only challenges for theorists. Both note that theorists modeling galaxy evolution must still rely on approximate models of star formation, which lie at the crux of the issue.

The observations demonstrate the power of ALMA, which has unprecedented spatial resolution at these far infrared wavelengths. In 1998, astronomers used the James Clerk Maxwell Telescope, a single 15-meter infrared dish on Mauna Kea in Hawaii, to spot an early galaxy, Elbaz says. But the telescope had such poor resolution it took 14 years to pinpoint the source on the sky. In contrast, Elbaz says, the ALMA dishes nailed down each new galaxy by focusing on it for less than 2 minutes.