GeminiFocus April 2018 | Page 13

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