Australian Sky Telescope MayJune 2017

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D. MARTÍNEZ-DELGADO (MPIA AND IAC), R. JAY GABANY (BLACKBIRD OBS.), K. CRAWFORD (RANCHO DEL SOL OBS.) ET AL.

that the pile-up exists. Cunningham, Deason and others

are collaborating to measure the spectra of another 350 or

so halo stars, while Hubble continues to observe those stars’

proper motions from space. In addition to 3D velocities, the

team will also measure and analyse spectra to measure each

star’s age and origin.

“I’m fascinated by the idea of piecing together the

assembly of our galaxy star-by-star,” Deason says. “I love the

idea that by studying distant stars in the Milky Way halo,

we can turn back the clock and reconstruct how these stars

became a part of our galaxy billions of years ago.”

Beyond the Milky Way

Reconstructing our past has shown the Milky Way’s halo

to be a surprisingly sedate place. Aside from the addition of

one massive dwarf early on, life has been pretty uneventful

for our galaxy until it only recently began accreting the

monstrous Magellanic Clouds.

The halo around the Andromeda Galaxy, on the other

hand, plays foil to our own. It’s larger, more massive and

messier all around; its stars are younger, too. Andromeda

has clearly been active from a young age, gobbling more and

bigger dwarf galaxies than the Milky Way has.

In fact, the halos surrounding the two galaxies are so

different, they actually span the full range that cosmological

simulations predict for galaxies of similar mass — a fact that

initially led theorists astray.

“We originally only had the Milky Way to compare

with,” says Roelof de Jong (Leibniz Institute for Astrophysics

Potsdam, Germany). “All the modellers basically tried to

model the Milky Way, because that’s where the data were.

They were sometimes frustrated that they missed it.”

Questions about our halo and its past have been

haunting astronomers. Now a second wave of

astronomical data promises a deeper view, and

perhaps some answers.

SENDLESS VARIETY These amateur images of nearby spiral galaxies show traces of devoured dwarfs, including wisps, plumes, stellar streams

and partially disrupted galaxies. In each composite image, the outer region shows the light in negative in order to allow better identification of the

faint structures surrounding these galaxies.

M63 NGC 1084 NGC 7531

NGC 4216 NGC 3521 NGC 5866

As astronomers began to see beyond Andromeda, they

realised that the Milky Way might not make the best

prototype. “If [the modellers] only had computing time for

one or two galaxies... they wouldn’t look like the Milky Way

at all,” de Jong explains. “Now we realise that if they’d had 20

models, maybe one or two would have hit the Milky Way.”

This realisation is motivating several teams to look beyond

our own galaxy. David Martínez-Delgado (University of

Heidelberg, Germany) has been collaborating for almost a

decade with a small team of amateurs. Another team, led

by Roberto Abraham (University of Toronto) and Pieter van

Dokkum (Yale University), takes a slightly different approach:

They’re coupling multiple Canon telephoto lenses to CCD

cameras, integrating them into a telescope that looks like

a dragonfly’s eye. Both amateurs’ scopes and the Dragonfly

Telephoto Array capture deep, wide-field images of stellar

streams around nearby spiral galaxies.

The variety is stunning. Around some galaxies, stars that

were once part of dwarf galaxies have been swept into circles,

shells, clouds and even spikes. Surprisingly, other galaxies,

such as the Pinwheel (M101), appear to have no stellar halos

at all. These images are filling in the gap between lively

Andromeda, the quiet Milky Way, and beyond.

These observations appear to confirm that galaxy halos,