http://www.skyandtelescope.com.au 29V. RUBIN & K. FORD /
ASTROPHYSICAL JOURNAL
FEBRUARY 1970 (2)
Our galaxy’s shadow
While the Milky Way’s spiral disk spans some 100,000 light-
years, its halo — mostly dark matter with faint, old stars here
and there — may extend as much as 10 times farther, almost
halfway to the Andromeda Galaxy. Yet the first indication
that the Milky Way even had a halo of largely invisible
material didn’t come until 1970 — and it came not from our
own galaxy but from its famous neighbour.
The late Vera Rubin and her colleague, Kent Ford (both
then at the Carnegie Institution of Washington), had clocked
the speed of hydrogen gas clouds whirling in the disk of
Andromeda. If the galaxy’s mass were largely concentrated
at its centre, where most of the light was coming from,
then the farther out a cloud, the slower its speed ought to
be. Instead, Rubin and Ford found that the clouds on the
outskirts were all flying along at the same speed — without
the galaxy whirling itself to pieces. Something was holding it
all together.
Theorists, such as Jeremiah Ostriker and James Peebles
(both at Princeton University), soon chimed in, agreeing
that disk galaxies must be couched in spherical cushions of
apparently invisible matter, or their fragile spiral patterns
would become unstable and collapse.
But over the next decades, astronomers proved the halo
to be far more complicated than it first appeared. In 1994
they first observed the strung-out stars of the disintegrating
Sagittarius dwarf galaxy. Then, a few years later, the Sloan
Digital Sky Survey (SDSS) began mapping very faint stars all
across the northern hemisphere sky. An iconic image, dubbed
the Field of Streams, showed that Sagittarius was far from
alone in its destruction.
“The SDSS really changed the way we viewed the halo,”
explains Ana Bonaca (Harvard-Smithsonian Center for
Astrophysics), who dived into SDSS data soon after the image
was published in 2006.
The swoops of stars in the Field of Streams — some ofthem from the tidal tails of disrupted dwarf galaxies, others
escaping from split globular clusters — point to a halo that’s
still in the act of growing. Moreover, SDSS provides spectral
information, which identifies stars’ motion toward or away
from Earth, and helps show which ones belong together.
With the advent of such large spectral surveys, measuring
the Milky Way halo became a whole new ball game.Ghosts of galaxies past
Look at the Field of Streams image (next page) and you’ll
see the ghosts that haunt the Milky Way today. The most
obvious is the massive Sagittarius Stream: These broad tidaltails of what was once a dwarf spheroidal galaxy wrap around
the poles of the Milky Way. Its stars lie between 20,000 and
300,000 light-years away from the Sun.
A smaller ‘orphan’ stream crosses Sagittarius; its parent
might be another torn-apart dwarf. (Except perhaps for
Sagittarius, all the other digested galaxies now swimming
in the halo were far less massive than the Milky Way’s
largest present-day dwarf, the Large Magellanic Cloud). Tiny
Palomar5isalsoclearlyseen:aglobularclusterintheact
of dissolution about 75,000 light-years away. Another tidally
destroyed globular is GD-1.
The list goes on, amounting to more than two dozen
potential streams — a cosmologist’s equivalent of a gold mine.
Each of these stars glides through the halo’s gravity well
like a marble rolling along the surface of a trampoline that’s
warped by a bowling ball resting in its centre. Even if we
know nothing about the bowling ball, the path a marble
takes tells us about the ball’s shape and weight.WSHE SPINS
RIGHT ROUND
Rubin and Ford
clocked the velocities
of gas clouds in the
Andromeda Galaxy
(marked by black
dots, left), measuring
from near the centre to
the galactic outskirts.
To the astronomers’
surprise, the clouds
farthest from the
centre were whipping
around much too
fast (right) — yet the
galaxy somehow still
held together.“The Milky Way is the one place — and this is
why I'm excited about it — where we can look
at the halo in three dimensions.”