Figure 14.
Three views of the
unusual dark-matter
deficient galaxy NGC
1052–DF2. The upper
left panel shows the sum
of the g and r images
taken with the Dragonfly
Telephoto Array, in
which the galaxy
appears as an extended,
low-surface-brightness
“blob.” The lower left
panel shows a sum of
g, r, and i images from
the Sloan Digital Sky
Survey (SDSS), revealing
a concentration of
compact objects overlaid
on a faint fuzz. The
panel at right shows the
Gemini North i-band
image of NGC 1052–
DF2, which provided
the best information on
the morphology of the
galaxy. Black ellipses
indicate the effective
radius (containing half
the total light) and twice
the effective radius;
white arrows mark
artifacts of the reduction
that become visible at
faint levels. The galaxy
has a regular elliptical
shape without any
significant variations
with radius.
38
would be much lower, meaning that a sub-
stantial amount of dark matter would then
be needed. Such a nearby distance would be
unlikely based on the velocity of the galaxy,
but perhaps not more unlikely than a galaxy
devoid of dark matter; moreover, the bright-
ness of the globular clusters suggested that
the distance might be only half as large as
assumed. Fortunately, the high-resolution
Hubble images enabled an independent
measure of the distance via analysis of the
galaxy’s surface brightness fluctuations, the
same statistical method that recently pro-
vided the most precise distance to the host
galaxy of GW170817, the first gravitational
wave event with an observed electromag-
netic counterpart. Using this technique, the
researchers found evidence that the UDG
was within the NGC 1052 group, reducing
this source of uncertainty.
Somewhat counterintuitively, the conclu-
sion that NGC 1052–DF2 lacks detectable
dark matter constitutes a strong argument
against theories that dispute dark matter’s
existence. Such alternative theories posit
that gravity simply works in a different way
than described by Einstein’s highly success-
ful General Relativity theory, and there is no
GeminiFocus
need for dark matter to provide additional
gravitational force to hold galaxies together.
But if this were the case, gravity should al-
ways act in the same alternative way for a
given amount of visible matter, such as the
stars observed in NGC 1052–DF2. Clearly this
is not the case, since other galaxies with the
same quantity of stars show very different
internal motions indicative of a much stron-
ger gravitational field, easily explained by
dark matter.
So far only a few UDGs have dynamically
measured masses, and most of these are
abundant in dark matter. However, the team
is continuing to follow up on others discov-
ered by the Dragonfly array. If more galax-
ies like NGC 1052–DF2 come to light, it will
provide much needed demographic infor-
mation to aid in understanding how such
galaxies form in the absence of dark matter.
The study appears in the March 28th issue of
the journal Nature.
John Blakeslee is the Chief Scientist at Gemini Ob-
servatory and located at Gemini South in Chile.
He can be reached at: [email protected]
January 2019 / 2018 Year in Review