Australian Sky Telescope MayJune 2017

http://www.skyandtelescope.com.au 31

COMPOSITE: V. BELOKUROV, D. ERKAL & S. E. KOPOSOV, DARK MATTER CLUMPS: VOLKER SPRINGEL, ANDROMEDA GALAXY: ADAM EVANS

Theuncertaintystemsinpartfromthenatureofcurrent

simulations.Theuniverseislarge,long-lived,andhugely

complex, so simulations that describe it accurately become

computationally expensive. Theorists have to simplify their

mathematical models as much as possible, but such measures,

Bonacaandothershavefound,canthrowoffmassestimates.

Uncertaintyalsoplaguesmeasurementsofthedarkmatter

halo’s shape. While some studies have reported the expected

squashedshape,morerecentmeasurementshavefoundthe

dark matter halo has a surprisingly spherical form. “I’d say

there isn’t much consensus at the moment,” says Alis Deason

(Durham University, UK), “but that’s likely due to a lack of

data rather than differences of opinion.”

Black gold

Still, there’s more to our galaxy’s dark matter halo than

just its mass and shape. Zoom into any state-of-the-

artcosmologicalsimulationandthedarkmatterhalos’

lumpyinteriorssoonbecomeapparent.Dependingonthe

simulation, the Milky Way’s halo might hold thousands of

dark, starless nuggets. The number of nuggets itself holds

clues to the nature of dark matter.

Though astronomers could never see these lumps directly,

they might spot them by the holes they punch in stellar

streams.Evenasinglefirmdetectionofsuchaholecouldtell

A definitive detection — the “holy grail” —

still eludes astronomers.

us a lot about what dark matter is like. “That’s the holy grail,”

says James Bullock (University of California, Irvine).

“I think we’re right on the cusp of being able to say

something about this,” Johnston adds. “If you ask an

astrophysicist what’s the most exciting thing, I think they

would say this is the most exciting thing.”

One of the best-studied stellar streams — where

researchers are making the most headway — is Palomar 5,

whose stars wend across 20 degrees on the sky. Two tails, one

leading and one trailing, come from a still-existing globular

cluster that only recently (that is, about 3.5 billion years ago)

began to disintegrate in the halo’s gravitational well. It’s a

perfect place to look for evidence of dark matter clumps. Yet

while initial studies were promising, debate has since ensued.

Picking out subtle variations along a barely visible stream

seen against a foreground rich with galactic stars isn’t easy.

So Rodrigo Ibata (Astronomical Observatory of Strasbourg,

France) led a team that collected the deepest data yet on the

Palomar 5 stream, using the Canada France Hawaii Telescope

(CFHT) to take sensitive images of Pal 5’s stars.

Ibata’s team found that, on scales of up to 1 degree on

SDARK LUMPS While dark matter halos are sometimes thought of as smooth, featureless things, they’re actually clumpy on small scales. This

illustration, based on the Aquarius dark matter simulation, shows how dark matter might clump around a Milky Way-mass galaxy (represented by an

image of the Andromeda Galaxy). Just how many clumps we’d expect depends on what goes into the simulation, such as the nature of dark matter.