“Detected” –Strange Dark-Matter Objects of the Milky Way

Milky Way Galaxy

 

A strange dark matter phenomena is speeding towards the Sun at 500 kilometers per second. In September of 2018, a team of astronomers detected 10 billion solar masses worth of dark matter from an ancient dwarf galaxy, Gaia-Enceladus also dubbed the Sausage Galaxy, swallowed by the Milky Way billions of years ago carried along by a vast star stream that harbors our Solar System.

The Gaia-Enceladus collision, around 10 billion years ago, would have taken millions of years to unfold, said Carme Gallart, lead author of a 2019 study published in Nature Astronomy. “It’s a very gradual process—it’s not something like a car crash—it’s something that has an effect on the galaxy as a whole. It’s very massive so it happens slowly in human terms, not so slowly in cosmic time.”

Milky Way’s ‘Dark-Matter’ Anomaly –“Explains Why Our Galaxy Appears to be Special”

A stellar stream is an association of stars orbiting a galaxy that was once a globular cluster, or in this case the S1 Stream of the ancient dwarf galaxy, that has been ripped apart and stretched out along its orbit by tidal forces. “There are tons of these streams all over the galaxy, some of them are really huge and you can see them in the sky,” said theoretical physicist Ciaran O’Hare from the University of Zaragoza in Spain who led the 2018 study.

O’Hare’s team determined that remnants of the Gaia-Enceladus dwarf galaxy eventually formed the halo of the present-day Milky Way, and that the collision contributed to “violent bursts” of star formation for around another four billion years, after which gas from those formations settled into the Milky Way’s thin disk that runs through the center of the galaxy.

“Mashup!” –Massive Ancient Collision ‘Puffed and Fractured’ the Milky Way

In a separate 2018 study, a team of astronomers led by Amina Helmi, University of Groningen, The Netherlands, looked at seven million stars from the Gaia Spacecraft data– those for which the full 3D positions and velocities are available – and found that some 30,000 of them were part of an ‘odd collection’ moving through the Milky Way. The observed stars in particular are currently passing by our solar neighborhood.

“Hiding in Plain Sight” –Milky Way’s Ancient Merger With Fossil Galaxy Gaia-Enceladus

We are so deeply embedded in this collection that its stars surround us almost completely, and so can be seen across most of the sky. Even though they are interspersed with other stars, the stars in the collection stood out in the Gaia data because they all move along elongated trajectories in the opposite direction to the majority of the Galaxy’s other hundred billion stars, including the Sun.

S1 Star Stream

The illustration below shows a small galaxy being torn apart as it is consumed by a larger galaxy, like the Milky Way. Gradually, the stars and dark matter that belong to the smaller galaxy were mixed into the Milky Way halo. Our galaxy is embedded in a cloud of dark matter, thought to consist of tiny particles traveling along orbits through the halo, permeating all regions of the galaxy, extending far beyond the edge of the bright central spiral, but also orbiting through our solar system. [Jon Lomberg in collaboration with David Martinez-Delgado for the Stellar Tidal Stream Survey]

 

S1 Star Stream

 

The European Space Agency’s billion-star survey using the Gaia spacecraft zoomed in on the S1 stream because its some 30,000 stars have a different chemical composition than those native to our galaxy. While there are over 30 such streams known in our galaxy, S1 captured the intense interest of astronomers because our solar system is actually inside this stream. Similar elliptical paths will intersect for millions of more years.

Gaia revealed the vast stream of stars  traveling along similar orbits around the center of the Milky Way: highly elongated, tipped to the plane of the galaxy’s disk, and weirdly in a direction backward relative to other stars. This structure is so big that we’re actually inside it — literally, it surrounds the Sun in all directions — and can be seen stretching nearly across the entire sky.

Hubble and Gaia –“Measure the Milky Way & Its Vast Envelope of Dark Matter”

“What we want to do is add the stream as part of our kind of main prediction for the types of signal that should show up in a dark matter experiment,” O’Hare said. According to a statement, current detectors searching for weakly interacting massive particles (WIMPs) (one popular idea of what dark matter might be) probably won’t see anything from S1, but future tech might.

O’Hare and colleagues looked at data captured by the liquified xenon detector, the LZ experiment located at the Sanford Underground Research Facility in South Dakota, US – and found the stream could be detected above the standard wind if it made up 10% of local dark matter, and the particles were between five and 25 times the mass of a proton.

The counter-rotating structure of the S1 stream will dramatically increase the amount of dark matter appearing to come from the same patch of sky as the standard dark matter wind, producing a tell-tale ‘ring’ like structure around this wind, something that directional dark matter detectors such as the multinational CYGNUS collaboration could easily detect in future.

Orbiting Dark-Matter Clumps

In January 2019, The Galaxy reported that theorists suspect that our Galaxy sits inside an enormous, roughly spherical halo of dark matter that, much like ordinary matter, has clumped together into smaller structures thanks to gravity. Cosmological simulations suggest that thousands of large dark-matter clumps orbit the Galaxy, occasionally getting eaten by a mass of dark matter at the center, in a process akin to the Milky Way’s consumption of its small satellite dwarf galaxies.

 

Dark Matter Clumps

 

The vast majority of the dark-matter substructures are thought to contain few or no stars make them hard to detect, but Gaia might have found a hint of one in GD-1, a long stream of stars discovered in 2006 that stretches across half of the northern sky. Gaia enabled astrophysicist Adrian Price-Whelan at Princeton University and astronomer Ana Bonaca at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, to pick out true members of the group. The two study how the tidal field of the Milky Way galaxy disrupts globular clusters, and what the resulting debris can tell us about the underlying distribution of dark matter

“Milky Way’s Hidden Past” –Gaia Unveils Dark-Matter Object 1-100 Million Times Mass of the Sun

In November 2018, Price-Whelan and Bonaca and two other colleagues identified a distinct gap, that could be the scar of an encounter with a massive object some 500 million years ago. “As the dark object sped past the stream, it might have separated the train of stars by gravitationally tugging on some, reports Mann, allowing them to pull ahead of their companions.

“The most likely culprit seems to be a dense dark-matter clump, probably somewhere between 1 million and 100 million times the mass of the Sun,” says Bonaca, an estimate could have implications for physical models of dark matter. A dark-matter particle’s mass helps to dictate how fast it can move and, in turn, the size of clusters it is liable to form. The GD-1 perturber’s size is in an interesting range, says Bonaca, that could eliminate hypothesized dark-matter candidates that are relatively low in mass.

“Warped & Twisted” –First 3D Map Unveils Milky Way’s Real Shape

The first accurate 3D image of the Milky Way at the top of the page reveals its true shape: warped and twisted. Astronomers from Macquarie University and the Chinese Academy of Sciences have used 1339 ‘standard’ stars to map the real shape of our home galaxy in a paper published in Nature Astronomy today. Artist’s impression above of the warped and twisted Milky Way disk. (Chen Xiaodian)

Bonaca and her team are now interested in using Gaia data to determine the orbit of the ancient dark-matter object. “If they can ascertain where it could be found today,” reports Nature, “they might be able to detect its gravitational effects on other material. Or perhaps they could train γ-ray telescopes on the spot to look for evidence of dark-matter particles annihilating one another or decaying, processes that could emit energetic photons. Either technique could offer a more-direct probe of the invisible substance’s physical properties.”

The Daily Galaxy via Nature, SLAC/Kavli Institute, PhysRevD, and AFP

The first accurate 3D image of the Milky Way at the top of the page reveals its true shape: warped and twisted. Astronomers from Macquarie University and the Chinese Academy of Sciences have used 1339 ‘standard’ stars to map the real shape of our home galaxy in a paper published in Nature Astronomy today. Artist’s impression above of the warped and twisted Milky Way disk. (Chen Xiaodian)

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