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From Scientific American:
Many major discoveries in astronomy began with an unexplained signal: pulsars, quasars and the cosmic microwave background are just three out of many examples. When astronomers recently discovered x-rays with no obvious origin, it sparked an exciting hypothesis. Maybe this is a sign of dark matter, the invisible substance making up about 85 percent of all the matter in the universe. If so, it hints that the identity of the particles is different than the prevailing models predict.
The anomalous x-rays, spotted by the European Space Agency’s orbiting XMM–Newton telescope, originate from two different sources: the Andromeda Galaxy and the Perseus cluster of galaxies. The challenge is to determine what created those x-rays, as described in a study published last month in Physical Review Letters. (See also an earlier study published in The Astrophysical Journal.) The signal is real but weak and astronomers must now determine whether it is extraordinary or has a mundane explanation. If that can be done, they can set about the work of identifying what kind of dark matter might be responsible. [Read more at Scientific American ]
The biggest black holes in the Universe reside at the centers of the largest galaxies. However, a new study suggests they may be proportionally even larger, compared with other galaxies. The bright cluster galaxies (BCGs) are huge galaxies found in the middle of galaxy clusters, where they grew by merging with and absorbing smaller galaxies. However, based on their X-ray and radio luminosity, their black holes may have grown much bigger—perhaps as much as ten times the mass in previous estimates. That means the largest black holes in the Universe are perhaps 60 billion times the mass of the Sun…or more.
A recent study has used an independent means of estimating black hole masses, based on their brightness in X-rays and radio light. J. Hlavacek-Larrondo, A. C. Fabian, A. C. Edge, and M. T. Hogan examined the massive central galaxies in 18 galaxy clusters and found that previous measurements could be off by as much as a factor of ten. In other words, if the luminosity-based measurements are correct, a black hole currently believed to be 6 billion times the mass of the Sun could actually be 60 billion times more massive.
That leaves two possibilities: either black holes in bright cluster galaxies behave differently by producing more light than we think they should, or the biggest black holes in the Universe might be astoundingly ultramassive. [Read more…]
I write articles and posts on a lot of different topics, both for my own blog and at Ars Technica. Many of those subjects drift pretty far from my putative area of expertise, but occasionally I get to write about something I know pretty well. To wit: last week, a group of researchers using the 4-meter Blanco telescope at Cerro Tololo (best known for its use in discovering dark energy in 1998) have measured distances to galaxy clusters very precisely. (Here’s my galaxy cluster primer, written as a podcast for 365 Days of Astronomy.) Their study, as with a major chunk of my thesis work, was intended to pin down the effect of dark energy—cosmic acceleration—on galaxy cluster formation and evolution. In particular, if dark energy’s effects change over time, that would have a profound influence on the number and size of galaxy clusters that form in a given era. To get a handle on this, we need a detailed census of clusters, dating back to the earliest times.
A new survey of galaxy clusters marks the beginning of a promising effort to map the birth and growth of galaxy clusters back to relatively early times. Jeeseon Song and colleagues used optical and infrared telescopes to measure the distances of 158 bright clusters in a large patch of the southern sky, looking back in time to when the Universe was less than one-third its current age. These observations provide the beginnings of a history of galaxy cluster evolution, which should help constrain models of dark energy. [Read more…]