Sky and Telescope - April 2017 - 25

CO MPOSITE: V. BELOK UROV, D. ERK A L & S. E. KOPOSOV, DA RK M AT TER CLU MPS:
VOLK ER SPRING EL, A NDRO MEDA G A L A X Y: A DA M E VA NS

p DARK 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.

mathematical models as much as possible, but such measures,
Bonaca and others have found, can throw off mass estimates.
Uncertainty also plagues measurements of the dark matter
halo's shape. While some studies have reported the expected
squashed shape, more recent measurements have found the
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-art cosmological simulation and the dark matter halos' lumpy interiors
soon become apparent. Depending on the 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. Even a single ﬁrm detection of such a hole could tell
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.

A definitive detection - the "holy grail" -
still eludes astronomers.
Ibata's team found that, on scales of up to 1 degree on
the sky, the stream appears surprisingly smooth. But new
simulations reveal that dark matter clumps ought to disturb
the stream on far larger scales - causing density variations
s k y a n d t e l e s c o p e .c o m

* APRIL 2017

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Sky and Telescope - April 2017

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