Australian Sky Telescope MayJune 2017

30 AUSTRALIAN SKY & TELESCOPE May | June 2017

Location

of Sun

Stellar

Bulge disk

100,000 light-years

Globular Stellar halo

clusters

DIAGRAM: GREGG DINDERMAN /

S&T

;^ FIELD OF STREAMS: V. BELOKUROV / SDSS

Similarly, the streams orbiting the Milky Way help

astronomersassessthemassandshapeofourgalaxy’sdark

matter halo. They can also help test standard cosmological

theory,inwhichdarkmatteranddarkenergydominatethe

natureandfateofouruniverse.Inthisframework,galaxies

and clusters (and the dark matter halos they live in) grow

largelybymergers.Ifthat’sthecase,thenpastcollisionswith

other galaxies might have shaped the Milky Way’s halo into a

slightly squashed football.

To understand how this shape develops, imagine balls

ofwaswithinalavalamp.Whentwoballsmerge,they

don’t form an instant sphere; first, they make a shape that’s

elongated like a football. Now watch another ball merge

and the football looks a bit bigger along that direction. Dark

matter simulations, no less mesmerising than a lava lamp,

show the same thing. Throw enough dark matter clumps

together,andeventuallytheproductofallthosemergersisa

flattened ellipsoid, a shape astronomers calltriaxial.

“TheMilkyWayistheoneplace—andthisiswhyI’m

excited about it — where we can look at the halo in three

dimensions,” says Kathryn Johnston (Columbia University).

Johnstonandmanyothershavetriedagainandagainto

estimate the mass and shape of the dark matter halo. At first,

groups used the Sagittarius Stream that jumps out of all-sky

surveys. However, this stream’s numerous stars have fanned

out across space. The very breadth that makes the feature easy

to see also makes it difficult to model mathematically.

AstronomershavehadbetterluckmeasuringtheMilky

Way’smassandshapeusingstreamssuchasPalomar5

and GD-1, which originated as globular clusters. The Milky

Way’sgravityhaspulledtheirstarsintolongarcsthatare

muchthinnerthanapulled-apartdwarfgalaxy.It’seasierto

calculatethepathofstarsstreamingfromasinglecompact

object:Sincethestarsareallcomingfromasmallerpointof

origin, they’re more or less on the same orbit.

Still,Johnstonsays,“Wedon’tknowthemassofthe

MilkyWaytobetterthanafactoroftwo,whichispretty

bad.” Estimates put our galaxy’s total mass around a trillion

times the mass of the Sun, but the uncertainties are large

enough that we’re still not sure if the Milky Way reigns as

heavyweight champion of the Local Group of galaxies, or if

that honour instead goes to Andromeda.

STHE HALO’S EXTENT The stellar halo reaches farther than the

disk, out to 600,000 light-years or so, as shown here. The dark matter

halo goes even farther — out to 1 million light-years — and outweighs

the stars 20 to 1. For simplicity, stellar streams aren’t shown.

SFIELD OF STREAMS This iconic image from the Sloan Digital Sky Survey maps individual stars across the Northern Hemisphere sky. The

combination of unprecedented data collection and immense sky coverage reveals stars streaming from swallowed dwarf galaxies, as well as from

globular clusters.

Sagittarius Stream

Orphan Stream

RA=12h

Dec=0°

Palomar 5

MILKY WAY HALO