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The universe’s stellar metropolises rend, chew, and merge with one another. But how important are these encounters in creating the galaxies we see today?

M any o f us have a rough-and-tumble view of galaxy within the universe’s first few billion years, making growth. In this savage landscape, the rules are simple: some wonder if mergers even had time to play a role and eat or be eaten. Spirals tear up dwarfs and munch them if the whole cosmological framework was wrong. like Fruit Roll-Ups. Big galaxies smash together and, Today, we occupy some sort of middle ground. “I in their cannibalistic fervor, gnash each other beyond think everyone would agree that merging is a fundamen­ recognition. Cosmic history can seem like the tale of tal and important process,” says Eline Tolstoy (University galaxies playing a grim game of king of the mountain. of Groningen, The Netherlands), “but I think we would In the very early universe, when protogalaxies disagree with each other about exactly how that mani­ reigned and things were more of a mishmash, a riotous fests itself from one galaxy to the next.” picture might have had some truth to it. But in their observations of the universe’s past 10 billion years Sorties and Wars astronomers have found that, when it comes to how On the largest scales, cosmic structure looks like a galaxies grow, the whopping crashes of whirligigs aren’t sponge. Veins of dark matter, gas, and galaxies outline as big a deal as you might think. The cosmos isn’t the sparse voids. This spongy structure formed thanks to Wild West; many stellar cities don’t land themselves in gravity, which exacerbated small blips in the nascent all-out brawls. universe’s density, pulling dark matter (which makes That hasn’t always been the thinking. Astronomers up most of cosmic particles) into a web of filaments have flip-flopped several times on how important merg­ and nodes. As this network coalesced, the first galaxies ers are in galaxies’ evolution, says Pieter van Dokkum also formed, like coffee spilled on a cobbled floor, James (Yale). In the mid-20th century, astronomers simply Geach (University of Hertfordshire, UK) writes in his weren’t thinking of mergers. Then, with the develop­ book Galaxy: Mapping the Cosmos. The sloshed coffee ment of modern cosmology in the 1980s, the idea arose collects in the grooves between cobbles, much the same that these unions were the main driver of galaxy growth. way the universe’s pristine gas drained into gravitational And in the late 1990s and early 2000s, astronomers valleys created by dark matter. discovered that the most massive galaxies already existed After that initial startup, galaxies continued to grow. For the first few billion years, they looked very little like MINOR SKIRMISH Arp 273 includes a spiral galaxy (top) that the ones we see today — they were clumpy and turbu­ has had its arms pulled askew by a second, smaller galaxy about one-fifth its mass (bottom). The smaller companion perhaps dove lent, a hodgepodge of objects. Star formation ramped up, through the larger one on an off-center trajectory, explaining the peaking across the universe about 10 billion years ago, stretched spiral. The encounter likely triggered the clusters of during what astronomers call cosmic high noon. Then it newly formed stars (blue). A small orange core embedded in the dwindled, to less than one-tenth that rate today. Even so, spiral’s outer right arm might be a third galaxy. about 95% of the stars shining now formed in the last 10 NASA / ESA /HUBBLE HERITAGE TEAM (5TSCI/AURA) billion years, van Dokkum says.

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GALACTIC CRASHES Major and minor mergers have different effects on the galaxies involved. NGC 6621 and 6622, also known collectively as Arp 81 (right), exemplify the interaction o f two evenly matched systems: the encounter has torn both galaxies apart. On the other hand, the large spiral NGC 5754 (below) is five times more massive than NGC 5752 (fuzz to its right) and survived their recent encounter relatively unscathed. Facing page: William Keel (University of Alabama) and Kirk Borne (now at George Mason University) endeav­ ored to reconstruct these systems’ major tidal structures (labeled images) in simula­ tions o f the interactions. In the simulation diagrams, the big dot marks the center of the contender that looped around, and orbits are orange where they passed behind the other galaxy from our perspective.

NGC 5752/4 AND NGC 6621/2: NASA / ESA / HUBBLE HERITAGE (STSCI / AURA) & ESA / HUBBLE COLLABORATION / W. KEEL (UNIV. OF ALABAMA, TUSCALOOSA); SCHEMATICS: S&T: GREGG DINDERMAN, SOURCE: W. KEEL AND K. BORNE / ASTRONOMICAL JOURNAL 2003 Some — including those that make for the most breath­ taking Hubble images — involve two large, well-matched adversaries, such as NGC 4038 and 4039, the Antennae Galaxies in Corvus. These major mergers happen when 6621 M jT j) one contender is at most four times more massive than the other. Major mergers spur dramatic changes, creating 'raMr f * a maelstrom of shock waves and turbulence in the galax­ ies’ gas clouds that subsequently triggers star formation. Toward the end of the merger, the interaction can also 6622 J Tidal spray^,* ♦ ## drive a large amount of gas into the new galaxy’s center, ...... • g§!p: feeding the supermassive black hole (or holes) in the core and fueling a starburst phase, during which hundreds to “The Milky Way wasn’t really around yet, as we see thousands of Suns form per year for a few million years. it now,” he says. Astronomers think that the types of These mergers are spectacular, but they’re not terribly systems present today — ellipticals, lenticulars (“lens­ common. In 2009 Shardha Jogee (University of Texas at shaped”), spirals, barred spirals, and all the siblings in Austin) and colleagues looked back 3 to 7 billion years each of those groups — took shape between 6 and 10 ago at galaxies slightly smaller than the Milky Way. The billion years ago. So if we can track what’s happened to team found that approximately two-thirds of these galax­ stars over the last 10 billion years, we’ll know how most ies had suffered a merger, but only 16% had clearly been of today’s stars ended up where they are. in a major duel. On the other hand, at least 45% had That’s why a lot of the action — and the mystery — weathered a minor merger, a merger with a galaxy one- centers on cosmic high noon. Astronomers are fairly con­ fourth to one-tenth the bigger one’s mass. fident that smaller clumps of gas combined to build galax­ Minor mergers often involve a big galaxy and a dwarf ies’ early precursors. The question is what came next: how galaxy. They’re more common than major ones because do you turn a galactic shantytown into a megalopolis? there are a lot of small galaxies in the universe and they Astronomers often tackle galaxy growth by looking tend to cluster around the larger ones, leaving them­ at star formation, because star formation essentially is selves vulnerable to cannibalism. growth. To amass extensive stellar suburbs, galaxies When the larger system is a spiral, a minor merger need to build upon the finite resources they first incor­ can warp its disk, creating pockets of star formation porated. That means they need to either nab stars from or puffing the pancake up and out. In some cases the elsewhere or find more gas to keep creating their own. merger barely leaves a mark. But the effect is hardly This sticking point is where mergers come in. When minor on the smaller contender: the encounter can dev­ astronomers talk about mergers, they generally mean astate a dwarf, ripping it apart and glomming its pieces the interaction of full-grown galaxies after the universe’s onto the rival’s outskirts. first couple billion years. But not all mergers are equal. Building Big Galaxies • Conventional wisdom — at least, of late — is that the Tidal arm most massive of the melon-shaped galaxies called ellip­ .. K i n k ^ / ticals form via mergers. These systems contain ancient stars moving along chaotic orbits, and they’re also the heftiest stellar cities, easily 10 times more massive than the Milky Way. Astronomers don’t see gargantuan ellip­ ticals at cosmic high noon, but they do see compact, bul­ **...*• bous galaxies, with the stars deep inside packed as tightly as they are in today’s leviathans. These compact cores can * * • * outweigh the Milky Way, even though they’re only about

MORE THAN ONE WAY TO SKIN A GALAXY

Mergers aren’t the only way galaxies Inter­ the technical term), when multiple high­ merge. But the galaxies can still end up act. There are also encounters that aren’t speed passes in a crowded cluster leave a looking not quite themselves. How com­ (or aren’t exactly) mergers. These include a galaxy looking frazzled. In harassment and mon such processes are depends on how galaxy sucking in gas from the space around other near encounters, galaxies move past crowded a galaxy’s neighborhood is, and it, and gravitational harassment (yes, that’s each other too quickly to grab hold and who its neighbors are.

SkyandTelescope.com September 2015 1 9 | Galactic Smashups Galactic | 20

September

NASA / ESA / HUBBLE HERITAGE (STSCI / AURA) & ESA / HUBBLE COLLABORATION tim e s m ore m assive than th e spiral, NGC 4647, w hich is on par par on is hich w 4647, NGC spiral, e th than assive m ore m s e tim w ith the M ilky Way. The galaxies lie roughly 10 m illio n light- light- n illio m 10 roughly lie 10 galaxies The roughly Way. is 60, ilky M M l, the tica ith w ellip t ian g The luster. C o irg V e th in sits their borders. “If you combine those two things — that that — things two those “If combine you borders. expand to their stars captured the using and mergers minor population eventually dominated by reddish dwarf stars. dwarf by reddish dominated eventually population stellar aging an early, out behind leaving burned ies that galax­ ith star-forming w small, as began size.They the pared a tenth com ry isto h ellipticals. erger-free assive m m r e oth telescopes. relatively a all had sm has in pair galaxy nice a make and er oth each m fro years D PO E SAM PEAS, ENT DIFFER they didn’t form a lot of new stars, but they’ve changed they’vechanged but stars, aof lot new form didn’t they via stars stealing by grew then cores, these as started cals. Simulations in 2007 by Frederic Bournaud (now at Bournaud 2007 Frederic by in Simulations cals. inescapable.” almost is Ithink, “That, says.Dolckum van mergers,” gone under­ have must look they — they how in dramatically M 60’s stars rotate coherently, w hich m ig h t indicate th a t the the t a th indicate t h ig m hich w coherently, rotate stars 60’s M And in a few billion years our own Milky Way is heading Milky Way own heading is our years a few billion in And — elliptical an into turn it will systems, other from mass initial of 30%its to 40% than more grabbed galaxya has after that, suggested CEA colleagues Saclay, and France) — everything will inevitably settle down and create a create and down settle inevitably will everything — bine to form a big, red elliptical elliptical red abig, form to bine Galaxy. Andromeda the spiral, sister its with for battle or minor. major were mergers its of whether regardless two galaxies — more than half of the contents, roughly roughly contents, of the half than galaxiesmore — two helter- to leading interactions, gravitational via one another from momentum angular steal can Stars are. they gassy ishow matters What disks. combatants’ the sug­ also observations and simulations Both astronomy. Computer mockups of that merger indicate they’ll they’ll com­ indicate merger of that mockups Computer disk. For major mergers, this might onlyhavehappened might this mergers, major For disk. pancakes. orderly initial, the like nothing layouts skelter don’t alwaysdestroy mergers major individual that gest But gas dilutes this effect. If there’s enough of it in the the of in it enough If there’s effect. this dilutes Butgas 5 1 0 2 Some astronomers think today’s massive ellipticals ellipticals today’smassive think astronomers Some In addition, we know that mergers can create ellipti­ create can mergers that we know addition, In But things are never straightforward in galactic in straightforward never are But things

S K Y &. &. Y K S TELESCOPE The galactic duo A rp 116 rp A duo galactic The (S&T: Oct. 2014,Oct. 20).p. were particularly gassy. particularly were have grown without outside help. The stars in these gal­ these in stars The help. outside without grown have tics, suggests these systems form from really massive, really massive, from form systems these suggests tics, characteris­ other with combined motion, ordered The a unit. as rotating together, move to seem they ellipticals; biggest the in do they as don’t randomly flyaxies about galaxies when years, few billion first universe’s the in likely at least two galactic breeds that can grow into the the into grow can that breeds galactic two likelyat least oids all on their own own their on all oids spher­ big grow into to stars enough out churn to poised look10 11that agoto from years billion shining galaxies massive detected have recently Astronomers mergers. (if from any)help much without galaxies, star-forming just in certain spots. That’s distinct from what astrono­ what from spots.That’sdistinct certain in just — they’re a mass lotlionthey years, in have increased but Milky like the Way, opaque.These iseven more picture the today. see we ellipticals their centers today centers as they 10were their billion yearsago. in dense as are ellipticals, massive which the see in mers not throughout, once denser they were,and than denser over 10 last the galaxies sizemuch bil­ in haven’t increased (butForspirals lessstill massive big)galaxies, including Confusion into Spiraling looked at about 6,500 local spiral galaxies from the Sloan the from galaxies spiral 6,500 local about at looked UK) of Hertfordshire, (University Kaviraj Sugata year, material the steadily,gas accrete with spirals mergers, significant via of assembling instead that, suspect omers astron­ Thus differently. material their acquire ellipticals bances — and the starbirth they induced — were from from — were induced they starbirth the and — bances preserve to unlikely are mergers major Since turbance. last study intriguing an In evolution. spirals’ play in laughing. adds, one,”he ous obvi­ most it’s the but it’sright, mean doesn’t “That says. galaxies’ edges. at the just not and at all radii deposited tussles instigate roughly 40% of all the star formation formation star of the 40% all roughly instigate tussles these that Kavirajestimates Given that, mergers. minor distur­ any that assumed he today’suniverse, in disks tic, suggesting that minor mergers might only ignite onlyignite might mergers minor that suggesting tic, optimis­ less are astronomers Other today’s in universe. formation of star all half around trigger might mergers minor that argues he galaxies, lenticular and of elliptical of dis­ signs for them checked SkySurvey and Digital it will spawn stars. “The dwarf essentially activates this this activates essentially dwarf “The stars. spawn will it before compressing needs gas that says,but gas,” he in galaxy. Milkya Way-type in stars 10% of about the did he study a previous with Coupled spirals. in on going gas into forming stars,” he says. “What gas the dwarf dwarf the gas says.“What he stars,” forming into gas “awash are spirals galaxy. These larger the in already gas itself brings into the system is pretty much irrelevant.” much ispretty system the into brings itself In addition, today’s lighter-weight ellipticals might might ellipticals today’s lighter-weight addition, In This difference might arise because spirals and big big and spirals because arise might difference This The main mystery today is the role minor mergers mergers minor role the is today mystery main The “That is the most obvious explanation,” van Dokkum Dokkum van explanation,” obvious most the is “That Kaviraj thinks dwarfs spur starbirth by stirring up up bystirring starbirth spur dwarfs Kavirajthinks (S&T: Aug. 2015, Aug. 14).p. are Sothere ist

ANTENNAE GALAXIES The iconic pair NGC 4038 and 4039 represent one of the nearest and most recent galactic interactio The two orange orbs are the original galaxii cores. The dust (brown), young star clust>. i (blue), and hydrogen gas (pink) make for a beautiful but messy picture.

NASA / ESA / HUBBLE HERITAGE (STSCI / AURA) &. ESA / HUBBLE COLLABORATION

SkyandTelescope.com Sept c mb r | Galactic Smashups

also suggest that cold streams can directly connect to the edge of a galactic disk and feed the galaxy like a fuel

Dark matter halo line (see right). Because the gas is so diffuse and faint, and stellar halo observing such accretion is difficult in practice. But there’s the problem of semantics: when is an interaction a minor merger, and when is it cold-gas Caseous Stellar accretion? It’s hard to tell when a clump of gas quali­ disk Bulge disk fies as a dwarf galaxy. Some astronomers draw the line based on whether the cloud sits in its own dark matter halo, as both dwarf and large galaxies do. But confirm­ ing such a halo poses its own challenges: astronomers are still debating whether a recent high-speed visitor to the Milky Way, the Smith Cloud, survived because of a magnetic sheath or because it has its own dark matter holding it together — and that gaseous object is a mere

20.000 light-years 40,000 light-years away, right in our backyard.

100.000 light-years Going It Alone 160.000 light-years Astronomers have other reasons to doubt mergers’ role. The 2009 study by Jogee’s team revealed that only a third MILKY WAY IN BRIEF Edge-on schematic o f our Milky Way (at most) of the stars being born 3 to 7 billion years ago Galaxy, with associated parts. Sizes are approximate. were in visibly merging galaxies; instead, most new stars were lighting up in galaxies that weren’t interacting. Enrico Di Teodoro (University of Bologna, Italy) Other observations support this picture: many of the stars agrees that the gas in dwarf galaxies can’t fuel the explo­ born in the last several billion years — and thus much of sion in stars: in a study of 148 nearby spirals, he and the growth that’s happening — arise in quiet galaxies. Filippo Fraternali (also at Bologna) estimated that dwarf Another cause of suspicion is stellar halos. These companions could supply at most one-fifth of the gas gigantic clouds of old stars surround galaxies like the needed to sustain the average starbirth rate in the galax­ Milky Way, extending far beyond the spiral disk. Some ies they observed. But he cautions that the gas in a spiral astronomers predict that most of the halo stars come has to come from somewhere. Spirals generally only have from dwarf galaxies the spiral has ripped up and eaten enough gas to sustain a “normal” star-formation rate of over time. Observers do see hints of stellar streams in one to two Suns per year for a few billion years at most. the Milky Way’s halo, including shreds of the dwarf “If no fresh gas is supplied from the outside, spiral gal­ Sagittarius, supporting this idea. And the Andromeda axies would have already exhausted their gas reservoir, Galaxy is “a big mess,” Tolstoy says, with a halo rich with and we would not observe star-forming galaxies in the substructures and clear evidence of a major accretion local universe,” he says. event. The merger debris could be from one of Androm­ Galaxies can also pull in gas directly from their sur­ eda’s dwarf companions or another unidentified galaxy. roundings, in a process called cold-gas accretion. Only But a merger-made halo is hard to reconcile with the relatively cold material can sink down deep into galax­ Milky Way’s chemistry, Tolstoy warns. The halo stars ies, cooling further until it can collapse to form stars. astronomers have studied in detail have fairly pristine Astronomers think such accretion was particularly hydrogen-and-helium compositions, with low levels important in the early universe, when there was so much of heavy elements. The latter build up as stars die and gas being channeled into galaxies that it could shove past seed the gas around them with things like carbon. The their hot halos (which would normally stymie accretion). chemistry suggests that any consumed dwarfs had to Simulation work by Dusan Keres (now at University of have stopped forming stars after only a billion years, California, San Diego) and colleagues in 2009, for exam­ then merged with the Milky Way. “That becomes quite ple, argued that a galaxy that today has roughly 10 times contrived,” she says. the Milky Way’s mass in stars built up most of that mass One unexpected turn comes from van Dokkum’s by accreting gas that’s never been substantially heated. team. The astronomers took a hard look at the nearby Spirals’ rotations, as well as how their rotation axes spiral M101, the Pinwheel Galaxy in Ursa Major. Using line up with the cosmic filaments they sit in, support a compound instrument called the Dragonfly Telephoto the idea that these galaxies form through a more peace­ Array, which combined eight Canon EF 400-mm f/2.8 ful accretion of gas drawn in from their surroundings. telephoto lenses on a common mount, the team studied And high-resolution simulations of the early universe M101 down to a magnitude per square arcsecond of 32,

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COLD-CAS SIMULATION In the universe’s first couple billion years (top), many protogalaxies grow by accreting cold gas along cosmic filaments (blue streams, forming galaxy in center). But as the gas falls into the gravitational well a galaxy sits in, the gas is shocked and heats up (red). At early times, the cold gas could push past the hot to accrete directly onto disks. But over time, the gas heats up and filaments become more diffuse, breaking up into smaller clouds that mix with the halo (bottom left, 10 billion years ago, and bottom right, 7 billion years ago). Astronomers observe cold accretion onto the Milky Way today in the form o f high-velocity clouds, potentially condensations from the galaxy’s gaseous halo. One estimate puts this accretion rate just shy o f one solar mass per year. To fuel the current starbirth rate, the Milky Way uses several times that amount o f gas. OSCAR ACERTZ (3)

SkyandTelescope.com September 2015 23 Galactic Smashups

similar to studies of the Milky Way’s galactic family, the is looking for these “chemical tags” in the Milky Way’s Local Group. To their surprise, the astronomers found halo stars, in order to identify debris from disrupted no stellar halo. stellar clusters and dwarf galaxies. Similarly, ESA’s Gaia If more spirals turn out to look like the Milky Way and spacecraft (S&T: Apr. 2014, p. 10) is hard at work compil­ M101, then galaxies might experience fewer mergers than ing a detailed map of the Milky Way’s stars and how astronomers think. But with only three big spirals studied they move. Preliminary Gaia data have turned up stars at this depth, it’s hard to say. “I have a completely open in the halo that do look like they’ve been snatched from mind on this particular question,” van Dokkum says. elsewhere. But the data also suggest that our galaxy hasn’t merged with anything larger than one-tenth its Making Sense of Mergers mass since its disk formed about 9 billion years ago. Astronomers are investigating the true role of mergers That would mean that between then and now, the Milky in several ways. One is the work Kaviraj and others are Way hasn’t scuffled with anything bigger than the Large doing, trying to determine observationally the impact Magellanic Cloud. minor mergers have across cosmic time. For that work, Learning more about how the Milky Way grew will astronomers are looking to deep, large-scale projects, shed light on how relatively isolated galaxies evolve. And such as the Dark Energy Survey and the upcoming because most stars today are in Milky Way-type galax­ Euclid mission and Large Synoptic Survey Telescope. ies, understanding our galaxy and those like it will help There are also chemical and dynamical clues closer us understand how important mergers have been in the to home. Stars born together should have similar universe overall. ♦ chemical makeups, revealing their common ancestry. A 5-year survey at Siding Spring Observatory in Australia, S&T Science Editor Camille M. Carlisle might trump the called GALAH (Galactic Archaeology with HERMES), universe’s galaxies in snackingfrequency.

SPIRALS DANCE The tw o How Astronomers Watch Galaxies Age galaxies o f Arp 238 in Ursa Major It's hard to track galaxies across cosmic time. As astrono­ hold each other at arm's length. mers study the universe at higher and higher redshifts, The arms are tidal tails o f gas and pushing farther back in cosmic time, they essentially take stars torn from the galaxies’ outer snapshots o f what galaxies looked like at a particular moment. edges by the encounter. But these snapshots show an assortment o f galaxies at dif­ NASA / ESA / HUBBLE HERITAGE (STSCI / AURA) & ESA / HUBBLE COLLABORATION / A. EVANS (UNIV. OF VIRGINIA, ferent look-back times; they don’t track the same set o f stellar CHARLOTTESVILLE / NRAO / STONY BROOK UNIV.) metropolises through their evolution across 10 billion years. Instead, astronomers have to connect the dots between epochs based on what they see. “ It’s like having the fossil record: you see all these dif­ ferent animals at different times, but then you have to say, ‘Okay, this dinosaur turned into this chicken,'” quips Pieter van Dokkum. In biology, scientists have genetic tools to help them understand the record. “We haven’t quite found the DNA o f galaxies yet.” Sans galactic DNA, astronomers often turn to mass as a proxy: they assume that the first, second, third, and so forth most massive galaxies today were also one o f the first, second, third, and so forth most massive galaxies 5 billion or 10 billion years ago — in other words, they rank them by mass, and then use that rank as a sort o f roll call to pick out the progenitors of today’s galaxies across cosmic time. So long as galaxies don’t on average hopscotch over one another in the mass pecking order, this trick works. Rank order is how astronomers deter­ mine that, compared with their appearance now, the progeni­ tors o f today’s disk galaxies were fluffier but about the same size several billion years ago. In fact, disk galaxies’ precursors seem to have a range o f appearances about 8 billion years ago, from smooth or unstable disks to interacting systems.

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