Astronomers Discover Milky Way–Sized Galaxy That's 99.99% Dark Matter

At left, a wide view of the Dragonfly 44 galaxy, and at right, a close-up of the same image, revealing its large, elongated shape and halo of spherical clusters of stars around its core, similar to the halo that surrounds the Milky Way. Image credit: Pieter van Dokkum, Roberto Abraham, Gemini Observatory/AURA

An unusual galaxy, made up almost entirely of exotic “dark matter," has left astronomers and physicists scratching their heads. The galaxy, known as Dragonfly 44, is located some 300 million light-years from our own Milky Way and is about the same size as our galaxy—but a mere 100th of 1 percent of it is made up of ordinary matter. The rest—99.99 percent—is dark matter.

Dragonfly 44 actually has about as much dark matter as our galaxy, but it has far fewer stars. As a result, the dark matter almost completely dominates. “It’s kind of a dark twin of the Milky Way,” lead researcher Pieter van Dokkum of Yale University tells mental_floss.

The findings were published today in Astrophysical Journal Letters [PDF].

First proposed in the 1930s, dark matter is a mysterious form of matter believed to account for more than one-quarter of the mass and energy in the universe. (A larger proportion—more than two-thirds—is the even-more-mysterious dark energy; a mere 5 percent of the universe is made of ordinary, visible matter.) Dark matter doesn’t interact with ordinary matter—it can’t be seen with optical or radio telescopes—but its presence can be deduced through the gravitational tug that it exerts.

The fact that dark matter dominates over regular matter is not by itself a surprise: In most galaxies, van Dokkum explains, there’s about 50 times as much dark matter as ordinary matter. But in Dragonfly 44, that ratio is even more extreme, thanks to the lack of stars.

The only other galaxies known to be this heavily skewed toward dark matter are the small dwarf galaxies that orbit the Milky Way. But Dragonfly 44 isn’t like those galaxies—rather, it’s just as large and massive as the Milky Way itself. How it ended up so dark matter–heavy, and with so few stars, is a mystery. “We thought we understood these [more massive] galaxies quite well,” says van Dokkum. “They usually have a relatively small amount of dark matter, in proportion to the number of stars that they have. This galaxy turns that on its head.”

Because of the paucity of stars, Dragonfly 44 is extremely faint. It’s one member of a new class of diffuse, dim galaxies discovered recently using the Dragonfly telescope array, an innovative imaging system that uses ultra-“fast” commercial telephoto lenses (the kind that sports photographers use) to find dim objects in the night sky. The brainchild of van Dokkum and University of Toronto astronomer Roberto Abraham, Dragonfly was tailor-made to detect objects with “low surface brightness”: While the light from stars is concentrated in specific points in the sky, galaxies are dim and their light is spread out—and these peculiar galaxies are even dimmer, and thus even harder to see. “These objects had always been missed, but with the Dragonfy telescope, we found them,” van Dokkum says.

Later, he and his colleagues aimed Hawaii’s Keck telescope at Dragonfly 44 for a closer look (because the galaxy is so dim, this required collecting data over six nights). They were able to measure the speeds of some of the galaxy’s stars, from which the total mass of the galaxy can be calculated. From the brightness and the mass, they determined how much mass is “missing”—that is, they inferred how much extra mass must be present in the form of dark matter, so as to keep the galaxy from flying apart. Observations with the Gemini North telescope, also in Hawaii, revealed a halo of spherical clusters of stars surrounding the galaxy’s core—similar to the halo known to surround our own Milky Way. “Ultimately, we may learn about the connection between dark matter and these mysterious star clusters,” van Dokkum says.

Meanwhile, the biggest mystery of all remains the identity of the dark matter itself. Physicists’ best guess is that it’s made up of some kind of primordial particle, perhaps created at the time of the big bang—but numerous attempts to detect such particles directly (including the most recent effort) have come up empty. And Dragonfly 44, being so far away, isn’t likely to help much—but in principle, other dark matter–dominated galaxies could still be awaiting detection, much closer to home. “If we found a galaxy like this, that’s close to us—that might be the ideal place to look, to make a direct detection of the dark matter particle,” van Dokkum says.

David Spergel, a Princeton University astrophysicist who was not involved in the current research, tells mental_floss that these low surface–brightness galaxies “are useful ‘laboratories’ both for studying the properties of dark matter and understanding galaxy formation.”

The Fascinating Device Astronauts Use to Weigh Themselves in Space

Most every scale on Earth, from the kind bakers use to measure ingredients to those doctors use to weigh patients, depends on gravity to function. Weight, after all, is just the mass of an object times the acceleration of gravity that’s pushing it toward Earth. That means astronauts have to use unconventional tools when recording changes to their bodies in space, as SciShow explains in the video below.

While weight as we know it technically doesn’t exist in zero-gravity conditions, mass does. Living in space can have drastic effects on a person’s body, and measuring mass is one way to keep track of these changes.

In place of a scale, NASA astronauts use something called a Space Linear Acceleration Mass Measurement Device (SLAMMD) to “weigh” themselves. Once they mount the pogo stick-like contraption it moves them a meter using a built-in spring. Heavier passengers take longer to drag, while a SLAMMD with no passenger at all takes the least time to move. Using the amount of time it takes to cover a meter, the machine can calculate the mass of the person riding it.

Measuring weight isn’t the only everyday activity that’s complicated in space. Astronauts have been forced to develop clever ways to brush their teeth, clip their nails, and even sleep without gravity.

[h/t SciShow]

Watch Astronauts Assemble Pizza in Space

Most everyone enjoys a good pizza party: Even astronauts living aboard the International Space Station.

As this video from NASA shows, assembling pizza in zero gravity is not only possible, it also has delicious results. The inspiration for the pizza feast came from Paolo Nespoli, an Italian astronaut who was craving one of his home country’s national dishes while working on the ISS. NASA’s program manager for the space station, Kirk Shireman, sympathized with his colleague and ordered pizzas to be delivered to the station.

NASA took a little longer responding to the request than your typical corner pizzeria might. The pizzas were delivered via the Orbital ATK capsule, and once they arrived, the ingredients had to be assembled by hand. The components didn’t differ too much from regular pizzas on Earth: Flatbread, tomato sauce, and cheese served as the base, and pepperoni, pesto, olives, and anchovy paste made up the toppings. Before heating them up, the astronauts had some fun with their creations, twirling them around like "flying saucers of the edible kind,” according to astronaut Randy Bresnik.

In case the pizza party wasn’t already a success, it also coincided with movie night on the International Space Station.

[h/t KHQ Q6]


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