Scientists might have defined the mysterious distribution of small satellite galaxies round our Milky Way that has lengthy puzzled astronomers, however not everyone seems to be in settlement.
The Milky Way galaxy is surrounded by smaller dwarf galaxies. The traditional mannequin of galaxy development proposes that these small galaxies have been trapped by a large halo of dark matter, materials that scientists cannot see however that seems to form the universe. The dwarf galaxies then got here collectively to type bigger galaxies, and the dwarfs that we see in the present day are the leftovers of that course of, the idea goes. Nonetheless, if so then astronomers would count on the surviving dwarf galaxies to be scattered randomly across the Milky Way’s dark matter halo. As an alternative, they appear to be confined to a single airplane that cuts by means of our galaxy at an angle, an oddity that threatens our understanding of the dark matter–dominated universe.
Now, astronomers led by Until Sawalha of the College of Helsinki in Finland have provided a proof. Utilizing measurements from the European House Company’s Gaia mission of how the satellite galaxies are transferring throughout the sky, they calculated the trajectories of the satellite galaxies ahead and backward in time. In doing so, they revealed that the satellite galaxies weren’t aligned up to now and will not be sooner or later — as a substitute, the airplane that they’re in now’s simply an opportunity, momentary association.
“In case you have a look at the velocities of the satellite galaxies in the present day, they’re transferring alongside the airplane, however what we did that is actually new and thrilling is forecast the place their orbits are taking them in a billion years time,” Carlos Frenk, a cosmologist on the College of Durham within the U.Ok. and a member of Sawalha’s group, advised House.com. “They only occur to be within the airplane in the present day, that is the coincidence of the entire thing, and in a billion years or much less they are going to go away the airplane.”
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Though it appears as if the percentages of us being round concurrently the satellite galaxies are all lined up in the identical airplane are slim, coincidences do occur. For instance, Saturn’s rings are young, and the truth that we exist concurrently the rings is a coincidence. One other unlikely prevalence is the flexibility of Earth’s moon to dam the sun simply exactly sufficient to trigger a total solar eclipse; because the moon is transferring slowly away from Earth, sooner or later will probably be too small within the sky to dam the sun completely. But, we’re coincidentally right here to see the spectacle of totality.
Nonetheless, this line of argument doesn’t win over everybody. Marcel Pawlowski, an astronomer on the Leibniz Institute for Astrophysics in Germany who didn’t participate within the analysis, is skeptical in regards to the coincidence of an opportunity alignment. “Why would we be right here at this particular time proper now when the satellite galaxies have this distribution?” he advised House.com. “There isn’t any actual purpose for that.”
The existence of the airplane of satellites can also be an necessary prediction of Modified Newtonian Dynamics, or MOND, which is a rival to the idea of dark matter. Astronomers got here up with dark matter to elucidate galaxies that keep collectively although their gravity seems too low to keep up them. MOND says as a substitute that dark matter doesn’t exist, and what we see as further gravity is brought on by a tweak to Newtonian gravity at low accelerations, comparable to these of stars orbiting galaxies or satellite galaxies orbiting the Milky Way.
In response to MOND, “the Milky Way and the Andromeda galaxy ought to have had a previous encounter about 8 billion years in the past,” Pawlowski stated. On this situation, the gravity of the Andromeda galaxy ought to have ripped materials from the Milky Way, creating what astronomers name a tidal tail, like we see within the interactions of different galaxies. The fuel and stars on this tidal tail may have coalesced right into a bunch of dwarf galaxies lined up with the trajectory of the Milky Way coming from its encounter with the Andromeda galaxy, MOND argues. If true, the identical course of ought to have fashioned a airplane of satellite galaxies across the Andromeda galaxy in addition to the Milky Way’s.
It seems that the Andromeda galaxy does have a airplane of satellites, as do a number of different close by galaxies, together with Centaurus A, which remains to be reeling from a galaxy interplay and merger that it skilled about 2 billion years in the past.
Nonetheless, Frenk stated he disagrees with the MOND clarification for the Milky Way’s companions. “After we discovered the airplane of satellites across the Milky Way, folks went on the lookout for planes round different galaxies and outlined them utilizing fully totally different standards,” he stated. “You actually must work exhausting and embrace a number of the satellites however throw some others out to outline this construction, however even if you take that into consideration, the airplane of satellites across the Andromeda galaxy is commonplace in simulations of galaxies with chilly dark matter.”
Whereas simulations are all properly and good, observational information is essential. Gaia is our most delicate instrument for measuring the positions and correct motions of stars and galaxies by means of space.
“Gaia information is fairly good — it is revolutionary — however it nonetheless has some uncertainties,” Pawlowski stated. Beforehand he had measured the correct motions of the satellite galaxies with not solely Gaia’s information, but in addition measurements from the Hubble Space Telescope for an much more exact evaluation. “For some purpose [the Hubble measurements] have been largely ignored on this research,” Pawlowski lamented.
Might the Gaia uncertainties actually be sufficient to result in an incorrect conclusion? Pawlowski attracts an analogy to a bunch of marbles all rolling in an ideal line, all in the identical route with no tangential pace away from their path. In case you measure the movement of the marbles, however you may have even a small quantity of uncertainty within the accuracy of your measurement of their tangential velocity away from the road, and also you then challenge your measurements and their uncertainties ahead in time, you’ll erroneously predict that the marbles will disperse over time.
The speculation of chilly dark matter remains to be the clear frontrunner general. There’s extra proof for it, though MOND supporters would say that isn’t shocking given the distinction within the quantity of funding spent on dark-matter analysis in comparison with MOND. Alternatively, MOND has had a number of wins, comparable to predicting the acceleration measured in galaxy rotation curves.
Nonetheless, dark matter theorists are in the end proud of this new end result.
“We’ve got been in a position to take away one of many most important excellent challenged to the chilly dark matter principle,” Frenk stated in a statement. “It continues to supply a remarkably devoted description of the evolution of our universe.”
The analysis is described in a paper (opens in new tab) revealed Monday (Dec. 19) within the journal Nature Astronomy.
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