ACT Reading Apr. 2017 74F - Passage IV

Astronomers sometimes describe the shape of our

home galaxy, the Milky Way, as a thin-crust pizza with

a plum stuck in the middle. The plum is the slightly

oblong central bulge, protruding about 3,000 light-
₅ years above and below the galactic plane, comprised

mostly of older stars; it makes up the core of the Milky

Way, and includes a black hole two and a half million

times the mass of the Sun. The crust of the pizza is the

galactic disk-the source of most of our galaxy's light.
₁₀ Thin and flat, the disk is 100,000 light-years across,

about 1,000 light-years thick, on average, and includes

more than 80 percent of the galaxy's hundred billion or

so stars.

The plum-and-pizza picture works well enough,
₁₅ but like most simple metaphors, it breaks down if you

push it. For one thing,. the galactic disk isn't a rigid

body, but a loose agglomeration of matter streaming

around a common center of gravity. (The swirling pat-

tern of a hurricane far better resembles our spinning
₂₀ galaxy.) For another thing, our galaxy's disk isn't flat;

it's warped. Picture a disk of pizza dough spun into the

air by a skilled chef: our galaxy goes through the same

kind of floppy, wobbly gyrations, though at a rate best

measured in revolutions per hundreds of millions of
₂₅ years.

Why does the Milky Way have such an odd-

looking warp? No definitive answer has emerged. One

thing we do know: when it comes to warps, our galaxy

is hardly unique. About half of all spiral galaxies are
₃₀ warped to some degree. Theoretical and computational

models have shown that a number of physical processes

can warp a galaxy, so it's a matter of figuring out which

scenario applies. An innovative analysis of the problem

by Jeremy Bailin, an astronomy graduate student at the
₃₅ University of Arizona in Tucson, has implicated a small

satellite galaxy, currently being ripped to shreds by the

gravity of the Milky Way.

The Sagittarius Dwarf Spheroidal Galaxy was dis-

covered in 1994. It appears to ·be in a roughly polar
₄₀ orbit around the Milky Way-that is, above and below

the galactic disk-about 50,000 light-years from the

galactic center. That orbit brings the dwarf galaxy far

too close to the huge gravitational tidal forces of the

Milky Way for the dwarf to remain intact. As a result,
₄₅ the Sagittarius Dwarf now looks something like strands

of spaghetti spilling from the front of a pasta-making

machine, the galaxy's matter being drawn out over hun-

dreds of millions of years by intergalactic tides.

Gravitational collisions between small satellite
₅₀ galaxies and big spiral galaxies have long been

regarded as possible culprits in the warping of a larger

galaxy's disk. The best known satellite galaxies orbit-

ing the Milky Way-the Large and Small Magellanic

Clouds-are too far away, and have the wrong orbital
₅₅ characteristics, to have warped our galactic home. The

Sagittarius Pwatf seems a much more likely candidate.

simply because it i.s only a third as far from the center

of the Milky Way as the Magellanic Clouds. But in

astronomy-unlike in real estate-location isn't every-
₆₀ thing; to show a direct connection between warp and

dwatf, the orbital motion of the Sagittarius Dwarf must

be linked to the rotation of the Milky Way's disk.

Bailin 's study is the first to find such a link. His

analysis of the galactic warp is based on angular
₆₅ momentum-a measure of how much a system is spin-

ning or rotating. Just as objects moving in a straight

line have momentum, objects spinning or orbiting

around an axis have angular momentum; and just as the

momenta of two objects combine when they collide, so
₇₀ too do their angular momenta. Imagine two figure

skaters coming together for a combination spin. When

they make physical contact, their individual spiraling

motions combine to produce a single, unified whirl.

Starting with the latest measurements of the struc-
₇₅ ture and spin of the Milky Way, Bailin deduced the

angular momentum of the warped portion of the Milky

Way's disk. He then compared that measure with the

angular momentum of the