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Astrotalk: Behind the News Headlines of September 2013

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Astrotalk: Behind the News Headlines of September 2013 AstroTalk: Behind the news headlines of September 2013 Richard de Grijs (何锐思) (Kavli Institute for Astronomy and Astrophysics, Peking University) Fountains, outflows and feedback: stellar and galactic winds in action If our eyes were sensitive to X-rays, we would see a very different Universe from the view we are used to. The observations in visible light we are most familiar with show the beauty of the night sky and its myriad of serenely-looking galaxies. Except for the few galaxy systems that have come too close to each other for their own good, most galaxies appear to be moving on quietly, without taking too much notice of their surroundings. Look again… But this time, use a telescope that allows you to study the Universe in ultraviolet or infrared light, at X-rays or in radio waves, and you will see that many galaxies affect their immediate environments quite strongly: they exhibit large-scale outflows of hot gas that reach enormous distances away from their source, a process referred to as ‘feedback’. In some galaxies, the outflows do not manage to escape their host galaxy’s gravitational pull, which causes a reversal of the flow and a plummeting of the gaseous matter back onto the galactic disk. This occurs in our own Milky Way, and we call such events ‘galactic fountains’. In fact, the most luminous galaxies in the Universe are not particularly bright in the visible. Most of their energy output (which can be hundreds or even thousands of times more than our Milky Way’s) is emitted at infrared wavelengths. The power source of these galaxies is related to either hyperactive bursts of star formation (called ‘starbursts’) and/or activity around a (super- )massive black hole at a galaxy’s nucleus, a so-called ‘active galactic nucleus’ or AGN. The radiation from these processes is absorbed by dust, which then re- emits it at infrared wavelengths. The manner and degree to which AGNs affect their surroundings is a critical part of a galaxy’s evolution, and astronomers try to quantify and understand the associated processes. In one scenario, a supermassive black hole (thought to be many thousands of times as massive as our own Sun, by definition one ‘solar mass’) can exert a strong but relatively brief influence to suppress star formation in its host galaxy. Of course, when we talk about timescales, we need to place this in the context of those governing astronomical evolution. The ‘relatively short’ quenching of star formation by such a black hole may last for 10 million years or longer... Such feedback is thought to occur when radiation from the rotating thin disk (a so-called ‘accretion disk’, where material captured by the black hole’s massive pull of gravity assembles before spiraling in and colliding with the central massive object itself) around the black hole drives a wind of gas particles into the galaxy that blows out the fuel needed for star formation. US astronomer Howard Smith joined a team of colleagues to use the Herschel Space Telescope to study 24 infrared-luminous galaxies in the radiation emitted by gas via the hydroxyl molecule, OH. This molecule is abundant, and a strong producer of infrared radiation at very long wavelengths that are not absorbed by dust and so can be seen unobscured. Two years ago, Herschel astronomers reported discovering strong galactic nuclear winds using this molecule as a diagnostic, and their new study of 24 luminous systems represents a systematic attempt to probe the nature of these winds with this new molecular tool. In September 2013, the team reported that the nature of the line in the spectra (measurements of the amount of light that the quasar emits at different wavelengths) that is indicative of the OH molecule’s energy varies considerably in detail between sources. However, overall it can be grouped into four categories, indentified by whether it is seen in emission, in absorption, in a mix of both, or is absent. When the lines are seen, however, they reveal the presence of very fast motions in the wind, of up to 2000 kilometers per second; there is as much as 200 million solar masses of material in these winds. The scientists speculate that the nuclear winds may be an early phase of disrupting the dust around the supermassive black hole. In this case, these objects may evolve before long into more prominent, optical sources like quasars (short for ‘quasi-stellar objects’ because of their bright, starlike appearance). Quasars are one type of AGN powered by gas falling into their central supermassive black holes. As the gas falls into the black hole, it heats up and emits light. The gravitational pull from the black hole is so strong, and it is pulling in so much gas, that the hot gas glows brighter than the entire surrounding galaxy. But with so much going on in such a small space, not all of the gas is able to find its way to the black hole’s accretion disk. Much of it escapes instead, carried along by strong winds blowing out from the centre of the quasar. “These winds blow at thousands of miles per second, far faster than any winds we see on Earth”, says Niel Brandt, a professor at Penn State University (USA). “The winds are important because we know that they play an important role in regulating the quasar’s central black hole, as well as star formation in the surrounding galaxy”. Just outside the centre of the quasar are clouds of hot gas flowing away from the central black hole. As light from deeper in the quasar passes through these clouds on its way to Earth, some of the light gets absorbed at particular wavelengths corresponding to the elements in the clouds. As gas clouds are accelerated to high speeds by the quasar, the Doppler effect spreads the absorption over a broad range of wavelengths, leading to a wide valley visible in the spectrum. The width of this ‘broad absorption line’ (BAL, for short) measures the speed of the quasar’s wind. Quasars whose spectra show such broad absorption lines are known as ‘BAL quasars’. But the hearts of quasars are chaotic, messy places. Quasar winds blow at thousands of kilometers per second, and the disks around the central black holes rotate at speeds that approach the speed of light. All this adds up to environments that can change quickly. Since 1998, the Sloan Digital Sky Survey (SDSS) has been regularly measuring spectra of quasars. Over the past few years, as part of SDSS III's Baryon Oscillation Spectroscopic Survey (BOSS),1 the survey has been specifically seeking out repeated spectra of BAL quasars through a programme proposed by Brandt and colleagues. Their persistence paid off: the research team gathered a sample of 582 BAL quasars, each of which had repeat observations over a period of between one and nine years, a sample about 20 times larger than any that had been previously assembled. The team then began to search for changes, and were quickly rewarded. In 19 of the quasars, the broad absorption lines had disappeared. There are several possible explanations for this observation, but the simplest is that, in these quasars, gas clouds that had been seen previously are literally “gone with the wind”: the rotation of the quasar’s disk and wind have carried the clouds out of the line-of-sight between us and the quasar. Separately, scientists are also trying to understand in detail the massive stellar outflows linked to the process of star formation and starburst activity in actively star-forming galaxies. A recently developed 3D computer animation is helping an international research team to get an unprecedented look at star-forming gases escaping from a nearby galaxy. Erik Rosolowsky, an astrophysicist at the University of Alberta (Canada), created the animation as part of a study that was recently published in the high-profile scientific journal Nature. The collaboration, led by Alberto Bolatto of the University of Maryland (USA), used the new and powerful Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile to discover billowing columns of cold, dense gas escaping from the disk of the nearby starburst galaxy NGC 253. Located 11.5 million lightyears away in the constellation Sculptor, this galaxy affords astronomers a rare and fortuitous view of several massive, young star clusters near its centre. These clusters represent areas where new stars are forming. They also mark the point of departure for material being ejected from the galaxy. The cosmic fireworks that characterize a starburst can abruptly fizzle out after only a relatively brief period of star formation. As a result, far fewer high-mass galaxies are evident, and astronomers want to know why. The new study shows in unprecedented detail how vigorous star formation can force hydrogen and other gases high into the surrounding galactic halo, leaving little fuel for the next generation of stars. “We couldn't see the wind before the new telescope”, Rosolowsky said. The ALMA telescope provided enough data to build a computer model that revealed a phenomenon that was difficult to discern by physical observation. To create the 3D animation, the scientists included data about the distance, brightness and velocity of carbon monoxide molecules in the starburst. The different colours represent the brightness of the gas at various points. The top of the structure is moving toward Earth; the bottom part is farther away. The solar wind appears as a yellow, peanut-shaped formation near the top of the structure. “Part of the complexity is seeing something very faint next to something very bright”, Rosolowsky said.
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