GeminiFocus October 2016 | Page 9

Nancy A. Levenson and Mischa Schirmer Science Highlights Recent significant scientific results based on Gemini data include determining the physical characteristics of a massive galaxy consisting almost entirely of dark matter, confirming the existence of several nearby Earth-sized exoplanets around a cool dwarf star, and obtaining the first-ever close-up images of Lyman-alpha blobs at low redshifts. A Dark Matter Milky Way Astronomers have discovered a massive galaxy that is almost entirely dark matter. The galaxy, called Dragonfly 44, has very low surface brightness and was discovered only in 2014. New Fast Turnaround program observations using the Gemini Multi-Object Spectrograph (GMOS) on Gemini North, as well as spectroscopy from the Keck II telescope also on Maunakea, reveal the galaxy’s physical properties. They show that it is like a “failed” Milky Way, in having similar total mass, size, and population of globular clusters, lacking only stars. Figure 1. The dark galaxy Dragonfly 44, observed using GMOSNorth, in wide- field (left) and close-up (right). Dragonfly 44 is very faint for its mass, and consists almost entirely of dark matter. Credit: P. van Dokkum et al., Gemini Observatory/AURA The Keck spectroscopy enabled Pieter van Dokkum (Yale University) and collaborators to measure the mass of Dragonfly 44. The deep images from Gemini (featured on the cover of this issue and in Figure 1) then yielded the galaxy’s mass-to-light ratio (48 within the half-light radius), and the Gemini imaging shows the large population of globular clusters in the halo. Considering theoretical models that include the halo, the researchers con- October 2016 GeminiFocus 7