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 noticed
that one of their UDGs, dubbed NGC1052-
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.
“We used several of the world’s premier
observatories, and the flexibility and fast
response time of Gemini were a key factor
April 2018
in the analysis,” said Pieter van Dokkum
of Yale University, lead author of the new
study of NGC1052–DF2. “We requested
Director’s Discretionary Time to observe
NGC1052–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 regular-
ity of the galaxy at low surface brightness
levels.” Visual inspection of the Gemini im-
ages (see Figure 3) prompted the team to
request a change in the scheduling of their
Hubble program targeting UDGs found
with Dragonfly; as a result, NGC1052–DF2
was given higher priority and observed
sooner. The GMOS images were also used
to select the globular clusters for spectros-
copy with the Keck I telescope (see this is-
sue’s cover image montage).
The spectroscopic observations revealed
remarkably little spread in the velocities
of the ten globular clusters observed in
NGC1052–DF2, and this narrow range of ve-
locities has major implications for the mass
of the galaxy. The researchers concluded
that the total 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 mat-
ter than in stars. “If there is any dark matter
at all, it’s very little,” van Dokkum 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
researchers considered several possible
sources of error in the analysis. One pos-
sibility was that NGC1052–DF2 is not ac-
tually in the NGC1052 group at a distance
of 20 megaparsecs (65 million light years),
but much closer to us. If so, the estimated
mass in stars would be much lower, mean-
GeminiFocus
11