A Fresh Look at Our Galaxy Points to a Chaotic Past and a Violent End

THE NEW MILKY WAY A FRESH LOOK AT OUR GALAXY POINTS TO A CHAOTIC PAST AND A VIOLENT END.

BY ANN FINKBEINER

rcing across the night sky is a pale band of light the Romans called the via lactea — the Milky Way. Astronomers have known since the 1920s that this band is an edge- on view of the Galaxy in which we Alive: a vast pinwheel teeming with nebulae, gas clouds and billions upon billions of stars. For most of the nine decades since, astronomers also thought that our Galaxy and others like it were rather quiet places: ponderous, slowly rotating structures that formed many eons ago and had settled into uneventful middle age. But then they began to see the Milky Way with fresh eyes (see ‘Galactic portrait’). Start- ing in the 1970s and 1980s, new generations of ground- and space-based telescopes began mapping the Milky Way at wavelengths rang- ing from microwaves to X-rays, revealing an unimagined richness. By the 2000s, systematic observing programmes were tracing galactic structures that sprawl across most of the heav- ens, and are so big that no one had noticed But no one today would argue that our cosmic them before. By the present decade, teams of home town is a quiet backwater. The emerging astronomers were racing to build ever-more picture of the Milky Way reveals that the Gal- powerful computer simulations to model the axy was born in chaos and shaped by violence, origins of galaxies on every scale from cosmos that it lives in a state of turbulent complexity, to star clusters. And by next year, the Atacama and that its future holds certain catastrophe. Large Millimeter/Submillimeter Array (ALMA) in Chile will be mapping the Galaxy at unprec- THE DARK-MATTER HALO Astronomers are still edented levels of detail. arguing about the precise sequence of events Astronomers are still struggling to assimilate during the Milky Way’s birth, but everyone all this new information. Disagreements, uncer- agrees that the story began with dark mat- tainties and unanswered questions abound. ter. The stuff is everywhere, even though it is

2012 Macmillan Publishers Limited. All rights reserved. GALACTIC PORTRAIT This artist’s impression, based on invisible and no one yet knows what it is. It dark. Finding such a galaxy-less clump of dark the latest data from telescopes and outweighs ordinary matter — stars, gas and matter means looking for its gravitational effect simulations, shows the Milky Way viewed from outside the Galaxy. everything else made of atoms — by a fac- on nearby dwarf galaxies or on streams of stars, tor of about five, and yet can be detected only and the signatures of such effects haven’t yet through its gravitational pull on visible stars been convincingly seen. “We’d love to find a and galaxies. Astronomers have known since dark-matter subhalo without a galaxy in it,” says the 1970s that the Milky Way, like every other Rockosi. “That’s high up on the list of things I galaxy, is wrapped in a vast cocoon of dark mat- hope I see.” ter; without it, the gravity generated by ordinary Still another possibility is that many more matter would be nowhere near enough to hold dwarf galaxies did form, but the first generation the Galaxy together. of stars were all so massive, hot and explosive In the immediate aftermath of the Big Bang, that they blasted all the gas and stars out of all some 13.7 billion years ago, gravity caused tiny but the largest subhalos2. irregularities in the dark matter to grow, forming denser and denser clumps on every size THE STELLAR HALO Either way, creation of the scale. Simulations show that this clumping pro- Galaxy continued apace, with gas and dwarf cess invariably becomes a chaos of collisions galaxies swirling inwards towards an ever- and mergers. But within a billion years of the increasing mass of gas and stars accumulating Big Bang things settle down slightly, and some at the dark-matter halo’s centre: the proto- of the dark-matter clumps begin to look much Milky Way. The dwarf galaxies were “whizzing like the one that surrounds the Milky Way: a all over the place”, says Heather Morrison, an roughly spherical halo several hundred kilopar- astronomer at Case Western Reserve University secs (1 kiloparsec is about 3,200 light-years) in Cleveland, Ohio. “Things were just a mess.” across, with a mass of about 1012 times that of Inevitably, some of them would have got too the Sun, and a multitude of subhalos all the way close to the ever-growing core and been pulled down to the mass of Earth. apart by its gravity. Inside this halo was a thin haze of primor- The region just outside today’s Milky Way dial hydrogen and helium gas that got pulled seems to be laced with remnants of such along by the dark matter’s gravity. After a few events: distinct streams of stars that loop hundred million years, as this gas cooled and around the galaxy along the dwarf galaxy’s condensed enough to start forming stars, it original orbit. These streams are tricky to iden- would become the raw material from which our tify, because they are faint and extend across Galaxy was created. But modelling that process much of the sky. But teams of observers are is anything but simple. “Dark matter answers finding more and more of them. In at least one only to gravity and we understand gravity,” says case, that of the Sagittarius dwarf galaxy and its Piero Madau, an astronomer at the University associated star stream, observers have found a of California, Santa Cruz. But marshalling dwarf galaxy in the act of disintegrating3. ordinary matter into today’s galactic structure These streams thread through a faint, diffuse involved collisions, dissipations, cooling, heat- halo of stars that extends outwards from the ing and explosions. “It’s very complicated,” Milky Way perhaps 100 kiloparsecs in every says Madau. direction, forming a rough sphere with a total mass of about 109 times that of the Sun (see DWARF GALAXIES One complication involves ‘The big picture’). This stellar halo may be noth- those dark-matter subhalos. Above a certain ing more than the remnant of all the dwarf gal- mass, yet to be determined, they would have axies that got disrupted over billions of years. pulled in enough gas to form stars and become But the story may be a little more complicated dwarf galaxies — irregular aggregations of stars than that. and gas with roughly 1% the mass of the mod- In 2007, a team led by Daniela Carollo, now ern Milky Way. But if that were the case, the at Macquarie University in Sydney, Australia, Milky Way ought to have thousands of dwarf and Timothy Beers, now director of the Kitt galaxies in orbit around it. So far, observers Peak National Observatory in Tucson, Arizona, have found some two dozen. confirmed earlier hints that the stellar halo is One possible explanation for this discrepancy divided into inner and outer components4. is that there are many more dwarf galaxies, but Stars in the outer halo generally have spectra they are vanishingly faint because they contain showing only trace amounts of heavy elements unusually high amounts of dark matter. The such as iron. This suggests that these stars are dwarf galaxy Segue 1, for example, has a thou- only one generation removed from the very sand times more dark than shining matter1. first stars to form in the Universe, less than “We are very, very interested in finding those a billion years after the Big Bang. If nothing vanishingly faint ones,” says Connie Rockosi, an else, this means that the precise distribution astronomer at the University of California, Santa of heavy elements in the outer halo should Cruz, “because they tell us the threshold mass give astronomers a record of what those long- below which dark-matter subhalos don’t form vanished first stars were like. stars and host galaxies at all.” Another possi The stars in the inner halo contain higher bility is that some subhalos are too small to amounts of heavy elements, and are some- ILLUSTRATIONS BY LYNETTE COOK have ever formed stars, and so are completely what younger — only about 11.4 billion years

old, according to work5 by Jason Kalirai of the flattened into a thin disk. Within the disk, mean- Space Telescope Science Institute in Baltimore, while, gravitational interactions caused the Maryland. In addition, the average motion of orbits of the stars and gas clouds to start piling the outer halo stars is opposite to that of the up and causing celestial traffic jams: coiling Galaxy, whereas the inner halo rotates in the ‘density waves’ that formed the spiral arms. same direction4. (In some galaxies, spiral arms also seem to be the result of shock waves propagating through THE DISK This pattern suggests that the outer interstellar gas.) halo formed from disrupted dwarf galaxies and The uncertainties arise when it comes to the the inner halo is a remnant of the maelstrom details. Did it take a billion years for the disk to at the centre, where the proto-Milky Way was form? Ten billion years? “Nobody really knows,” collapsing into its modern pinwheel form. The says James Bullock, an astronomer at the Uni- dynamics of this collapse have been under- versity of California, Irvine. stood for decades: every collision among the And how is it that the Milky Way can keep on incoming gas and dwarf galaxies dissipated making stars, when it probably should have run some of their orbital energy, so that they fell out of raw material billions of years ago? To do farther inwards. As they approached the centre, that, the Galaxy has to maintain itself as a com- what started out as a small, random amount plex ecosystem in which matter cycles back of rotation became magnified. And as the con- and forth between stars and interstellar gas. tracting mass spun faster and faster, it steadily Much of that gas is quite sparse, perhaps a few

2012 Macmillan Publishers Limited. All rights reserved. the disk as high-velocity clouds. “We see stuff from the distance of the Moon. They found that going out and stuff coming in,” says Weinberg, Andromeda and the Milky Way — now about “but we don’t know if they’re the same things.” 770 kiloparsecs apart and moving towards each other at 109 kilometres per second — will THE BULGE AND BAR At the Galaxy’s centre, collide head on in about 6 billion years. They roughly 8 kiloparsecs from Earth, is the bulge: will then pass through each other and mutu- a collection of mostly elderly stars, around ally orbit until, at 7 billion years, the two spiral 10 billion years old, arranged in a sphere hold- galaxies will merge to form one elliptical galaxy. hundred atoms per cubic metre, and, thanks to ing around 1010 solar masses. Bisecting the Ellipticals are one of the two main shapes of ultraviolet light from stars, it drifts through the bulge is a roughly linear ‘bar’ of younger stars galaxy. In contrast to the lively spiral galaxies, disk in a hot, ionized form. But in the 1970s, some 2–4 kiloparsecs long. Its origins are a which tend to be actively forming stars, the ellip- astronomers discovered that sometimes, for matter of debate, but similar features are com- ticals are more like featureless blobs containing reasons that still aren’t completely clear, the gas monly seen in other ‘barred’ spiral galaxies. little gas and few new stars. Oddly, only a minor- can gather itself into clouds so dense that their And at the heart of the bulge is an enormous ity of galaxies seem to be in transition; on the interiors are shielded from starlight. The gas black hole, which sits at the precise centre of whole they’re either lively or quiescent. Theo- on the inside can get as cold as 10–30 K, allow- the Galaxy. At 4 million solar masses, our local rists’ best explanation for this is that the merger ing atoms in the gas to form molecules such black hole is on the small side as such objects between two big galaxies leads to a burst of star as molecular hydrogen and carbon monoxide; thus the name ‘molecular clouds’. But because of gravity, this density also brings instability. No “IN THE UNIVERSE, STAR FORMATION IS sooner do the molecular clouds form than their thickest clumps collapse, heating up and ignit- ing by thermonuclear fusion to become stars. GRADUALLY SHUTTING DOWN AND THE These star-forming regions of the clouds, often called the Galaxy’s stellar nurseries, are tumultuous: newborn stars eject matter in the DEAD ARE BUILDING UP.” form of fierce stellar winds, along with floods of ultraviolet radiation. The most massive of them go: most galaxies seem to have one, and they formation, which quickly uses up the available also quickly die in supernova explosions; oth- often reach billions of solar masses. Ours also gas. Or maybe the merger reactivates the black ers end their lives by expanding into red giants happens to be inactive at present — that is, holes in the galaxies’ centres, and the resulting and shedding their outer layers. All of these nothing is falling into it. high-energy shocks and jets either drive the gas processes blast gas back into the wider galaxy, It was once livelier. In 2010, Douglas Fink- out of the galaxies or keep the gas stirred up where it will eventually cool, condense and start beiner (no relation to this writer) at the Har- and so hot that it can’t form stars. One way or the cycle again. vard-Smithsonian Center for Astrophysics in another, says Tim Heckman of Johns Hopkins The problem is that the Milky Way turns gas Cambridge, Massachusetts, found two bubbles University in Baltimore, Maryland, “infalling gas into stars at the rate of a few solar masses every on either side of the bulge and perpendicular to is cut off and the galaxy uses up the gas it’s got”. year, a pace that by now should have used up the disk11. The bubbles were each 7,600 parsecs The Universe holds only so much gas, and all the available gas. But the Galaxy has been long and outlined by X-ray emissions; shooting sooner or later — maybe as long again as forming stars for at least the past 10 billion into them from the galactic centre were small, galaxies have already existed — galaxies will years. “It’s got to get gas from somewhere,” faint γ-ray jets. Both bubbles and jets are sig- have converted all their gas into stars. In the says Ken Freeman of the Australian National natures of an active black hole, formed when Universe, “star formation is gradually shutting University in Canberra. matter falling into the black hole sends out jets down”, says Heckman, “and the dead are build- That somewhere may be an outside reservoir: of energy and creates shockwaves in the sur- ing up”. Little stars, one-tenth of a solar mass, a halo of gas that’s been observed in X-ray and rounding gas. Active black holes in the centres can live quietly for a trillion years. But even they extreme-ultraviolet wavelengths surrounding the of galaxies are fairly common and are probably will eventually burn out — and that will be it. Milky Way’s stellar halo6,7. Such reservoirs of gas a stage through which all galaxies pass. Fink- The end. ■ have also been seen around other galaxies8. It is beiner estimates that the Milky Way’s black hole mostly ionized hydrogen at maybe 1,000,000 K, was active somewhere around 10 million years Ann Finkbeiner is a freelance writer in and extends a few hundred kiloparsecs from the ago, and probably at intervals before that too. Baltimore, Maryland. centre. It is low density, around a hundred hydro- “The black hole didn’t get to be 4 million solar gen atoms per cubic metre, but so large that its masses if nothing fell into it,” he says. 1. Geha, M. et al. Astrophys. J. 692, 1464–1475 (2009). 2. Governato, F. et al. Nature 463, 203–206 (2010). mass should be at least that of all the stars in the 3. Yanny, B. et al. Astrophys. J. 700, 1282–1298 (2009). Galaxy — “a terrific reservoir”, Freeman says, THE FUTURE Observers have known for decades 4. Carollo, D. et al. Nature 450, 1020–1025 (2007). “and just a little of it coming in would kick off star that the nearest large galaxy, the spiral M31 5. Kalirai, J. S. Nature 486, 90–92 (2012). 6. Shull, J. M., Jones, J. R., Danforth, C. W. & Collins, J. formation” for billions of years. in the constellation Andromeda, is heading A. Astrophys. J. 699, 754–767 (2009). If halo gas does cool and condense enough towards the Milky Way. But they didn’t know 7. Gupta, A., Mathur, S., Krongold, Y., Nicastro, F. & to fall into the Galaxy — “like dew settling out of whether a collision was inevitable because they Galeazzi, M. Astrophys. J. Lett. 756, L8 (2012). a fog”, says David Weinberg of Ohio State Uni- hadn’t been able to measure its movement 8. Daddi, E. et al. Astrophys. J. 713, 686–707 (2010). 9. Wakker, B. P. & van Woerden, H. Astron. Astrophys. versity in Columbus — it may give rise to what sideways across the sky — a quantity known as 250, 509–532 (1991). observers see as high-velocity clouds9, falling ‘proper motion’. In May, Roeland van der Marel 10. Shapiro, P. R. & Benjamin, R. A. Publ. Astron. Soc. towards the disk. These clouds, in turn, may be at the Space Telescope Science Institute and Pac. 103, 923–927 (1991). 11. Su, M., Slatyer, T. R. & Finkbeiner, D. P. Astrophys. J. related to the ‘fountains’ that result when stars his colleagues compared Andromeda’s posi- 724, 1044–1082 (2010). explode into supernovae and kick gas 10–100 tion over time with background galaxies, and 12. Sohn, S. T., Anderson, J. & van der Marel, R. P. kiloparsecs out of the disk10. The theory is that measured its proper motion to an accuracy of Astrophys. J. 753, 7 (2012). 12–14 13. van der Marel, R. P. et al. Astrophys. J. 753, 8 (2012). the fountains soar up into the gas halo, pick up 11 microarcseconds per year — roughly 14. van der Marel, R. P., Besla, G., Cox, T. J., Sohn, S. T. & some of the ionized gas, and fall back towards equivalent to watching human fingernails grow Anderson, J. Astrophys. J. 753, 9 (2012).