fection together. Both the most luminous gi-
ants and the faintest dwarfs have especially
large fractions of dark matter, outweighing
the stars by factors of a hundred or more.
Middling galaxies like our Milky Way gener-
ally have the highest proportion of stars by
mass, but still about a factor of 30 less than
the mass in dark matter.
Ultra-diffuse galaxies (UDGs) are a recently
identified class of extended low-surface-
brightness objects with sizes that may be as
large as the disk of the Milky Way but total lu-
minosities typical of low-mass dwarfs. These
galaxies have turned up in large numbers
in recent imaging surveys by the Dragon-
fly Telephoto Array, a custom-built array of
telephoto lenses with anti-reflection nano-
structured coatings coupled with commer-
cial CCD cameras. The array is located in New
Mexico and operated robotically. Follow-up
studies with large-aperture telescopes of
several UDGs spotted by Dragonfly have
found that the ghostly galaxies generally
have large reservoirs of dark matter.
Because the UDGs themselves are so faint,
in many cases the easiest way of estimat-
ing their total masses is from the motions
of the globular star clusters associated with
the galaxies, as long as the galaxies are near
enough to allow spectroscopic observations
of the globulars in a reasonable amount of
time. These compact star clusters move in
response to the total gravity field, regard-
less of whether it is produced by luminous
or dark matter. Values of the total mass de-
rived from the speed of orbiting stars or star
clusters are referred to as “dynamical mass”
estimates. When the Dragonfly team no-
ticed that one of their UDGs, dubbed NGC
1052-DF2 because of its association with the
NGC 1052 galaxy group, contained multiple
bright compact points of light likely to be
globular clusters, they knew it was a prom-
ising candidate for further study with larger
telescopes.
January 2019 / 2018 Year in Review
“We used several of the world’s premier ob-
servatories, and the flexibility and fast re-
sponse time of Gemini were a key factor in
the analysis,” said Pieter van Dokkum of Yale
University, lead author of the new study of
NGC 1052–DF2. “We requested Director’s
Discretionary Time to observe NGC 1052–
DF2, and it was observed nine days later.
The Gemini image showed us that we had
found a truly unusual galaxy.” According to
researchers, the Gemini data, taken with
GMOS-North, provided “the best available
information on the regularity of the galaxy
at low surface brightness levels.” Visual in-
spection of the Gemini images (see Figure
14, next page) prompted the team to re-
quest a change in the scheduling of their
Hubble program targeting UDGs found with
Dragonfly; as a result, NGC 1052–DF2 was
given higher priority and observed sooner.
The GMOS images were also used to select
the globular clusters for spectroscopy with
the Keck I telescope.
The spectroscopic observations revealed re-
markably little spread in the velocities of the
ten globular clusters observed in NGC 1052–
DF2, and this narrow range of velocities has
major implications for the mass of the gal-
axy. The researchers concluded that the to-
tal dynamically determined mass was very
close to the observed mass of the stars in the
galaxy. This is unusual because UDGs of this
size typically have hundreds of times more
mass in dark matter than in stars. “If there is
any dark matter at all, it’s very little,” van Dok-
kum explained. “The stars in the galaxy can
account for all the mass, and there doesn’t
seem to be any room for dark matter.”
Because their result was so surprising, the re-
searchers considered several possible sourc-
es of error in the analysis. One possibility
was that NGC 1052–DF2 is not actually in the
NGC 1052 group at a distance of 20 mega-
parsecs (65 million light years), but much
closer to us. If so, the estimated mass in stars
GeminiFocus
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