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Evidence for a Dark Matter Clump in the Milky Way – Sky & Telescope

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There are ghosts in our galaxy’s previous. Now astronomers have discovered a solution to detect them.

Ana Bonaca (Middle for Astrophysics, Harvard & Smithsonian) introduced June 11th on the assembly of the American Astronomical Society in St. Louis that she and colleagues have discovered proof of a darkish matter clump within the outskirts of the Milky Method. If their end result, which is able to seem within the Astrophysical Journal (full preprint available here), pans out, it should assist astronomers perceive the character of darkish matter.

Darkish Clumps of the Cosmos, Unite!

A lot of the universe is invisible: Darkish matter gives the gravitational pressure that holds collectively galaxies and clusters of galaxies, shaping large-scale cosmic construction. In actual fact, many cosmological simulations tracing our universe’s historical past focus completely on darkish matter, which lumps collectively early on. These huge darkish matter halos appeal to extraordinary matter, which comes collectively to make stars, galaxies, and clusters.

Cold dark matter vs warm dark matter

Whereas chilly darkish matter fashions (left) predict that the Milky Method’s halo can be filled with lumpy darkish matter, heat darkish matter fashions (proper) predict fewer — and extra huge — lumps.
Lovell et al. / Month-to-month Notices of the Royal Astronomical Society 2014

However not each darkish matter blob holds a galaxy. Within the favored darkish matter situation, the place particles are fairly huge (referred to as chilly darkish matter), many smaller lumps type. Darkish blobs with lower than 10 million photo voltaic plenty wouldn’t appeal to sufficient extraordinary matter to make stars — the one solution to see them is by their gravitational results. That’s what Bonaca and colleagues got down to do after they tracked down stars belonging to a stream of stars dubbed GD-1.

The Olden-Day Globular and Its Misplaced Pearls

GD-1 was once a globular cluster, considered one of greater than 160 tightly packed spheres of stars that orbit the Milky Method. From time to time considered one of these clusters comes too near the galactic heart for gravitational consolation, and the group tears aside. Beginning three billion years in the past, tidal results started pulling stars from GD-1. Now they make a 30,000-light-year-long stellar stream, resembling a string of pearls.

GD-1 and Palomar 5

An all-sky view from the Sloan Digital Sky Survey exhibits the lengthy stream of stars that make up GD-1, in addition to a shorter however extra distinguished stream referred to as Palomar 5.

However it seems just a few of these pearls are lacking. Information from the European House Company’s Gaia satellite tv for pc present two gaps within the seemingly steady stream, in addition to a so-called spur, whose stars orbit on orbits barely altered from the remainder of the cluster. One of many gaps most likely represents the situation of the globular cluster earlier than it dissolved beneath gravitational forces. However the different hole and the spur related to it require rationalization.


The noticed GD-1 stream of stars (high) in comparison with what simulations predict GD-1 would appear like if there have been no perturbing object (backside).
Bonaca et al. / Astrophysical Journal

Bonaca and colleagues suggest huge object punched by way of GD-1 about 500 million years in the past, kicking a few of its stars out of their orderly orbits. In line with the group’s simulations, the article would have been hefty — not less than 5 million photo voltaic plenty — however would have spanned between 60 and 130 light-years.

Dark matter punch

A mannequin of a large, compact object punching by way of GD-1 reproduces its present hole and spur options
Bonaca et al. / Astrophysical Journal

“What we infer in regards to the mass and measurement of this object is according to one thing like a globular cluster,” Bonaca acknowledges. “Nonetheless, now we have calculated the orbits of the recognized globular clusters, and so they don’t come very near the stream.” It is nonetheless potential, although, cluster at the moment hiding inside or behind the galactic airplane could possibly be accountable for GD-1’s options.

Repeating Historical past?

Palomar 5

The still-dissolving globular cluster Palomar 5 (white blob in heart) with its 5°-long tidal tails, proven utilizing Sloan Digital Sky Survey knowledge. The gaps within the tidal tails could come from interactions with the Milky Method’s bar.
M. Odenkirchen and E. Grebel

Earlier than GD-1, there was Palomar 5 — one other torn-apart globular with apparent gaps inside its stream of stars. Finally, Sarah Pearson (then at Columbia College) and colleagues confirmed that these options might need had a far simpler explanation: The bar on the heart of our galaxy rotates in the identical course as Palomar 5, and may work together with its stars to barely alter their orbits. No darkish matter clumps required.

However that straightforward rationalization doesn’t work for GD-1, which orbits in the other way because the galaxy’s bar. “As a result of GD-1 strikes in the other way in our galaxy with respect to the spin of the galactic bar, and since GD1’s orbit takes it fairly removed from the bar at its closest strategy, it’s much less seemingly that the bar has had any impact on the GD-1 stream,” Pearson notes.

Bonaca and colleagues additionally handle potential interactions with the Milky Method’s disk. GD-1 would have handed by way of the disk removed from the galactic heart, coming no nearer than 40,000 light-years. The chance of working into stars or clusters out there’s low, and an interplay with a big gasoline cloud wouldn’t have made the identical sort of hole and spur.

“Whether or not the hole is certainly a results of a darkish matter subhalo remains to be to be decided, however the hole and spur are certainly intriguing,” Pearson says.

Watch the simulation of a darkish object crossing the GD-1 stream right here:

Credit score: Bonaca et al.

A Shot within the Darkish

Bonaca notes that, even when a darkish clump is accountable for GD-1’s options, it’s troublesome to attract conclusions about darkish matter based mostly on a pattern of 1 disrupted stellar stream. However, she provides, even the existence of a single 5 million-solar-mass clump can be sufficient to rule out many heat darkish matter fashions.

There’s much more work to be completed, with GD-1 and with the opposite greater than 40 stellar streams astronomers have found within the Gaia knowledge. For now, Bonaca says, “That is an thrilling demonstration, a proof of precept.”

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