Only 70 of 15,000 simulated stellar streams stayed smooth, and the galaxy alone bent the rest with no dark matter in the model.<\/p>\n
Torn-apart star clusters leave long, thin star streams looping around the Milky Way. For years, astronomers have read the gaps and kinks in those trails as fingerprints of dark matter. <\/p>\n
New research suggests the galaxy produces the same features on its own. Judged on shape alone, the two causes look identical.<\/p>\n
Arpit Arora is a postdoctoral scholar in astronomy at the University of Washington<\/a>. He and his colleagues grew roughly 15,000 of these streams inside four computer models of Milky Way\u2013scale galaxies. <\/p>\n The group left every clump of dark matter out on purpose.<\/p>\n \u201cWe found that almost all of the streams had some sort of structural variation,\u201d Arora said. \u201cSo this idea that streams are naturally thin and smooth wasn\u2019t really necessarily true.\u201d<\/p>\n Smooth streams were the assumption<\/p>\n A big galaxy pulls apart any dense ball of stars that wanders too close. The stars spread out along the path that the ball was already traveling along. <\/p>\n The filament<\/a> that remains behind can wrap a long way around the galaxy. Astronomers call these stellar streams.<\/p>\n Most of the ones visible from Earth are ragged. Stretches of them bend off course, and a mysterious gap<\/a> can interrupt an otherwise even band.<\/p>\n For two decades the leading explanation has been dark matter. The stars are thought to be pulled out of line by small, invisible clumps<\/a> of it that pass near a stream. <\/p>\n Astronomers can\u2019t weigh a clump that gives off no light any other way. This is why star streams became the tool of choice.<\/p>\n \u201cDark matter makes up most of the mass in the universe and forms the scaffolding that galaxies grow on, but we still don\u2019t know what it is,\u201d said co-author Nora Shipp, a UW assistant professor of astronomy. <\/p>\n \u201cThe Milky Way is one of the best laboratories we have for figuring that out, and stellar streams are one of the sharpest tools inside it.\u201d<\/p>\n Simulations left dark matter clumps out<\/p>\n The team started with four galaxies about as heavy as the Milky Way. Each one was grown from scratch in an earlier round of cosmological simulations. <\/p>\n One stayed fairly isolated and one built an unusually large disk. The other two had recently swallowed something big, in one case a dwarf galaxy<\/a> about as massive as the Large Magellanic Cloud.<\/p>\n None of those galaxies keeps the same shape. The disk warps, the halo reacts to whatever fell in last, and a stream orbiting inside feels a different pull as billions of years go by.<\/p>\n Into that, the researchers dropped about 15,000 star clusters on orbits ranging from nearly circular to sharply stretched. They then let them come apart over five billion simulated years. <\/p>\n They took out the small stuff by hand, leaving no dark matter clumps or giant gas clouds in the model. Anything that turned up in a stream had to come from the galaxy itself.<\/p>\n Only 70 streams stayed smooth<\/p>\n About a quarter of the streams, 26 percent, finished with no significant features off their track. Another 34 percent were a mess. Roughly three-quarters departed in some way from a clean, narrow line.<\/p>\n Even the tidy ones weren\u2019t tidy. Their width varied by about 22 percent from one segment to the next, and no stream in the sample held steadier than 10 percent.<\/p>\n Out of roughly 15,000, about 70 came through free of detectable wiggles at any scale. In a galaxy with nothing small in it to blame, a featureless stream is close to a fluke.<\/p>\n Distance mattered most. Streams with orbits that carried them within about 49,000 light-years (15 kiloparsecs) of the galactic center came out the most disturbed. The calmest ones kept to wide, nearly circular paths farther out.<\/p>\n \u201cIn our simulations, the host galaxies alone caused the same kinds of irregularities that we observe in real stellar streams,\u201d Arora said. <\/p>\n \u201cNow that we can predict what the host galaxy does on its own, we can start isolating the part that dark matter is responsible for.\u201d<\/p>\n Feature sizes overlap with dark matter<\/p>\n Size is what\u2019s awkward here. Among the smoothest streams that still showed clumping along their length, the typical smallest feature spanned about 2 degrees of sky, roughly four times the width of the full moon. <\/p>\n Dark matter clumps are predicted to leave gaps of 1 to 5 degrees. The galaxy and the clumps produce features of the same size, so shape alone can\u2019t tell them apart. <\/p>\n The team checked that against a control, running 1,000 more streams through a model galaxy that was frozen in place and perfectly symmetric. <\/p>\n About 35 percent of those finished with no detectable clumping along their length, compared with about 2 percent of the smooth streams in the realistic galaxies.<\/p>\n Two features astronomers have argued over turned up in the simulation too. The stream called GD-1 carries a spur, and the ATLAS\u2013Aliqa Uma stream carries a sharp kink. <\/p>\n Both have been read as the work of dark matter, and both have look-alikes in a catalog built with no dark matter clumps in it.<\/p>\n Motion may separate the two causes<\/p>\n The simulation is deliberately simple in places. Stars leave the clusters at a steady rate rather than in bursts, and nothing happening inside a cluster is modeled, so real streams may carry more structure than these do. <\/p>\n The fractions also belong to the orbits the team sampled, rather than to the real Milky Way.<\/p>\n What nobody has tested is whether a dark matter clump leaves a different mark in how a stream\u2019s stars move, as opposed to how the stream looks from here. That\u2019s the next run: the same four galaxies, with the clumps put back in.<\/p>\n \u201cSadly there\u2019s no magic wand to reveal the structure of dark matter,\u201d said James Davenport, a research assistant professor of astronomy at the UW. <\/p>\n \u201cStreams are complex systems, but they\u2019re still the most interesting way to study the dark matter close to home.\u201d <\/p>\n In June, the Vera C. Rubin Observatory<\/a> released its first images<\/a>, and it should find hundreds more streams. Each one will now arrive with a prediction of how bent it ought to be with no dark matter involved at all.<\/p>\n The full study was published in the journal The Astrophysical Journal<\/a>.<\/p>\n \u2014\u2013<\/p>\n Like what you read? Subscribe to our newsletter<\/a> for engaging articles, exclusive content, and the latest updates.<\/p>\n Check us out on EarthSnap<\/a>, a free app brought to you by Eric Ralls<\/a> and Earth.com.<\/p>\n \u2014\u2013<\/p>\n","protected":false},"excerpt":{"rendered":"Only 70 of 15,000 simulated stellar streams stayed smooth, and the galaxy alone bent the rest with no…\n","protected":false},"author":2,"featured_media":629035,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[23],"tags":[85,46,141,145],"class_list":["post-629034","post","type-post","status-publish","format-standard","has-post-thumbnail","category-space","tag-il","tag-israel","tag-science","tag-space"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/posts\/629034","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/comments?post=629034"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/posts\/629034\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/media\/629035"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/media?parent=629034"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/categories?post=629034"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/tags?post=629034"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}