News of science:
Astronomers using the Hubble Space Telescope are mystified by a merging galaxy cluster known as Abell 520 in which concentrations of visible matter and dark matter have apparently come unglued.
A
report on the Hubble observations,
published in the Astrophysical Journal, raises more questions than answers about a cosmic pile-up that's occurring 2.4 billion light-years away.
"We were not expecting this," the study team's senior theorist, Arif Babul of the University of Victoria, said in a
news release. "According to our current theory, galaxies and dark matter are expected to stay together, even through a collision. But that's not what's happening in Abell 520. Here, the dark matter appears to have pooled to form the dark core, but most of the associated galaxies seem to have moved on."
In the case of Abell 520, the situation is completely different: The galaxies sailed through the collision, but the dark matter piled up in the middle, along with the hot gas.
Researchers were hoping that Hubble would resolve the mystery first posed by the detection of the dark core in 2007. No such luck.
"We know of maybe six examples of high-speed galaxy cluster collisions where the dark matter has been mapped. But the Bullet Cluster and Abell 520 are the two that show the clearest evidence of recent mergers, and they are inconsistent with each other," James Jee, an astronomer at the University of California at Davis who is the lead author of the Astrophysical Journal paper, said in a
news release from the Space Telescope Science Institute. "No single theory explains the different behavior of dark matter in those two collisions. We need more examples."
Jee, Babul and their colleagues propose several possible explanations for the discrepancy. One explanation might be that the dynamics of the Abell 520 collision are more complex than the Bullet Cluster's crash. Maybe multiple collisions, involving three or four galaxy clusters, have led to the dark matter pile-up.
Another possibility is that there's actually lots of ordinary galactic material in the core, but it's just too dim to be seen, even by Hubble. That would suggest that the super-dim galaxies in the core have somehow formed far fewer stars than normal galaxies.
Update for 5:40 p.m. ET: The picture of Abell 520 served as this week's "Where in the Cosmos" picture puzzle on the
Cosmic Log Facebook page this morning, and it took only a few minutes for Ryan Marquis to figure out what the image was all about. "It appears the dark matter and galaxies aren't anchored as previously believed," he wrote.
I'm sending Ryan a pair of 3-D glasses as a token of my appreciation. It turns out Ryan's a fellow space blogger who posts his items on
46BLYZ. We're glad to have him as a Cosmic Log correspondent, and hope that more of you will join our Facebook community. That's where you'll find the next "Where in the Cosmos" puzzle, a week from now.
News source:cosmiclog.msnbc.msn