OUT THERE 



Slammin' 



the Milky Way 



The majestic calm in the stellar disk of our galaxy 

 may conceal a history of intergalactic collisions. 



By Charles Liu 



New Galaxy Slams into 

 the Milky Way, blared the 

 headlines in January. At the 

 winter meeting of the American As- 

 tronomical Society, a research team led 

 by Mario Juric at Princeton University 



shaped like a pancake with a plum 

 stuck through the middle. The plum is 

 called the galactic bulge; the pancake, 

 known as the galactic disk, is a mighty 

 flat flapjack — about 100,000 light- 

 years across but only a hundredth that 



How did the thick disk form? The 

 leading hypothesis says a small dwarf 

 galaxy collided with the Milky Way. 



As I noted earlier, you can imagine 

 such galaxy mergers as cannibalistic 

 collisions. The gravitational effects of 

 a merging dwarf galaxy, it turns out, 

 can seriously disrupt the Milky Way's 

 disk. If a small dwarf galaxy had 

 fallen into the Milky Way long ago, it 

 might have "splashed" up and down 

 through the thin galactic disk several 

 times as it settled gravitationally into 

 our galaxy. Those disruptions might 

 well have given some of the stars extra 

 random motion and created a thicker, 

 puffier disk. Or, even after billions of 

 years, some of the dwarf's stars may 

 still not have settled into the more 

 massive and stable Milky Way disk. 

 In that case, the thick disk could 



The bulge and disk of our home galaxy, the Milky Way, are shown in this composite image, 

 assembled from infrared images collected with the Two Micron All Sky Survey. 



announced that they'd found a dwarf 

 galaxy, a gathering of millions of stars, 

 crashing into our home galaxy. 



Exciting news, yes — but maybe not 

 as dramatic as it seemed in the newspa- 

 pers. We astronomers are pretty sure 

 that a lot of dwarf galaxies have fallen 

 into the Milky Way in the past 10 bil- 

 lion years. And because the stars are all 

 so far apart that they don't actually hit 

 each other, and since the dwarfs are 

 eventually fully incorporated into the 

 Milky Way, they're really more like 

 mergers than crashes. Even so, they 

 can have a noticeable impact on the 

 structure of our galaxy, even billions of 

 years after the fact. 



I low about an example? First, a few 

 bits of background: The Milky Way is 



thick. Together, the galactic disk and 

 bulge are embedded in a sparse stellar 

 sphere, a "galactic halo" some 150,000 

 light-years across. 



About twenty years ago, yet another 

 component of our galaxy's stellar 

 structure was identified. A substantial 

 number of stars was discovered near, 

 but not in, the superthin galactic 

 disk — stretching more than 2,000 

 light-years above and below the main 

 plane of stars. The extra stars also orbit 

 the galactic center, but unlike stars 

 belonging to the disk, they move 

 both faster and more randomly. As- 

 tronomers concluded that the galaxy 

 had both a main, thin disk as well 

 as a "thick" disk — a sparse, pufted-up 

 structure that envelops its companion. 



be made up of those "unsettled" stars. 



Mary-Margaret Brewer at William 

 Jewell College in Liberty, Missouri, 

 and Bruce W. Carney at the University 

 of North Carolina, Chapel Hill, are 

 studying long-lived, sunlike stars in 

 both the thin and the thick disks of the 

 Milky Way. Their goal is to determine 

 whether stars in the two disks formed 

 in common or in disparate settings. 



Stars, like people, are what they 

 eat — or rather ate. Each star is born 

 with the same elements, in the same 

 ratios, that were present in the vast 

 clouds of gas and dust out of which the 

 star formed. So in principle, to deter- 

 mine whether two stars were born in 

 (Continued on page 74) 



NAT U R A I HISTORY March 2006 



