Velocit yDark matter
(in red)Globular
cluster-30 0 30
Velocity (km/s)Sample^si
ze Velocity
dispersionVelocity with respect to average-7 07
Velocity (km/s)Sample^si
ze Velocity
dispersionVelocity with respect to averageWWW.ASTRONOMY.COM 47
Debating DF2’s distance
“Something that caught my attention very early on was the fact
that the galaxy was not only anomalous for not having dark
matter, but also for having an extraordinarily bright population
of globular clusters,” says Ignacio Trujillo of the Instituto de
Astrofísica de Canarias, who led the main study challenging the
bizarre nature of DF2. “I remember thinking: ‘Two anomalies
at the same time really looks odd.’ ” This led Trujillo to consider
whether DF2 is really as distant as van Dokkum’s team thinks.
If DF2 were roughly 65 million light-years away, like van
Dokkum suspects, then it would be a strong candidate for the
first example of a galaxy without dark matter. But Trujillo says
that if DF2 is closer, the galaxy’s observed properties would fall
in line, more or less, with what’s expected from your typical
dark-matter-dominated galaxy.
To test their theory, Trujillo and his team set off to indepen-
dently determine the distance to DF2 using five different
methods — all with varying degrees of trustworthiness.
Two of the methods relied on analyzing the brightnesses
and sizes of DF2’s globular clusters. According to the short-
distance camp, if DF2 is closer than initially thought, then the
galaxy’s globular clusters no longer would be weirdly big and
bright. Trujillo’s team also compared DF2’s properties to aGALAXIES WITH DARK MATTER. In diffuse
galaxies, globular star clusters typically do not orbit
around their galaxies in circular paths like they do in
spiral galaxies. Instead, they have a range of radial
orbits that carry them every which way. If dark matter
dominates a diffuse galaxy, the globulars within it
should be moving relatively fast, as seen above.
ASTRONOMY: ROEN KELLYGALAXIES WITHOUT DARK MATTER.
However, in diffuse galaxies without dark matter, the
lack of hidden mass means that globular clusters within
the galaxies move much more slowly. In the case of
DF2 and DF4, researchers found that the globulars are
moving so slowly that the mass of the galaxies’ visible
matter alone is enough to account for their motions.
ASTRONOMY: ROEN KELLYThe basic idea of the
surface brightness
fluctuation method
for determining
distance is that the
farther away a galaxy
is, the smoother its
overall brightness
profile. This is
because each pixel
of an image contains
a lot more stars for
more distant
galaxies. This image
uses resampled
Hubble data to
show what M32
would look like at
increasingly greater
distances (nearest is
at top, farthest is at
bottom). NASA/ESA/W. KEEL
(UNIVERSITY OF ALABAMA)DARK MATTER’S EFFECT ON GLOBULARS
Hubble captured this beautiful
image of the transparent
galaxy DF2 with its Advanced
Camera for Surveys. NASA/ESA/
P. VAN DOKKUM (YALE)