Why is the Milky Way 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.”

 “Something that nobody has seen before”

“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.

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Satellite Galaxy Invader?

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, 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.

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 ‘Architects’ of the Milky Way’s central bar and spiral arms

“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 

Asymmetry appears as a vertical “ripple”

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.

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 14-foot long 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.


Dark Energy Camera

“There are all kinds of discoveries waiting to be found in the data,” said Yanny. “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.”

Eleven New Star Streams Detected

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 University of Chicago. “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.

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Milky Way’s disk is being bombarded on a continuing basis

In an email to The Daily Galaxy, Brian Yanny wrote: “Theorists predict that for a symmetrical, rotating disk like our Milky Way,  vertical oscillations would naturally die down or ‘damp out’  on a timescale of about 50 million years.  The fact that the oscillations seem to be on-going even with no evidence of a recent large stellar merger of a dwarf galaxy into our Milky Way suggests that the arrangement of matter in an around our  Milky Way is *not* as symmetrical as first thought , or that the disk is being bombarded on a continuing basis to the present day by seen or unseen perturbers, or both.

“Recent studies  have found that the Large Magellanic Cloud system is significantly more massive than previously thought and it could be responsible for a significant asymmetry of the Milky Way — allowing oscillations to persist over billion year time scales.  Also, researchers have uncovered Gaia data has uncovered evidence for an enormous merger slapping vertically onto the face of the Milky Way’s disk from above some billions of years ago, whose effects are still persisting to this day.

“The large Sagittarius dwarf galaxy,”  concludes Yanny, “which is in a  orbit over the pole of the Milky Way, also can provide some influence on the vertical motions of stars in the solar neighborhood — though at a lessor magnitude that the other effects.  Whether or not there are large invisible clumps of dark matter roaming around the halo of the Milky Way remains unfortunately, unclear, as astronomers have not been able to confidently identify the existence of one or more of these blob dissociated from luminous matter such as stars.”

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.

Avi Shporer, Research Scientist, MIT Kavli Institute for Astrophysics and Space Research via Brian Yanny, University of Kentucky  and University of Chicago.

[Editor’s note: Brain Yanny’s sources –Widrow et al 2012 ApJL 750, L41 : Evidence for Vertical Oscillations of the Milky Way Disk; Erkal, D et al. 2019 MNRAS 487, 2685;  A Heavy LMC influences the Dark Matter halo of the Galaxy; Gardner, Hinkel and Yanny 2020 ApJ 890,110 ;Asymmetries in star counts imply time non-steady state disk dynamics; Vasiliev, et al. MNRAS 2021 501, 2279: Both LMC and Sagittarius are influential on our Galaxy; Helmi et al 2018 Nature 563, 85 and Belokurov et al 2018 MNRAS 478, 611:  The existence of the Gala Enceladus/Sausage structure; Donlon et al. 2020 ApJ 902, 119 on more recent timescale for when Gaia Enceladus Sausage hit the Milky Way; deBoer et al. 2020 MNRAS 494, 5315 on gaps, spurs in stellar streams caused by dark matter blobs or passing close to Sagittarius dwarf]

Image credit: Dark Energy Survey


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