Australian Sky Telescope MayJune 2017

32 AUSTRALIAN SKY & TELESCOPE May | June 2017

PALOMAR STREAM: V. BELOKUROV, D. ERKAL, S. E. KOPOSOV (UNIV. OF CAMBRIDGE, UK); DIAGRAM: NASA / ESA / A. FEILD (STSCI)

the sky, the stream appears surprisingly smooth. But new

simulationsrevealthatdarkmatterclumpsoughttodisturb

thestreamonfarlargerscales—causingdensityvariations

uptoseveraldegreeslong.Indeed,teamsexaminingthesame

CFHT images on these larger scales argue that flybys of one

kind or another have indeed had a profound impact.

The most recent result in this back-and-forth comes from

Denis Erkal (University of Cambridge, UK) and colleagues,

whodetectedtwobig‘gaps’inthePal5stream:Oneis2

degrees across and plausibly generated by something other

than dark matter — a run-in with a giant molecular gas

cloud, perhaps. The other, at 9 degrees across, is harder to

explain with ordinary interactions, but a dark matter clump

roughly 10 million times the Sun’s mass could be responsible.

This result is exactly what theorists would expect, considering

Pal 5’s relatively short lifespan.

Not everybody’s convinced, and a definitive detection

— the ‘holy grail’ — still eludes astronomers. Calculations

of clump-stream interactions remain in early stages, says

Andrew Wetzel (Caltech), who works on a simulation called

Feedback in Realistic Environments (FIRE). “I still interpret

any claimed detections with a grain of salt,” he says. That’s

why he and colleagues are in the midst of modifying FIRE to

test, for the first time, clump-stream interactions in a ‘live’

dark matter halo that’s still growing and spewing gas. “It will

be interesting to see how all this comes together.”

Stellar archaeology: Digging up the past

Stellar motions that tell us about the current Milky Way

dark matter halo also reveal its past: Each star in the

galaxy’s outskirts has its own unique history that informs

its movement. Whether a star is moving toward or away

from Earth is easy enough to determine from the Doppler

shift of its spectrum. But to understand a star’s 3D

SANCIENT MERGER? This diagram shows the Milky Way’s blue

disk of stars surrounded by a larger halo of faint, older stars. Hubble

Space Telescope observations have revealed that 13 of these halo stars

are travelling in a sideways motion, rather than in and out toward the

galaxy’s centre as was expected. These stars could be part of a larger

shell, a relic from the long-ago accretion of a massive dwarf galaxy.

Andromeda

Galaxy

Sun Shell

Stellar Halo

Disk

SMIND THE GAP(S)The top stream (coloured blue) shows the Palomar

5 stellar stream as it’s observed on the sky. The green stream is a

simulated version of Palomar 5, in which the stream has not encountered

any dark matter clumps. The red stream is likewise a simulation, this time

one that has been struck by two dark matter clumps, one with the mass

of 1 million Suns, which created the small gap at the left, and one with

the mass of 50 million Suns that created the larger gap on the right. The

observed stream appears to match the second, perturbed simulation,

hinting at otherwise invisible clumps of dark matter in the halo.

Observed Stream

Unperturbed Stream

Perturbed Stream

Small gap Large gap

movement takes time — lots of it.

For several years, the Hubble Space Telescope has been

watching distant halo stars’ proper motions — their movement

across the sky. “The motion of stars across the face of

[background] galaxies is analogous to watching human

hair grow on the surface of the Moon as seen from Earth,”

explains Puragra Guha Thakurta (University of California,

Santa Cruz).

But it’s worth the wait — that slow motion is key to

understanding halo dynamics.

With a pilot project studying 13 faint halo stars, Emily

Cunningham (University of California, Santa Cruz) and

Deason combined Hubble proper motions with spectra to

obtain 3D velocities for each halo star. Full knowledge of the

stars’ motions is crucial because, while stars in our part of

galactic suburbia make orderly circles around the galaxy’s

centre, halo stars don’t follow the same rules.

Most stars in the halo come from dwarf galaxies that

fell into the Milky Way, so their orbits are typically radial —

travelling toward or away from the galactic centre. But, to

Cunningham’s surprise, the orbits of these 13 halo stars are

more circular than radial. The stars, Cunningham argues,

might be part of a pile-up roughly 80,000 light-years from the

Milky Way’s centre. This pile-up could mark the point where

stars from a single massive dwarf (or perhaps from several

smaller dwarfs), accreted early in the Milky Way’s history,

slow down and turn around in their radial orbits.

Even with impeccable data, though, 13 stars provide an

extremely limited view into the Milky Way’s halo. The team

needs many more stars along other sight lines to confirm

MILKY WAY HALO