Ghosts of the Milky Way –“Have Our Galaxy Ringing Like a Bell”

Dark Energy Survey


Our galaxy is still ringing like a bell from a galactic collision that possibly occurred within the last 100 million years, when a small satellite galaxy or clump of invisible dark matter plowed through the Milky Way, leaving behind the ringing echoes, according to observations from the Sloan Digital Sky Survey (SDSS) telescope in New Mexico and the 2018 Dark Energy Survey that discovered ghostly streams rippling through the Milky Way.

“Our part of the Milky Way is ringing like a bell,” said Brian Yanny, of the Fermi National Accelerator Laboratory (Fermilab). “But we have not been able to identify the celestial object that passed through the Milky Way. It could have been one of the small satellite galaxies that move around the center of our galaxy, or an invisible structure such as a dark matter halo.”

“We have found evidence that our Milky Way had an encounter with a small galaxy or massive dark matter structure perhaps as recently as 100 million years ago,” said Larry Widrow, a professor at Queen’s University in Canada. “We clearly observe unexpected differences in the Milky Way’s stellar distribution above and below the Galaxy’s midplane that have the appearance of a vertical wave — something that nobody has seen before.”

About 60 miniature “dwarf galaxies” have been discovered orbiting the Milky Way. Theory suggests that many invisible dark matter satellites also circle our galaxy, though these would only be detectable by their gravitational effect.

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

It’s likely that one of these may have slammed into the Milky Way, though even that is not certain. “The perturbation need not have been a single isolated event in the past, and it may even be ongoing,” said Susan Gardner of the University of Kentucky. “Additional observations may well clarify its origin.”

In their 2017 paper, “Milky Way Tomography with K and M Dwarf Stars: the Vertical Structure of the Galactic Disk,” using observations from the Sloan Digital Sky Survey (SDSS), Gardner and Yanny analyzed the spatial distribution of 3.6 million stars and found ripples that confirm the evidence of the Milky Way’s ancient impacts, which could include an impact with the massive Sagittarius dwarf galaxy some 0.85 billion years ago.

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

“These impacts are thought to have been the ‘architects’ of the Milky Way’s central bar and spiral arms,” Gardner said. “Just as the ripples on the surface of a smooth lake suggest the passing of a distant speed boat, we search for departures from the symmetries we would expect in the distributions of the stars to find evidence of ancient impacts. We have found extensive evidence for the breaking of all these symmetries and thus build the case for the role of ancient impacts in forming the structure of our Milky Way.”

This new paper continues Gardner’s earlier studies with Yanny and others of the breaking of north/south symmetry in the stellar disk of the Milky Way. Their earlier work revealed an asymmetry that appears as a vertical “ripple” in the number counts of the stars as one samples in vertical distance away from the center of the galactic disk. In the new paper, the team analyzed the largest sample yet, and confirmed their earlier interpretation of the north/south asymmetry and found evidence for symmetry breaking in the plane of the galactic disk as well.

“A Fractured Cosmos?” –Was a Cosmic String Detected at Milky Way’s Black Hole

In 2018, scientists have released the preliminary cosmological findings from the Dark Energy Survey—research on about 400 million astronomical objects, including distant galaxies as well as stars in our own galaxy, including the discovery of 11 new stellar streams—remnants of smaller galaxies torn apart and devoured by our Milky Way.

The data cover about 5,000 square degrees, or one-eighth of the entire sky, and include roughly 40,000 exposures taken with the Dark Energy Camera. The images correspond to hundreds of terabytes of data and are being released along with catalogs of hundreds of millions of galaxies and stars.

This image shows the entire Dark Energy Survey field of view – roughly one-eighth of the sky – captured by the Dark Energy Camera, with different colors corresponding to the distance of stars. (Blue is closer, green is farther away, red is even farther.) Several stellar streams are visible in this image as yellow, blue and red streaks across the sky.

“There are all kinds of discoveries waiting to be found in the data,” said Brian Yanny of Fermi National Accelerator Laboratory, Dark Energy Survey data management project scientist. “While DES scientists are focused on using it to learn about dark energy, we wanted to enable astronomers to explore these images in new ways, to improve our understanding of the universe.”

The Dark Energy Camera, the primary observation tool of the Dark Energy Survey, is one of the most powerful digital imaging devices in existence. It was built and tested at UChicago-affiliated Fermilab, the lead laboratory on the Dark Energy Survey, and is mounted on the National Science Foundation’s 4-meter Blanco telescope in Chile. The DES images are processed by a team at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.

One new discovery enabled by the data set is the detection of 11 new streams of stars around our Milky Way. Our home galaxy is surrounded by a massive halo of dark matter, which exerts a powerful gravitational pull on smaller, nearby galaxies. The Milky Way grows by pulling in, ripping apart and absorbing these smaller systems. As stars are torn away, they form streams across the sky that can be detected using the Dark Energy Camera. Even so, stellar streams are extremely difficult to find since they are composed of relatively few stars spread out over a large area of sky.

“It’s exciting that we found so many stellar streams,” said astrophysicist Alex Drlica-Wagner of Fermilab and the Kavli Institute for Cosmological Physics at UChicago. “We can use these streams to measure the amount, distribution and ‘clumpiness’ of dark matter in the Milky Way. Studies of stellar streams will help constrain the fundamental properties of dark matter.”

Prior to the new discoveries, only about two dozen stellar streams had been discovered. Many of them were found by the Sloan Digital Sky Survey we covered above, a precursor to the Dark Energy Survey.

The Daily Galaxy, Max Goldberg, via University of Kentucky  and University of Chicago