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Home , Andromeda Galaxy, Leo A, Local Group, Sextans

5 billion years A.D. Our ’s date with destruction The is on a collision The Milky Way’s course with its neighbor, the Galaxy. What will the night sky look like after the date with crash? ⁄ ⁄ ⁄ BY abraham loeb and t.j. cox

ur home galaxy, the Milky Way, and its nearest neighbor, the , are on a collision course. Billions of years from now, the merger will drastically alter the struc- ture of both and spawn a new city of we have dubbed Milkomeda (“milk-AHM-mee-da”). The event will also radically transform the night sky. But into what? destruction Currently, the Milky Way’s thin disk of stars, dust, and gas appears as O a nebulous strip arching across the sky. As Andromeda grazes the Milky Way disk, we will see a second strip of stars looming across the night sky. After the final merger between these galaxies, the stars will no longer be confined to two narrow strips, but instead get scattered across the entire sky. In our research, we have explored the Milky Way’s fate by simulating the birth of Milkomeda in a supercomputer. The simulations are at a suf- ficient level of detail, or “resolution,” to learn much about the coming merger and how it will change our perspective on the . Although we will not be alive to witness the event — nor to take responsibility for whether our forecast proves accurate — this is the first research in our

24 ⁄ ⁄ ⁄ June 08 Our galaxy’s date with destruction

BILLIONS OF YEARS FROM NOW, the night sky will blaze with stars, dust, and gas from two galaxies: the Milky Way, in which we live, and the encroaching Andromeda Galaxy (M31). Lynette Cook for Astronomy

www.Astronomy.com 25 THE ANDROMEDA GALAXY (M31) is a typical spiral of stars, dust, and gas — the type of galaxy that dominates the night sky in the Local Universe. Fourteen small satellite galaxies axies are barreling toward each other at accompany Andromeda, including the two visible in this image: M32 (above Andromeda) nearly 270,000 mph (120 kilometers per and NGC 205 (below). TONY AND DAPHNE HALLAS second). We know this because the spectral lines of Andromeda’s light appear to be careers that has a chance of being cited in 5 Andromeda are the celebrity couple of the blueshifted — displaced toward the blue billion years. . end of the — by the Doppler In astronomical jargon, a effect. This is in sharp contrast to most gal- The Local group comprises two or more massive galaxies in axies in the universe, which are flying away The vastness of the night sky might suggest relatively close proximity. As the headlights from the Milky Way. This spreading the Milky Way resides in a relatively remote on a dark country road indicate the exis- motion induces a in the light from part of the Universe. However, astronomers tence of an entire car, the luminous stars of distant galaxies, a fact used to establish the know the Milky Way to be the second larg- a galaxy indicate the existence of an expanding universe since the time of the est member of the Local Group of galaxies. extended halo of “.” The close American astronomer (1889 The largest in the Local Group is Androm- proximity of galaxies in groups suggests – 1953). eda. It contains somewhat more than that their dark halos are gravitationally the Milky Way, resides nearly 2.5 million bound and dynamically coupled to each Timing is everything light years away, and is visible in the north- other. “Dynamically coupled” simply means Nearly 50 years ago, Franz Kahn and ern sky with the . The remaining the haloes attract each other via their gravi- Lodewijk Woltjer pioneered the “timing members of the Local Group — more than tational fields, and a change in one galaxy argument.” This hypothesis held that the 30 galaxies — are a bevy of much smaller affects the fate of the other. Milky Way and Andromeda formed close satellite galaxies. The satellites cluster near Evidence of the dynamical connection to each other, during the dense, early stages the Milky Way or Andromeda like celebrity between the Milky Way and Andromeda of the universe. Subsequently, they were entourages. Thus, the Milky Way and comes from their relative motion. The gal- pulled apart by the general cosmological expansion. Later, the Milky Way and Abraham Loeb is professor of astronomy at Harvard University, a visiting professor at the Andromeda reversed their outward paths Weizmann Institute of Science, and director of the Institute for Theory and Computation at the owing to mutual gravitational attraction. Harvard-Smithsonian Center for Astrophysics. T.J. Cox is a postdoctoral fellow at the Institute Since then, they have now traced out nearly for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics. a full orbit of each other.

26 Astronomy ⁄ ⁄ ⁄ June 08 The timing argument, along with knowledge of the current separation, rela- tive velocity, the , yields an estimated total mass for the Local Group of more than 3 trillion times the ’s mass (solar-). In addition, it suggests the Milky Way and Andromeda will make a close pass in about 4 billion years. The mass estimate, in particular, generated significant amount of interest at the time, because it is more than 10 times the mass of all visible matter. This result became one of the first pieces of evidence for the existence of dark matter. Kahn and Woltjer inspired a generation of studies that further constrained the mass of the local group and revealed important FROM , we see the Milky Way from an insider’s perspective. Only one of the galaxy’s characteristics of Andromeda’s orbit, such spiral arms is visible. JOHN CHUMACK as its total energy of motion, or “angular momentum.” But the timing argument does structure of the galaxies, but the sheer yields. This implies there is additional mass not have the ability to follow the complex number of atoms of matter interacting over in the Local Group. The missing mass dynamics that accompany the merger of time makes it difficult or impossible to turned out to be the diffuse “intergalactic extended galaxies. Therefore, it cannot pre- solve without massive computer power. medium” of atoms, gas, and dust between dict the future arrangement of the Local To simulate the evolution of the Local the galaxies. Galaxies are simply the visible Group. For processes as complex as galaxy Group, first we construct a physical model peaks of massive icebergs of matter. Much mergers, astronomers need more powerful describing its present state. This task is of the mass is not readily apparent, just as tools: supercomputers. straightforward for the Milky Way and most of an iceberg’s bulk lies beneath the Andromeda, since several decades of obser- waves. Simulating the Local Group vations enable us to estimate the amount of Numerical simulations are an indispensable gas and stars involved and the contribution When galaxies collide tool to understand astronomical processes from dark matter. Combined with cosmo- After we construct a model that includes all too complex to solve with pen and paper. A logical simulations, a plausible mass model the stars, gas, and dark matter in the Local perfect example is the merger of two galax- for the Milky Way and Andromeda can be Group, we evolve the system over time in a ies. Simple gravitational forces govern the determined to well beyond the visible inner computer and see what happens. Full- portion of each galaxy. scale simulations typically require 2 weeks However, the combined mass of the of number crunching, using the equivalent B Milky Way and Andromeda is still less than of 16 fully loaded desktop computers. nearly every number the timing argument Galaxy interactions are spectacular The IN THE LOCAL GROUP OF GALAXIES, the Milky Way, Andromeda Local Group (M31), and the (M33) are the largest in the group. Dozens of smaller satellite galaxies accompany them. The group’s of galaxies A members are all bound by mutual gravitational attraction. It’s total NGC 3109 filled space spans 6 million light-years. ASTRONOMY: ROEN KELLY

NGC 147 FPO Milky Way Sextans Dwarf IC 10 LMC SMC Sagittarius Dwarf NGC 185 Dwarf NGC 205 Andromeda (M31) NGC 6822 Pinwheel M32 (M33) 1 million light-years

IC 1613 www.Astronomy.com 27 GALAXY MERGERS IN CYBERSPACE Astronomer astronomers don’t simulate galaxy mergers just to create Cox’ web site, depict the complex merger of the Milky Way and Androm- pretty pictures. The simulations are experiments to test hypotheses about eda. These frames highlight important features of the galaxies and the how mergers work whether they are a significant process in the formation merger process. UNLESS OTHERWISE NOTED, merger images by t.j. cox (HARVARD-SMITHSONIAN SFA) and evolution of galaxies. These images, taken for an animated film on T.J

2 billion years 2.5 billion years 4.5 billion years 5.5 billion years from now from now from now from now

2 billion years from the pres- In 2.5 billion years, the In 4.5 billion years, the galax- In 5.5 billion years, Milkom- ent, the galaxies loop around galaxies are still moving ies loop around again and come eda is born. Tidal swirls,tails, each other in a close pass. Mutual apart. A ghostly bridge of back together to finally merge. and eddies left over from the attraction draws tenuous “tidal gas and stars connects the Their dense cores, each harboring violent merger slowly relax and tails” of stars and gas. Tidal tails galaxies. Stars in the a supermassive , gradu- dissipate. Individual stars are hallmarks of merger in the bridge, perhaps some with ally combine. The merging galax- spread out, forming a more real universe (see image at right). , could end up liter- ies experience a brief pulse of smooth, internally homogenous ally lost of intergalactic formation as the two black holes similalr to the space bridge dissipates. merge. barrEd elliptical galaxy IN CEN- NGC 2207 at right.

NGC 2207 (LEFT) merging with Montreal astronomy John Dubinski created this image of the merger of the Milky Way with smaller IC 2163. Andromeda. It highlights to fine, elegant contours of Milkomeda in the making, although at a cost of less NASA/ESA/Hubble Heritage Teams (STScI) detailed scientific contting in the images. The Dubinsky image reflects the skeletal structure of the margin events. Since the early days of astronomy, axies, such as the Milky Way and Androm- amazing story. One of the distinguishing merging galaxies have remained curiosities eda, into smooth spheroidal or “elliptical” characteristics of galaxy interactions is the owing to their complex and irregular galaxies. (see “What happens when galaxies appearance of long streams of stars and gas shapes. But astronomers now appreciate collide?,” March 2008.) that stretch from one or both of the partici- that mergers are one of the most significant You can view numerous spectacular pant galaxies. These features are typically drivers of galaxy evolution. For example, images of interacting galaxies captured by referred to as “tidal tails,” and result from galaxy mergers touch off huge bursts of star the and other great the powerful gravitational forces at work formation (starbursts), trigger the birth of observatories. These images are snapshots between merging galaxies. As the tails , and transform rotating spiral gal- of the dynamic merger process and paint an form, they rip material from the host galaxy

28 Astronomy ⁄ ⁄ ⁄ June 08 FPO Is this available in a hi-res?

THE MERGER OF SPIRALS often produces a single, new sphere-shaped type of galaxy called an elliptical. The galaxy above, A (NGC 5128), is a “peculiar elliptical” visible in the Southern Hemisphere. NOAO/AURA/NSF

and hurl it into intergalactic space. Change of fortune that future astronomers will be able to wit- As the Local Group evolves, the Milky However, the Sun’s peaceful orbit around ness some, if not all, of the Local Group Way and Andromeda will begin to have a the center of the Milky Way — which it has evolution we have simulated. dynamical impact upon each other owing traversed nearly 20 times since its birth — Although the Milky Way and Androm- to their mutual gravitation. As a result, it’s will forever change. Its new path will be far eda will merge, stars within the two galax- possible the Sun — and Earth and the other more chaotic owing to the rapid fluctua- ies, such as our Sun, will not physically planets — will be dragged into a tidal tail. tions in gravity induced by the merger. collide. The reason is the extremely large During this period, an observer will have What would this mean for the Earth and its distances between individual stars in galax- one of the most unique vantage points ever residents? ies. For example, if the Sun was the size of a imagined. Torn shreds of the Milky Way Our computer studies suggest the Milky ping pong ball, the nearest star (Proxima will fill a large fraction of the night sky as Way and Andromeda will begin to strongly Centauri) would be another ping pong ball our galaxy experiences its gravitational interact 2 billion years from now, and then nearly 1,000 miles (1,600 km) away. dance with Andromeda. complete the merger in about 5 billion Because only a small fraction of a gal- years. The latter date is notable, because it Our final resting place axy’s mass ends up in tidal tails, it is more coincides with the Sun’s remaining lifespan. The Sun’s orbit will follow a chaotic path likely the Sun will go for a much less dra- Currently, our Sun is about halfway until the merger concludes and the system matic ride. Most of the stars in merging through its lifetime and will soon begin to relaxes and expands. At this point, the Sun galaxies remain relatively close to their host expand as it slowly consumes all available will reside inside a new galaxy, Milkomeda. galaxies. The chance of the Sun being ban- and evolves toward a It will look very different from either the ished to the tidal-tail boondocks is rela- phase within 5 billion years. In short, the Milky Way or Andromeda. tively small, based on our simulations. Sun will be in its death throes on Milkome- The Milky Way and Andromeda are There is also a 3 percent change the Sun da’s birthday. spiral galaxies, with most stars concen- will end up in Andromeda after its second The Sun’s red-giant stage will make trated into a disk and moving in nearly cir- close encounter with the Milky Way. In that on Earth rather uncomfortable. Indeed, it cular orbits around the . In case, earthly observers could gaze across will spell the end of life (as we know it). contrast, Milkomeda will be nearly spheri- space at their own former home, seeing it However, it does not preclude the possibil- cal in shape and much smoother in appear- truly for the first time. ity for colonization of habitable planets ance than any . Stars within around nearby stars, and thus it is possible Milkomeda will follow more complex

www.Astronomy.com 29