The galaxy M87 (lower left) might be a dead ringer for COSMOS 11494 today (black spots are deleted foreground stars) Chris Mihos (Case Western Reserve University)/ESO

Great galaxies can grow up fast. Extreme quantities of magnesium in a quiet specimen far away reveal it was once one of the wildest star creators in the universe.

The galaxy, named COSMOS 11494, is 10.7 billion light years away, so we see it when the universe was just 3.1 billion years old. It’s the most distant galaxy in which astronomers have measured the abundances of both magnesium and iron.

Together, the two elements indicate how quickly the galaxy has spawned stars. Magnesium is cast into space when short-lived massive stars blow up, whereas most of the iron in your blood ultimately comes from white dwarfs that explode after stealing material from a companion star, a process that takes much longer. Having more magnesium relative to iron means a galaxy has run through its star-forming cycle more quickly.


“We found that the magnesium abundance is really extreme,” says Mariska Kriek of the University of California at Berkeley. Using a new spectrograph on the giant Keck I telescope in Hawaii, her team measured a magnesium-to-iron ratio four times that of the sun – the largest ever found in a massive galaxy.

Brief growth spurt

This high ratio means that the galaxy created all its stars in a cosmic eye-blink, before any iron-producing white dwarfs exploded. The growth spurt lasted only 100 to 500 million years.

“This galaxy just went off like crazy, and it had a huge vigorous star-forming period,” Kriek says. Then, for unknown reasons, COSMOS 11494 suddenly quit making new stars.

The galaxy’s stellar mass is five times the Milky Way’s, or about 320 billion solar masses. Divide that number by the duration of the star-forming spurt, and it suggests the galaxy once converted 600 to 3000 solar masses’ worth of gas into stars a year. That far exceeds the Milky Way’s current star formation rate of about 2 solar masses per year.

Chiaki Kobayashi at the University of Hertfordshire in Hatfield, UK, is excited by the finding but warns that it raises lots of problems. “For example, it doesn’t match with the current understanding of galaxy evolution,” she says.

The unusual magnesium-to-iron ratio now needs to be confirmed, Kobayashi says – perhaps by searching the Milky Way’s central bulge for a star that has exactly the same spectrum and comparing its elemental abundances with that of the distant galaxy.

If we could magically see COSMOS 11494 as it is today, Kriek says it would probably resemble M87, a giant elliptical galaxy in the constellation Virgo. By now, COSMOS 11494 has probably swallowed smaller galaxies, gaining mass but reducing its magnesium-to-iron ratio.

Journal reference: Nature, DOI: 10.1038/nature20570