Australian Sky Telescope MayJune 2017

34 AUSTRALIAN SKY & TELESCOPE May | June 2017

ESA / HUBBLE & DIGITIZED SKY SURVEY 2 / DAVIDE DE MARTIN (ESA / HUBBLE); INSET: NASA / ESA / T. M. BROWN (STSCI)

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).

To complement holistic views of galaxy halos, another

team, led by de Jong, employs the Hubble Space Telescope to

zoom in on the streams around 18 galaxies for the Galaxy

Halos, Outer disks, Substructure, Thick disks, and Star

clusters (GHOSTS) survey.

Hubble’s field of view is so small that even in its first

proposals for telescope time, the team was already asking

the telescope to point far off of a galaxy’s disk to capture

halo stars. “I remember getting the feedback from the Space

Telescope Science Institute saying, ‘Are you sure you want to

put your field here? You’ll miss the galaxy completely!’” de

Jong recalls. “And I said, ‘Yes, this is exactly the point.’”

Like the other teams, GHOSTS found a remarkable

diversity of halo shapes and sizes. But the team also

uncovered something entirely unexpected: middle-aged stars.

Called asymptotic giant branch (AGB) stars for their stage of

evolution, they’re typically a couple billion years old and not

exactly spring chickens anymore. But they’re positively spry

compared to halos’ more typical ancient stellar populations.

Dwarf galaxies formed most of their stars early in the

SSTARS IN THE STREAM This Hubble image (above), which covers

the area outlined by the yellow box (top), resolves a smattering of faint,

red stars in Andromeda’s halo that were once part of a companion

galaxy. The stars’ colours reveal their age and composition.

SANDROMEDA’S HALO Though this groundbased image

is too shallow to show the stellar stream that crosses over

our sister galaxy, Hubble Space Telescope images of areas

far off the stellar disk, such as the field marked by the

yellow box, reveal individual stream members.

MILKY WAY HALO