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Have astronomers pinpointed dark matter Galaxy X?

In 2009, astronomers predicted an unseen dark-matter-dominated dwarf galaxy orbiting our Milky Way – nicknamed Galaxy X. Now they say they’ve found it.

This image is a 2007 supercomputer simulation illustrating dark matter satellites as bright clumps around our Milky Way galaxy. Image via stanford.edu

This image is from a 2007 supercomputer simulation. The bright clumps illustrate dark matter satellite galaxies around our Milky Way galaxy. Many large galaxies like the Milky way are now believed to have dark matter satellites. Image via stanford.edu

As recently as January 2015, astronomers were speculating about Planets X beyond Pluto – extra planets within our solar system – but so far none have been found. Dark matter hunters this week announced what may be better success at locating a previously unseen dark-matter-dominated dwarf galaxy, which they believe lies on the far side of our home galaxy, the Milky Way. They’ve been speculating about this little galaxy, nicknamed Galaxy X, for several years. Evidence for it came originally from a 2009 analysis of ripples in the Milky Way’s outer disk. The latest study by these same astronomers – led by Sukanya Chakrabarti of Rochester Institute of Technology – points to a cluster of four young, pulsating stars approximately 300,000 light-years away. Those stars might mark the location Galaxy X. The team announced their discovery in a paper accepted for publication in the Astrophysical Journal Letters on February 5, 2015. Chakrabarti said:

I decided to see if I could actually find the thing. It was a difficult prediction to test because it was close to the plane [of our Milky Way galaxy], and therefore difficult to see in the optical.

If it exists, Galaxy X is a satellite galaxy to our Milky Way. Large galaxies like the Milky Way are thought to have many satellite galaxies that are too dim to see, dominated by dark matter, the mysterious unseen substance thought to make up 23 percent of the mass of the universe.

Chakrabarti and her team found the four stars – known as Cepheid variable stars – by analyzing data collected by the European Southern Observatory’s survey VISTA, whose infrared capability lets astronomers peer into unexplored regions close to our galaxy’s plane that are inaccessible via visible light. The stars are located in the direction of the southern constellation Norma. Cepheid variables are a well known species of star to astronomers, who use them as standard candles to measure distances across intergalactic space. According to Chakrabarti, these are the most distant Cepheid variables found close to the plane of the Milky Way.

The team believes the four newly discovered Cepheid variable stars are associated with the dark-matter dominated dwarf galaxy predicted in Chakrabarti’s 2009 analysis of ripples in the outer disk of the Milky Way. That study had specified a mass and location for the unseen galaxy. Radiation from the four Cepheid variables let Chakrabarti derive accurate distances and test her prediction. Chakrabarti said:

These young stars are likely the signature of this predicted galaxy. They can’t be part of our galaxy because the disk of the Milky Way terminates at 48,000 light-years.

The discovery of the Cepheid variables shows that our method of finding the location of dark-matter dominated dwarf galaxies works.

It may help us ultimately understand what dark matter is made up of. It also shows that Newton’s theory of gravity can be used out to the farthest reaches of a galaxy.

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Bottom line: In 2009, astronomers led by Sukanya Chakrabarti of Rochester Institute of Technology predicted an unseen dark-matter-dominated dwarf galaxy orbiting our Milky Way – nicknamed Galaxy X. Now they say they’ve found four Cepheid variable stars – 300,000 light-years away – that might mark Galaxy X’s location.

Deborah Byrd

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