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Some Of Our Baryons Are Missing

Galaxy clusters don’t seem to have enough visible matter in them. Now astrophysicists think they know where it’s hiding

There’s a puzzle emerging from the great x-ray observatories in the sky that in the last few years have been piecing together a map of the universe through x-ray eyes.

This x-ray map gives a good idea of the large scale distribution of gas and stars throughout the universe, which is important because it tells us how the universe must have evolved and the forces that have shaped it.

The largest structures in these maps are clusters of galaxies which can each represent significant fractions of the entire universe. Their mass is such that escape ought to be difficult for ordinary baryonic matter, the stuff from which us and all visible stuff is made. So the fraction of visible matter to dark matter in these clusters ought to be close to the universe’s average, since there’s nowhere else for it to hide.

And yet the maps made by orbiting x-ray telescopes such as Chandra and XMM-Newton indicate that the amount of visible matter is significantly lower than the average. So the question that many astrophysicists are scratching their heads over is where this missing matter could have gone.

Now, Bilhuda Rasheed, Neta Bahcall, and Paul Bode at Princeton University in New Jersey say they have solved this embarrassing conundrum. The problem is that astronomers have been looking in the wrong place for the missing mass.

It turns out that most mass surveys have concentrated on looking for heavy stuff in the central regions of galaxy clusters. This, after all, is where stars tend to cluster. But the Princeton crew say this ignores the outer regions of clusters where their analysis indicates that significant amounts of gas is sitting. “This suggests that the baryons are not missing, they are simply located in cluster outskirts,” they say.

That neatly solves the mystery. Or almost solves it. All that remains is for somebody to go and have look for this gas and report back. The Princeton guys are confident of success: “Upcoming observations should be able to detect these baryons,” they say.

Time to get polishing with those lens cloths.

Ref: arxiv.org/abs/1007.1980: Where Are The Missing Baryons In Clusters?

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