Astronomers discover a strange galaxy without dark matter

Astronomers have mapped the stars (shown here in blue) and gas (green) in the strange galaxy known as AGC 114905.
Enlarge / Astronomers have mapped the stars (shown here in blue) and gas (green) in the strange galaxy known as AGC 114905.

Javier Román and Pavel Mancera Piña

Three years ago, Filippo Fraternali and his colleagues spotted half a dozen mysteriously diffuse galaxies, which looked like sprawling cities of stars and gas. But unlike almost every other galaxy we’ve ever seen, including our own Milky Way, they didn’t appear to be surrounded by huge masses of dark matter, which would normally hold these stellar metropolises together with their gravity. Scientists chose one to zoom in, a modestly sized galaxy about 250,000 light years away, and pointed the 27 antennas of the Very Large Array radio telescope in New Mexico at it.

After collecting 40 hours of data, they mapped the stars and gas and confirmed what previous snapshots had suggested: Fraternali, an astronomer at the Kapteyn Institute of Astronomy at the University of Groningen in the Netherlands. If the team or its competitors find more such galaxies, it could be a challenge for scientists. view dark matter, the dominant perspective in the field for at least 20 years. Fraternali and his team published their findings in December in the Monthly notices from the Royal Astronomical Society.

Based on decades of telescope observations and computer simulations, scientists have come to view dark matter as the hidden skeleton of the cosmos; its “joints” are massive clusters of invisible particles that house galaxies large and small. But Fraternali is not the first to see an exception to this rule. A few years ago, Pieter van Dokkum, astronomer at Yale, and his colleagues discovered similar galaxies with the hubble telescope which also seemed to lack dark matter. “These galaxies that we found in 2018, they created a lot of controversy, discussion and follow-up work because they were unexpected and hard to explain,” van Dokkum said.

These other galaxies lived in a crowded environment, where larger neighboring galaxies frequently flew, possibly dragging dark matter with them. In contrast, the Fraternali galaxy is quite isolated, with no such troublesome neighbors, so its scarcity of dark matter cannot be explained that way. “It could be very important,” van Dokkum said. “How do you bring the stars and gas together in this place without the help of dark matter?” “

These strange objects are now called “ultra-diffuse galaxies”. These are extreme outliers: in terms of mass, they are tiny, but they are spread over vast distances. Some are as large as the Milky Way, but with only a hundredth as many stars, if not fewer. They are so close to being transparent that they are difficult to spy on in the night sky. “They’re slightly weaker in the middle, so they’re hard to spot. Now, with better telescopes and deeper observations, they have become better known, ”says Mireia Montes, an astronomer at the Space Telescope Science Institute in Baltimore and an expert on these galaxies.

Beginning in the 1960s, American astronomer Vera Rubin and others first revealed the probable existence of invisible or “dark” matter, by measuring the speed at which stars in galaxies revolve around the center. , showing that the inner stars orbit at different speeds than the outer ones. Based on the rotation of these stars, scientists calculated the mass the galaxy must have to keep them constantly in orbit, rather than being thrown into space. For many galaxies, this mass was several times greater than that of all the stars added together. Scientists solved the problem by deducing the presence of some kind of dark matter, which neither emits nor reflects light, and which must constitute the rest of the mass that holds the galaxy together.

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