At first glance, XMM-2599 appears to be a rather boring galaxy (because it’s dying). But an international research team has recently discovered that it’s really a sleeping monster. XMM-2599 formed more than 12 billion years ago, when the universe was still very young, only 1.8 billion years old. At first the galaxy was extremely active. “Even before the universe was 2 billion years old, XMM-2599 had already formed a mass of more than 300 billion suns, making it an ultramassive galaxy,” says Benjamin Forrest, lead author of the study in Astrophysical Journal.
“More remarkably, we show that XMM-2599 formed most of its stars in a huge frenzy when the universe was less than 1 billion years old – and then became inactive,” explains Forrest. The team found that XMM-2599 produced 1000 solar masses in stars per year during its most active period. In contrast, the Milky Way produces only one new star per year.
“In this epoch, very few galaxies have stopped forming stars, and none of these are as massive as XMM-2599,” says Gillian Wilson, a professor of physics and astronomy at the lab where Forrest works. “The mere existence of such large galaxies is a challenge to our models. They do predict that they would exist, but they would be expected to be actively forming new stars. That makes XMM-2599 and the reason why it became inactive all the more interesting. Did it run out of fuel or was its central black hole active? Models will need to take into account that also early galaxies can turn off their growth.”
How the galaxy will continue to evolve is unclear. “We’ve caught XMM-2599 in an inactive phase,” says Wilson. “But the galaxy cannot lose mass. What will happen with it? Could it attract nearby star-forming galaxies and form a bright galaxy cluster?” Maybe XMM-2599 will become one of the largest galaxy clusters of the local universe – or maybe it will remain isolated. Or it’s all already happened. Due to the huge distance, we will only learn what’s happened many billions of years from now.