Scientists baffled by the “missing” dark matter in a galaxy

A team of astronomers have examined a galaxy far, far away and found evidence that it contains little, if any, dark matter, so a paper published this month in the monthly bulletin of the Royal Astronomical Society. The confusion is that until now, scientists have never thought it was possible.

The galaxy AGC 114905 is about 250 million light years away and about the size of the Milky Way. It is also much more diffuse than the Milky Way and contains about a thousand times fewer stars. According to theoretical models, gas-rich ultra-diffuse galaxies like AGC 114905 should be held together by dark matter, an invisible type of matter believed to make up most of the universe.

Galaxy AGC 114905, which puzzled scientists with its apparent lack of dark matter.  (Photo credit: Javier Román & Pavel Mancera Piña)

Galaxy AGC 114905, which puzzled scientists with its apparent lack of dark matter. (Photo credit: Javier Román & Pavel Mancera Piña)

Pavel Mancera Piña and his co-authors previously observed AGC 114905 and five other ultra-diffuse galaxiesto find hardly any evidence of dark matter among them. Following the publication of these results, the Mancera Piña team took 40 hours of measurements of AGC 114905, the most extreme of the six galaxies, using the Very Large Array, an observatory that enables higher-resolution data. They found that the movement of the galaxy’s gases can be explained by regular matter without the need for dark matter.

NASA Black Hole Gallery

“Of course we thought and hoped for that, because it confirms our measurements so far,” said Mancera Piña. in a statement. “But now the problem remains that the theory predicts there must be dark matter in AGC 114905, but our observations say it is not. In fact, the difference between theory and observation is getting bigger and bigger.”

There was some discussions in recent years about the existence of other ultra-diffuse galaxies that appear to have little or no dark matter. Other researchers have also studied galaxies that are apparently deficient in dark matter, but AGC 114905 is much more isolated, and the Mancera Piña team say their techniques and measurements are more robust. The team says errors that would interfere with their results, such as the miscalculation of the galaxy’s angle, are unlikely.

Astronomers are not yet sure how dark matter-free galaxies might form, but further study of galaxies like AGC 114905 could help them understand more about the nature of dark matter.

Kait Sanchez is a freelance writer for IGN. Find her on Twitter @crisp_red.

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