GeminiFocus 2016 Year in Review | Page 25

October 2016
Figure 4 . A sample of Io images obtained from the Gemini North telescope using NIRI with the Altair adaptive optics system for tracking volcanic activity over a 29 month period . Image credit : Katherine de Kleer and Imke de Pater , UC Berkeley / Gemini Observatory / AURA / W . M . Keck Observatory
October 2016
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 .
The Keck spectroscopy enabled Pieter van Dokkum ( Yale University ) and collaborators to measure the mass of Dragonfly 44 . The deep images from Gemini ( Figure 5 ) 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-
Figure 5 . The dark galaxy Dragonfly 44 , observed using GMOS- North , in wide-field ( left ) and close-up ( right ). Dragonfly 44 is very faint for its mass , and consists almost entirely of dark matter . Image credit : P . van Dokkum et al ., Gemini Observatory / AURA
January 2017 | 2016 Year in Review GeminiFocus
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