Astronomers using the Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) at the W. M. Keck Observatory have discovered an ultramassive galaxy 12 billion light-years away from Earth. The unusual galaxy, named XMM-2599, formed stars at a very high rate and then died; why it suddenly stopped forming stars is unclear.
An artist’s impression of what an ancient star-forming galaxy would look like in visible light. Image credit: NRAO / AUI / NSF / B. Saxton.
“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,” said Dr. Benjamin Forrest, a postdoctoral researcher at the University of California, Riverside.
“More remarkably, we show that XMM-2599 formed most of its stars in a huge frenzy when the Universe was less than one billion years old, and then became inactive by the time the Universe was only 1.8 billion years old.”
Dr. Forrest and colleagues used spectroscopic observations from the MOSFIRE instrument to make detailed measurements of XMM-2599 and precisely quantify its distance.
“In this epoch, very few galaxies have stopped forming stars, and none are as massive as XMM-2599. The mere existence of ultramassive galaxies like XMM-2599 proves quite a challenge to numerical models,” said University of California, Riverside’s Professor Gillian Wilson.
“Even though such massive galaxies are incredibly rare at this epoch, the models do predict them. The predicted galaxies, however, are expected to be actively forming stars.”
“What makes XMM-2599 so interesting, unusual, and surprising is that it is no longer forming stars, perhaps because it stopped getting fuel or its black hole began to turn on. Our results call for changes in how models turn off star formation in early galaxies.”
The astronomers found XMM-2599 formed more than 1,000 solar masses a year in stars at its peak of activity.
“XMM-2599 may be a descendant of a population of highly star-forming dusty galaxies in the very early Universe that new infrared telescopes have recently discovered,” said Dr. Danilo Marchesini, an astronomer at Tufts University.
“We have caught XMM-2599 in its inactive phase. We do not know what it will turn into by the present day. We know it cannot lose mass. An interesting question is what happens around it. As time goes by, could it gravitationally attract nearby star-forming galaxies and become a bright city of galaxies?” Professor Wilson said.
“This outcome is a strong possibility. Perhaps during the following 11.7 billion years of cosmic history, XMM-2599 will become the central member of one of the brightest and most massive clusters of galaxies in the local Universe,” added University of California, Irvine’s Professor Michael Cooper.
“Alternatively, it could continue to exist in isolation. Or we could have a scenario that lies between these two outcomes.”
The discovery is reported in a paper in the Astrophysical Journal Letters.
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Ben Forrest et al. 2020. An Extremely Massive Quiescent Galaxy at z = 3.493: Evidence of Insufficiently Rapid Quenching Mechanisms in Theoretical Models. ApJL 890, L1; doi: 10.3847/2041-8213/ab5b9f
