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.”

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iStock
Astronomers Discover 12 New Moons Around Jupiter
iStock
iStock

As the largest planet with the largest moon in our solar system, Jupiter is a body of record-setting proportions. The fifth planet from the Sun also boasts the most moons—and scientists just raised the count to 79.

A team of astronomers led by Scott S. Sheppard of the Carnegie Institute for Science confirmed the existence of 12 additional moons of Jupiter, 11 of which are “normal” outer moons, according to a statement from the institute. The outlier is being called an “oddball” for its bizarre orbit and diminutive size, which is about six-tenths of a mile in diameter.

The moons were first observed in the spring of 2017 while scientists looked for theoretical planet beyond Pluto, but several additional observations were needed to confirm that the celestial bodies were in fact orbiting around Jupiter. That process took a year.

“Jupiter just happened to be in the sky near the search fields where we were looking for extremely distant solar system objects, so we were serendipitously able to look for new moons around Jupiter while at the same time looking for planets at the fringes of our solar system,” Sheppard said in a statement.

Nine of the "normal" moons take about two years to orbit Jupiter in retrograde, or counter to the direction in which Jupiter spins. Scientists believe these moons are what’s left of three larger parent bodies that splintered in collisions with asteroids, comets, or other objects. The two other "normal" moons orbit in the prograde (same direction as Jupiter) and take less than a year to travel around the planet. They’re also thought to be chunks of a once-larger moon.

The oddball, on the other hand, is “more distant and more inclined” than the prograde moons. Although it orbits in prograde, it crosses the orbits of the retrograde moons, which could lead to some head-on collisions. The mass is believed to be Jupiter’s smallest moon, and scientists have suggested naming it Valetudo after the Roman goddess of health and hygiene, who happens to be the great-granddaughter of the god Jupiter.

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ESA/Herschel/SPIRE; M. W. L. Smith et al 2017
Look Closely—Every Point of Light in This Image Is a Galaxy
ESA/Herschel/SPIRE; M. W. L. Smith et al 2017
ESA/Herschel/SPIRE; M. W. L. Smith et al 2017

Even if you stare closely at this seemingly grainy image, you might not be able to tell there’s anything to it besides visual noise. But it's not static—it's a sliver of the distant universe, and every little pinprick of light is a galaxy.

As Gizmodo reports, the image was produced by the European Space Agency’s Herschel Space Observatory, a space-based infrared telescope that was launched into orbit in 2009 and was decommissioned in 2013. Created by Herschel’s Spectral and Photometric Imaging Receiver (SPIRE) and Photodetector Array Camera and Spectrometer (PACS), it looks out from our galaxy toward the North Galactic Pole, a point that lies perpendicular to the Milky Way's spiral near the constellation Coma Berenices.

A close-up of a view of distant galaxies taken by the Herschel Space Observatory
ESA/Herschel/SPIRE; M. W. L. Smith et al 2017

Each point of light comes from the heat of dust grains between different stars in a galaxy. These areas of dust gave off this radiation billions of years before reaching Herschel. Around 1000 of those pins of light belong to galaxies in the Coma Cluster (named for Coma Berenices), one of the densest clusters of galaxies in the known universe.

The longer you look at it, the smaller you’ll feel.

[h/t Gizmodo]

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