The hunt is over.<\/p>\n
After more than 50 years of searching, astrophysicists at Northwestern University have finally discovered evidence of a powerful wind blowing from the Milky Way\u2019s central supermassive black hole, Sagittarius A* (Sgr A*).<\/p>\n
According to theoretical physics and a long-accepted understanding of galaxies evolution, as black holes consume materials, they should produce wind or jets. Even a small amount of gas falling into a black hole should generate enough energy to push material outwards. Without wind, Sgr A* would be a unique outlier.<\/p>\n
But, until now, no one could find it.<\/p>\n
By providing the most detailed view yet of how Sgr A* interacts with and transforms its surrounding environment, the scientists resolved one of the longest-standing mysteries in astronomy. It also opens a new window into the physics at play in the center of the Milky Way.<\/p>\n
The study was published in The Astrophysical Journal Letters<\/a>.<\/p>\n \u201cUnless a black hole exists in a perfect vacuum, it must blow a wind somehow,\u201d said Northwestern\u2019s Mark Gorski<\/a>, who co-led the study. \u201cAnd there is no perfect vacuum in the universe. With new observations, this is the first time we\u2019ve had a clean enough view to see the wind\u2019s imprint. We looked at the data and said, \u2018There it is. There is the thing that everybody\u2019s been looking for for 50 years.\u2019\u201d<\/p>\n \u201cWe were the first to show that molecular gas very, very close to the black hole is feeding it,\u201d said\u00a0Elena Murchikova<\/a>, who co-led the study with Gorski. \u201cThe wind is not powerful, and its direction probably wanders with time. It shows that our black hole is not unique, and our place in the universe is not unique.\u201d<\/p>\n Focused on the evolution of galaxies, Gorski is a research assistant professor at Northwestern\u2019s Center for Interdisciplinary Exploration and Research in Astrophysics<\/a> (CIERA). An expert on black hole astrophysics, Murchikova is an assistant professor of physics and astronomy at Northwestern\u2019s Weinberg College of Arts and Sciences<\/a> and member of CIERA.<\/p>\n Elusive wind at the galaxy\u2019s heart<\/p>\n Although black holes are infamous for swallowing anything that ventures too close, they don\u2019t just pull matter in. They also push material out. For decades, theorists have predicted that all actively feeding black holes launch powerful outflows. As material spirals inward toward a black hole, it moves faster and faster \u2014 until it reaches close to the speed of light. This creates enough energy and pressure to fling some of the hot, fast-moving material outward in the form of winds or jets.<\/p>\n While astronomers have spotted evidence of past eruptions from Sgr A*, they struggled to detect currently occurring outflows. The Northwestern team says this is likely because Sgr A* is in a quieter phase and just incredibly difficult to see.<\/p>\n \u201cTo observe our own black hole, we have to look through the plane of our galaxy,\u201d Murchikova said. \u201cThat means we have to peer through gas, dust and ionized structures, and you can\u2019t really see through all of that easily.\u201d<\/p>\n A cone-shaped cavity<\/p>\n Now, with new tools and observations, the team finally was able to take a closer look. Using five years of extraordinarily deep observations from the Atacama Large Millimeter\/Submillimeter Array (ALMA) radio telescopes in Chile, Gorski and Murchikova constructed the sharpest image ever devised of cold molecular gas surrounding the black hole.<\/p>\n The image reflected the gas located incredibly close to Sgr A* \u2014 within just one parsec (or about three light-years) of the black hole. Then, the duo applied a calibration method to remove the black hole\u2019s bright radio signals. The resulting image is 100 times deeper and 80 times sharper than previous maps of the region. With this level of detail, it revealed structures that were completely invisible in previous observations.<\/p>\n But one newly revealed, unmistakable feature left Gorski and Murchikova gobsmacked. A vast, cone-shaped cavity \u2014 nearly one parsec long and 45 degrees wide \u2014 was devoid of cold molecular gas. According to the researchers, only hot, energetic wind blowing from Sgr A* could have created this hollowed-out region. Wherever the hot wind travels, it either sweeps cold gas away or heats it up.<\/p>\n \u201cIf you blow hot material from the black hole, it\u2019s not going to want to exist with the cold material,\u201d Gorski said. \u201cIt\u2019s either going to push the cold material out or heat it up. And, if it\u2019s too hot, you will no longer see the cold gas.\u201d<\/p>\n Exceptional claims, exceptional evidence<\/p>\n While stars, too, create winds, stellar winds are not powerful enough to carve out a cleanly swept region of this size. Even the combined power of all the nearby stars falls short.<\/p>\n \u201cIt\u2019s a huge absence of material,\u201d Gorski said. \u201cWe calculated how much energy was needed to create this cavity. It is more than can be provided by the stars in that area. Basically, there has to be input from the supermassive black hole. And, if you follow the shape of the cone, it\u2019s pointed directly at the black hole.\u201d<\/p>\n Before declaring they solved a long-standing mystery, Gorski and Murchikova continued to analyze data to further confirm their results. NASA\u2019s Chandra X-ray Observatory previously pinpointed bright X-ray emissions in the exact same region. In the same location as the bright X-rays, a hollow, cone-shaped region appeared where cold gas was missing.<\/p>\n \u201cExceptional claims require exceptional evidence,\u201d Gorski said. \u201cWe wanted to make sure that we weren\u2019t just looking at some sort of imaging artifact. Then, the X-ray image from Chandra just slotted in perfectly. The molecular features lined up.\u201d<\/p>\n \u201cWhen you find something that no one has seen before, the first thought that runs through your mind is not \u2018Oh my god, we made a discovery,\u2019\u201d Murchikova said. \u201cIt\u2019s \u2018Oh my god, what\u2019s wrong with my analysis?\u2019 But when we overlaid our image with the X-ray image, it started to make sense.\u201d<\/p>\n A quiet phase of life<\/p>\n Based on how far its effects extend into a nearby stream of ionized gas, the astrophysicists estimate the wind has been active for at least 20,000 years. The discovery also confirms that Sgr A* is relatively quiet compared to other galaxies\u2019 central supermassive black holes.<\/p>\n \u201cThe majority of other galaxies spend most of their lives in a state where they are not particularly active,\u201d Murchikova said. \u201cBut we can only see them when they are in a fireworks stage. It is very attractive to study black holes when they are in the fireworks stage, but that\u2019s not actually their dominant state. Sgr A* finally gives us a window into the life of a black hole in this quiet state.\u201d<\/p>\n The study was supported by the Weinberg College of Arts and Sciences and CIERA.<\/p>\n","protected":false},"excerpt":{"rendered":"The hunt is over. 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