Figure 3 . Three views of the unusual dark-matter deficient galaxy
NGC1052 – 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 , lowsurface-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 NGC1052 – 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 .
ing that a substantial 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 brightness 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 provided the most precise distance to the host galaxy of GW170817 , the first gravitational wave event with an observed electromagnetic 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 conclusion that NGC1052 – 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 successful General Relativity theory , and there is no need for dark matter to provide additional gravitational force to hold galaxies together . But if this were the case , gravity should always act in the same alternative way for a given amount of visible matter , such as the stars observed in NGC1052 – 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 stronger 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 discovered by the Dragonfly array . If more galaxies like NGC1052 – DF2 come to light , it will provide much needed demographic information 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 Observatory and located at Gemini South in Chile . He can be reached at :
jblakeslee @ gemini . edu
12 GeminiFocus April 2018
Figure 3.
Three views of the
unusual dark-matter
deficient galaxy
NGC1052–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 NGC1052–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.
12
ing that a substantial amount of dark mat-
ter 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 mat-
ter; moreover, the brightness 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 provided the most precise distance
to the host galaxy of GW170817, the first
gravitational wave event with an observed
electromagnetic 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 NGC1052–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
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