Australian Sky Telescope MayJune 2017

28 AUSTRALIAN SKY & TELESCOPE May | June 2017

our galaxy’s past

IN THE LATE 19TH CENTURY,new photographic techniques

revealed ‘spirits’ among us. First generated through an

accidental trick of exposure, supposed ghosts soon became

regular features in Victorian-era spirit photography.

Now, more than a century later, new photographs are

once more revealing ghosts — but this time they’re real.

They’re made of the stars in our galaxy’s halo, which

surrounds the disk in a diffuse sphere, and they have a

greatdealtosayabouttheMilkyWayanditspast.

The halo glows dimly with the light of only a couple

billion suns, a number that pales in comparison to the

hundreds of billions of stars that shine in our galaxy’s

spiral disk and central bulge. Yet the halo contains about

20timesasmuchmassasallofthosestarscombined—it’s

just that we can’t see most of it. The lion’s share of the halo

isintheformofdarkmatter,themysteriousinvisiblestuff

that surrounds every galaxy and governs its growth and

evolution, yet so far has avoided detection in physicists’ labs.

Since we can’t see dark matter, and would dearly like to,

we must turn to what we can see. For more than a decade,

ambitious all-sky surveys have revealed dozens of wraith-

likestellarstreamsglidingthroughthehalooftheMilky

Way Galaxy; meanwhile, new techniques have revealed

similar features around other nearby galaxies. These

streams are mostly the stellar remains of dwarf galaxies

torn apart during gravitational encounters with their

bigger siblings.

When two large galaxies meet, they typically merge

intoasingleobject.However,whenadwarfencounters

a far larger galaxy, it (counterintuitively) isn’t entirely

subsumed. The gravitational friction induced by the larger

galaxydragsinefficientlyonthedwarf’sstars,whichend

up ‘remembering’ their dwarf galaxy’s initial orbit.

Becauseofthisuniquefeature,stellarstreamsgivevoice

to the past and present of the dark matter all around us. In

addition to understanding the halo and what it says about

our galaxy’s ancient history, we also learn how the Milky

Wayfitsin(ordoesn’t)withtheuniverseatlarge.

of

ABDUL AZIS / GETTY IMAGES

Ephemeral streams of stars

are helping astronomers probe

our galaxy’s dark matter halo,

raising more questions than they

answer about the Milky Way

and its past.

MILKY WAY HALO by Monica Young