5 billion years A.D. Our galaxy’s date with destruction The Milky Way is on a collision The Milky Way’s course with its neighbor, the Andromeda 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 Andromeda Galaxy, are on a collision course. Billions of years from now, the merger will drastically alter the struc- ture of both galaxies and spawn a new city of stars 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 universe. 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 Astronomy ⁄ ⁄ ⁄ 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. Local Group. end of the spectrum — by the Doppler In astronomical jargon, a galaxy group 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 redshift 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 “dark matter.” The close American astronomer Edwin Hubble (1889 The largest in the Local Group is Androm- proximity of galaxies in groups suggests – 1953). eda. It contains somewhat more mass 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 naked eye. 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 age of the universe, yields an estimated total mass for the Local Group of more than 3 trillion times the Sun’s mass (solar-masses). 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 EARTH, 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 Sextans B Sextans A 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 Pinwheel Galaxy (M33) are the largest in the group. Dozens of smaller satellite galaxies accompany them. The group’s of galaxies Leo 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 Fornax 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.