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 (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 (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 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. “This is the first time we’ve used this type of visualization

1 See my AstroTalk contribution to The Amateur Astronomer magazine of January 2013 for more details. for these data. Usually, we use these methods to visualize computer simulations.” Rosolowsky says that he is looking forward to using the ALMA data for more research. He notes that ALMA has been used to obtain similar data for other molecules, and further study should help determine how much gaseous material is carried away by stellar winds and understand how these clouds create the starbursts seen in this and other galaxies, such as the prototype starburst galaxy (the ‘Cigar galaxy’).

Star-forming clusters are particularly conducive to forming huge stellar outflows, because they are composed of young ‘stellar populations’ that also contain massive stars. Massive stars are relatively rare, but play a very important role in recycling materials in the Universe. They burn their nuclear fuel much more rapidly than stars like the Sun, living only for millions of years before exploding as a supernova and returning most of their matter to space. But even during their brief lives, they lose a significant fraction of their mass through fierce winds of gas driven off their surfaces by the intense light emitted from the star.

The winds from massive stars are at least a hundred million times stronger than the solar wind emitted by our own Sun and can significantly shape their surrounding environment. They might trigger the collapse of surrounding clouds of gas and dust to form new stars or, conversely, blast the clouds away before they have the chance to get started. Despite their important role, however, the detailed structure of the winds from massive stars remains poorly understood. Are they steady and uniform, or broken up and gusty?

Astronomers have recently gained an unprecedented, detailed glimpse into this wind structure by taking observations with the European XMM–Newton X-ray satellite, spread over a decade, to study variability in the X-ray emission from the massive star ζ (zeta) Puppis. One of the nearest massive stars to Earth, it is bright enough to be seen with the naked eye in the constellation of Puppis, in the southern hemisphere. The X-rays arise from collisions between slow- and fast- moving clumps in the wind, which heats them to a few million degrees. As individual colliding clumps in the wind are heated and cooled, the strength and energy of the emitted X-rays vary. If only a small number of large fragments are present, variations in the combined emission could be large. Conversely, as the number of fragments grows, a change in the X-ray emission from any given fragment becomes less important, and the overall variability decreases.

In ζ Puppis, the X-ray emission was found to be remarkably stable over short timescales of just a few hours, pointing to a very large number of fragments. There must still be clumps in the wind to make X-rays in the first place, but there must be many of them to yield such low variability. However, unexpected variation in the emission was seen to occur on the order of several days, implying the presence of a few very large structures in the wind, possibly spiral- arm-like features superimposed on the highly fragmented wind co-rotating with the star. “Studies at other wavelengths had already hinted that the winds from massive stars are not simply a uniform breeze, and the new XMM–Newton data confirm this, but also reveal hundreds of thousands of individual hot and cool pieces”, says Yaël Nazé of the Université de Liège (Belgium), who led the analysis. “This is the first time constraints have been placed on the number of fragments in a stellar wind of an adult massive star, a number which far exceeds theoretical predictions.”

To fully understand these observations, improved models of stellar winds will be needed, taking into account both the large-scale emission structures and the highly fragmented wind, in order to understand how they affect mass-loss in stellar giants. “ζ Puppis also goes by the name ‘Naos’, which in antiquity was the name given to the innermost sanctuary of a temple, accessible to only a few people; thanks to XMM–Newton, scientists have been able to unlock the secrets of this mysterious stellar object”, adds Nazé. “This long-term XMM–Newton study of ζ Puppis has provided the first constraints on the number of fragments in a stellar wind from a massive star. There is no dataset with comparable sensitivity or time and or spectral coverage currently available for any other massive star”, says Norbert Schartel, the European Space Agency’s XMM–Newton project scientist. Studies of massive galactic winds and outflows have seen something of a resurgence in recent years. This is an area that is currently ‘hot’, but as yet poorly understood. We can look forward to many new discoveries and insights related to feedback processes—the interactions between galactic winds and the intergalactic medium—in years to come.

Figure 1: An optical image of the luminous galaxy Markarian 231 taken by the . Infrared observations of this galaxy taken in the radiation from the molecule OH have discovered evidence of powerful winds blowing from the nucleus. (Credit: NASA/Hubble Space Telescope.)

Figure 2: 3D rendering of carbon monoxide (CO) in the starburst galaxy NGC 253. Colours correspond to the intensity of emission – and therefore the amount of CO – from red (faint) to purple (bright). (Credit: Erik Rosolowsky, University of Alberta.)

Figure 3: ALMA carbon monoxide image of outflowing gas in NGC 253. Colour represents the brightness of the emission (red is faint, purple is bright). The bright central lane, coloured in green to purple, is the densest concentration of molecular gas, associated with the bar in this galaxy. The fainter (red) pillars of emission perpendicular to it correspond to ejected molecular gas. The areas where molecular, expanding shells are observed, likely caused by young stellar clusters, are illustrated by stars and arrows to give an idea of the expanding motions. The dashed line illustrates the cone of the hot outflow seen in X-rays. (Credit: Alberto Bolatto, University of Maryland.)

Figure 4: Artist’s impression comparing a smooth stellar wind (left) with a highly fragmented stellar wind (right) of a massive star like ζ Puppis. A decade’s worth of observations with the XMM–Newton have revealed that the wind of ζ Puppis is fragmented into hundreds of thousands of individual hot (red) and cool (blue) clumps. Studying stellar winds is vital not only to understand mass loss from the star itself and thus its expected lifetime, but also how the winds inject material and energy into the surrounding environment and influence the birth and death of other stars. (Copyright: ESA, C. Carreau/Nazé et al.)

Figure 5: Artist’s impression of a quasar. The black dot in the centre represents the supermassive black hole at the centre of the quasar. The red-and-yellow spiral surrounding it shows the accretion disk of hot gas falling into the black hole. Some of this gas is ejected as the quasar’s wind, which is shown in light blue. The size of the accretion disk shown is comparable to the size of our solar system. The inset at the top right shows two Sloan Digital Sky Survey (SDSS) spectra for the same quasar (named SDSS J093620.52+004649.2). The top spectrum (blue) was taken in 2002, while the bottom spectrum (red) was taken in 2011. The deep, wide valley in the 2002 spectrum is a so-called ‘broad absorption line’ — a feature which has disappeared from its spectrum by 2011. (Credit: NASA/Chandra X-ray Center/M. Weiss, Nahks Tr'Ehnl, Nurten Filiz Ak.)

Figure 6: SDSS image of the quasar SDSS J093620.52+004649.2, one of the 19 quasars with disappearing BAL troughs. The constellation map at the bottom left shows the quasar’s position in the constellation Hydra. Three successive views zoom in closer and closer to the quasar. Credit: Jordan Raddick (Johns Hopkins University) and the SDSS-III collaboration. Hydra constellation chart from The , produced by the International Astronomical Union and Sky and Telescope magazine (Roger Sinnott, Rick Fienberg, and Alan MacRobert)

Figure 7: Matter blasts out of the starburst galaxy M82 in this composite image from three observatories: the Hubble Space Telescope (visible light; orange and yellow-green), the Spitzer Space Telescope (infrared radiation; red) and the Chandra X-ray Observatory (blue). (Credit: NASA, ESA, Chandra X-ray Center and JPL/Caltech.)

Figure 8: The NGC 253. This galaxy is host of massive starbirth. Credit: Jay Gallagher (University of Wisconsin-Madison), Alan Watson (Lowell Observatory, Flagstaff, AZ), NASA/ESA

Figure 9: This mosaic image of the magnificent starburst galaxy, Messier 82 (M82) is the sharpest wide- angle view ever obtained of M82. It is a galaxy remarkable for its webs of shredded clouds and flame-like plumes of glowing hydrogen blasting out from its central regions where young stars are being born 10 times faster than they are inside in our Milky Way galaxy. Credit: NASA, ESA and the Hubble Heritage Team STScI/AURA). Acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (NSF).

Figure 10: Resembling a swirling witch’s cauldron of glowing vapours, the black hole-powered core of a nearby active galaxy appears in this colorful NASA/ESA Hubble Space Telescope image. The galaxy lies 13 million lightyears away in the southern constellation Circinus. Credit: Andrew S. Wilson (University of Maryland); Patrick L. Shopbell (Caltech); Chris Simpson (Subaru Telescope); Thaisa Storchi-Bergmann and F. K. B. Barbosa (UFRGS, Brazil); and Martin J. Ward (University of Leicester, U.K.) and NASA/ESA

Figure 11: These NASA/ESA Hubble Space Telescope snapshots reveal dramatic activities within the core of the galaxy NGC 3079, where a lumpy bubble of hot gas is rising from a cauldron of glowing matter. The picture at left shows the bubble in the centre of the galaxy’s disk. The structure is more than 3, 000 lightyears wide and rises 3, 500 lightyears above the galaxy’s disk. The smaller photo at right is a close-up view of the bubble. Astronomers suspect that the bubble is being blown by winds (high-speed streams of particles) released during a burst of star formation. Gaseous filaments at the top of the bubble are whirling around in a vortex and are being expelled into space. Eventually, this gas will rain down upon the galaxy’s disk where it may collide with gas clouds, compress them, and form a new generation of stars. The two white dots just above the bubble are probably stars in the galaxy. Credit: NASA/ESA, Gerald Cecil (University of North Carolina), Sylvain Veilleux (University of Maryland), Joss Bland-Hawthorn (Anglo- Australian Observatory), and Alex Filippenko (University of California at Berkeley).