Sky and Telescope - April 2017 - 27

M63

NGC 1084

NGC 7531

NGC 4216

NGC 3521

NGC 5866

p ENDLESS 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.

"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."

D. M A RTÍNE Z-DELG A DO (MPIA A ND IAC), R. JAY G A BA N Y (BL ACK BIRD OBS.), K . CR AWFORD
(R A NCHO DEL SOL OBS.) E T A L.

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 modelers basically tried to model
the Milky Way, because that's where the data were. They were
sometimes frustrated that they missed it."
As astronomers began to see beyond Andromeda, they realized that the Milky Way might not make the best prototype.
"If [the modelers] only had computing time for one or two
galaxies . . . they wouldn't look like the Milky Way at all," de
Jong explains. "Now we realize that if they'd had 20 models,
maybe one or two would have hit the Milky Way."

This realization 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 (see box on page 29). 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.

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.
These observations appear to conﬁrm that galaxy halos,
where they exist, are largely dwarf galaxy graveyards. But the
variety astronomers see as they image galaxy after galaxy
doesn't have a clear explanation yet.
"It's still a bit early to say what a 'typical' stellar halo for
Milky Way-mass galaxies looks like," says Allison Merritt
(Yale University).
s k y a n d t e l e s c o p e .c o m

* APRIL 2017

http://www.skyandtelescope.com

Sky and Telescope - April 2017

Table of Contents for the Digital Edition of Sky and Telescope - April 2017