{"197291":{"#nid":"197291","#data":{"type":"news","title":"An Active Milky Way","body":[{"value":"\u003Cp\u003EThese days the core of the Milky Way galaxy is a pretty tame place...cosmically speaking. The galactic black hole at the center is a sleeping giant. Existing stars are peacefully circling. Although conditions are favorable, there doesn\u2019t even seem to be much new star formation going on.\u003C\/p\u003E\u003Cp\u003EBut there is growing evidence that several million years ago the galactic center was the site of all manner of celestial fireworks. A pair of assistant professors \u2013 Georgia Tech\u0027s Tamara Bogdanovi\u0107\u0026nbsp; and Kelly Holley-Bockelmann at Vanderbilt\u0026nbsp; \u2013 have come up with an explanation that fits these \u201cforensic\u201d clues.\u003C\/p\u003E\u003Cp\u003EWriting in the March 6 issue of the \u003Cem\u003EMonthly Notices of the Royal Astronomical Society\u003C\/em\u003E, the astronomers describe how a single event \u2013 a violent collision and merger between the galactic black hole and an intermediate-sized black hole in one of the small \u201csatellite galaxies\u201d that circle the Milky Way \u2013 could have produced the features that point to a more violent past for the galactic core.\u003C\/p\u003E\u003Cp\u003E\u201cTamara and I had just attended an astronomy conference in Aspen, Colorado, where several of these new observations were announced,\u201d said Holley-Bockelmann. \u201cIt was January 2010 and a snow storm had closed the airport. We decided to rent a car to drive to Denver. As we drove through the storm, we pieced together the clues from the conference and realized that a single catastrophic event \u2013 the collision between two black holes about 10 million years ago - could explain all the new evidence.\u201d\u003C\/p\u003E\u003Cp\u003EThe most dramatic of these extraordinary clues are the Fermi bubbles. These giant lobes of high-energy radiation - caused by particles moving nearly the speed of light - extend some 30,000 light years above and below the Milky Way center. If they were glowing in visible light they would fill about half of the night sky. But they radiate X-ray and gamma-ray light, so you need X-ray vision to see them. The discovery was reported by astronomers at the Harvard-Smithsonian Center for Astrophysics.\u003C\/p\u003E\u003Cp\u003EAnother puzzling characteristic of the GC, the astronomer\u2019s abbreviation for the galactic center, is the fact that it contains the three most massive clusters of young stars in the entire galaxy. The Central, Arches and Quintuplet clusters each contain hundreds of young, hot stars that are much larger than the Sun. These stars typically burn out in \u201conly\u201d a few million years because of their extreme brightness, so there had to have been a relatively recent burst of star formation at the GC.\u003C\/p\u003E\u003Cp\u003EThe supermassive black hole that dominates the center of the Milky Way weighs in at about four million solar masses and is roughly 40 light seconds in diameter: only nine times the size of the sun. Such an object produces intense gravitational tides. So astronomers were surprised to discover a number of clumps of bright new stars closer than three lights years from the black hole\u2019s maw. It wouldn\u2019t be that surprising if the stars were being sucked into the black hole, but they show every sign of having formed in place. For this to happen, the clouds of dust and gas that they formed from must have been exceptionally dense: 10,000 times thicker than the other molecular clouds in the GC.\u003C\/p\u003E\u003Cp\u003EWhile there is an excess of young hot stars in the galactic core, there is also a surprising dearth of older stars. Theoretical models predict that the density of old stars should increase as you move closer to the black hole. Instead, there are very few old stars found within several light years of the sleeping giant.\u003C\/p\u003E\u003Cp\u003EWhen she got home from the conference, Holley-Bockelmann recruited Vanderbilt graduate student Meagan Langto work on the problem with them. With the assistance of Pau Amaro-Seoane from the Max Planck Institute for Gravitational Physics in Germany, Alberto Sesana from the Institut de Ci\u00e8ncies de l\u0027Espai in Spain, and Vanderbilt Research Assistant Professor Manodeep Sinha, they came up with a theoretical model that fits the observations and makes some testable predictions.\u003C\/p\u003E\u003Cp\u003EThe scenario began about 13 billion years ago, when the path of one of the smaller satellite galaxies orbiting the Milky Way is diverted so that it began drifting inward toward the core. According to a recent study, this may have happened dozens of times in the lifetime of the Milky Way. As the satellite galaxy \u2013 a collection of stars and gas with an intermediate-sized black hole with a mass equal to about 10,000 suns \u2013 spiraled in, most of its mass was gradually stripped away, finally leaving the black hole and a handful of gravitationally bound stars.\u003C\/p\u003E\u003Cp\u003EAbout 10 million years ago, the stripped down core of the satellite galaxy finally reached the galactic center. When two black holes merge, they first go through an elaborate dance. So the smaller black hole would have circled the galactic black hole for several million years before it was ultimately consumed.\u003C\/p\u003E\u003Cp\u003EAs the smaller black hole circled closer and closer, it would have churned up the dust and gas in the vicinity and pushed enough material into the galactic black hole in the process to produce the Fermi bubbles.\u003C\/p\u003E\u003Cp\u003EThe violent gravitational tides produced by the process could easily have compressed the molecular clouds in the core to the super densities required to produce the young stars that are now located on the central black hole\u2019s doorstep.\u003C\/p\u003E\u003Cp\u003EIn addition, the vigorous churning would have swept out the existing stars from the area surrounding the massive central black hole. In fact, the astronomer\u2019s model predicts that the black holes\u2019 merger dance should have flung a large number of the missing old stars out into the galaxy at hyper velocities, thus explaining the absence of old stars immediately around the super-massive black hole.\u003C\/p\u003E\u003Cp\u003E\u201cThe gravitational pull of the satellite galaxy\u2019s black hole could have carved nearly 1,000 stars out of the galactic center,\u201d said Bogdanovi\u0107, a faculty member in the College of Sciences. \u201cThose stars should still be racing through space, about 10,000 light years away from their original orbits.\u201d\u003C\/p\u003E\u003Cp\u003EIt should be possible to detect these stars with large surveys like the Sloan Digital Sky Survey because these stars would be traveling at much higher velocities than stars that have not undergone this type of interaction. So discovery of a large number of \u0022high velocity stars\u0022 racing outward through the galaxy would strongly support the proposed scenario of the Milky Way and satellite galaxy merger.\u003C\/p\u003E\u003Cp\u003EThe research was supported by National Science Foundation Career Grant AST-0847696 and National Aviation and Space Administration grants NNX08AG74G and PF9-00061 as well as an NSF Graduate Research Fellowship.\u003C\/p\u003E\u003Cp\u003E\u003Cem\u003EArticle written by David Salisbury, Senior Research Writer, Vanderbilt University \u003Cbr \/\u003E\u003C\/em\u003E\u003C\/p\u003E","summary":null,"format":"limited_html"}],"field_subtitle":"","field_summary":[{"value":"\u003Cp\u003EEvidence suggests that the Milky Way was very active several million years ago. Astronomers from Georgia Tech and Vanderbilt propose that a single event -- a black hole collision -- can explain clues about the galaxy\u0027s current state.\u003C\/p\u003E","format":"limited_html"}],"field_summary_sentence":[{"value":"Georgia Tech and Vanderbilt researchers propose that a black hole collision several million years ago may explain clues about the Milky Way."}],"uid":"27560","created_gmt":"2013-03-06 12:40:40","changed_gmt":"2016-10-08 03:13:48","author":"Jason Maderer","boilerplate_text":"","field_publication":"","field_article_url":"","dateline":{"date":"2013-03-06T00:00:00-05:00","iso_date":"2013-03-06T00:00:00-05:00","tz":"America\/New_York"},"extras":[],"hg_media":{"197271":{"id":"197271","type":"image","title":"Milky Way Black Hole","body":null,"created":"1449179918","gmt_created":"2015-12-03 21:58:38","changed":"1475894848","gmt_changed":"2016-10-08 02:47:28","alt":"Milky Way Black Hole","file":{"fid":"196456","name":"milky_way_final_3d-web.jpeg","image_path":"\/sites\/default\/files\/images\/milky_way_final_3d-web_0.jpeg","image_full_path":"http:\/\/hg.gatech.edu\/\/sites\/default\/files\/images\/milky_way_final_3d-web_0.jpeg","mime":"image\/jpeg","size":254798,"path_740":"http:\/\/hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/images\/milky_way_final_3d-web_0.jpeg?itok=X8xN0qGb"}},"197311":{"id":"197311","type":"image","title":"Tamara Bogdanovic","body":null,"created":"1449179918","gmt_created":"2015-12-03 21:58:38","changed":"1475894848","gmt_changed":"2016-10-08 02:47:28","alt":"Tamara Bogdanovic","file":{"fid":"196458","name":"tamara.jpg","image_path":"\/sites\/default\/files\/images\/tamara_0.jpg","image_full_path":"http:\/\/hg.gatech.edu\/\/sites\/default\/files\/images\/tamara_0.jpg","mime":"image\/jpeg","size":3991353,"path_740":"http:\/\/hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/images\/tamara_0.jpg?itok=Qtouc1sQ"}}},"media_ids":["197271","197311"],"related_links":[{"url":"http:\/\/arxiv.org\/abs\/1107.2923","title":"PDF of study"}],"groups":[{"id":"1183","name":"Home"}],"categories":[{"id":"135","name":"Research"}],"keywords":[{"id":"60491","name":"Black hole"},{"id":"4896","name":"College of Sciences"},{"id":"60501","name":"Milky Way"},{"id":"167146","name":"space"}],"core_research_areas":[{"id":"39541","name":"Systems"}],"news_room_topics":[],"event_categories":[],"invited_audience":[],"affiliations":[],"classification":[],"areas_of_expertise":[],"news_and_recent_appearances":[],"phone":[],"contact":[{"value":"\u003Cp\u003EJason Maderer\u003Cbr \/\u003EMedia Relations\u003Cbr \/\u003E\u003Ca href=\u0022mailto:maderer@gatech.edu\u0022\u003Emaderer@gatech.edu\u003C\/a\u003E\u003Cbr \/\u003E404-385-2966\u003C\/p\u003E","format":"limited_html"}],"email":["maderer@gatech.edu"],"slides":[],"orientation":[],"userdata":""}}}