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The Ring of Darkness

posted Feb 26, 2018, 10:11 AM by Jeremy Drake
The speed with which stars orbit around the centre of a galaxy - the ``rotation curve'' - tell us about the distribution of mass within the galaxy.  One of the most remarkable discoveries made in astronomy employed this technique of examining the orbital speeds of stars in galaxies that are viewed nearly edge-on.  The data indicated that most of the mass of a typical galaxy lies in an extended halo beyond the visible stars and gas .  This otherwise unseen mass was dubbed ``dark matter".   

The importance of stellar kinematics for understanding galaxies gives the impression that our own Milky Way rotation curve would be well-defined and comprehensively observed.  Instead, the kinematics of the Galaxy outside the Solar Circle - the orbit of the Sun about the Galactic centre -  is quite poorly known. University of Hertfordshire PhD student Amy Harris set out to show that stars slightly more massive than the Sun, of spectral type A to F, can be uniquely exploited to remedy the situation.  These types of star can be efficiently selected from photometric surveys.  Being relatively bright, they can be seen to large distances, and being relatively young they still reflect the kinematics of Galactic rotation.  

Amy used Multiple Mirror Telescope observations of a sample of about 800 A/F stars to probe a narrow 1 degree sightline across our Galaxy and found a surprising increase in orbital speed with increasing distance from the Galactic centre. The results support earlier suggestions that the Milky Way has ring of dark matter lying between 13 and 18 kpc from the centre. These are just the first pilot study results and Amy's method is now set to be deployed to test these findings in much more extensive upcoming surveys.  Amy's work was published in the 2018 January 5 edition of the Monthly Notices of the Royal Astronomical Society.