Galaxies grow up fast — or at least they used to. Astronomers have discovered from the young universe a bevy of massive galaxies that are not only grown-up but retired. The galaxies, each roughly the size of the Milky Way, formed most of their billions of stars in the first 1.5 billion years after the Big Bang, when the universe was just 11 percent of its current age. Astronomers can’t explain how the galaxies formed and shut down so quickly.
The universe was more vibrant in its youth. Distant galaxies, whose light travels for billions of years before reaching Earth’s telescopes, are typically compact and furiously churn out new stars. So finding a distant group of retired heavyweights came as a surprise. The researchers identified the silent sentinels by measuring the galaxies’ brightness in visible and infrared light. The data indicate the distances to the galaxies as well as the makeup of their stars.
Before they mysteriously turned star formation down to a trickle, these galaxies must have been creating about 100 suns per year, the team calculates. Astronomers should find such productive star factories in even more distant galaxies from an earlier part of the universe. But those more prolific galaxies are nowhere to be found. One possibility for the source of the mature galaxies, the authors speculate in the March 1 Astrophysical Journal Letters, is another known group of dust-enshrouded galaxies. The dust, possibly blown out by intense star formation, could be blocking UV light from stellar nurseries.
