GeminiFocus April 2018 | Page 14

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
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