H2 - observed velocity and evidence for dark matter

image: Sofue, Honma & Omodaka (2009)

• the observation shows:

  • $v\propto r^{-1/2}$ as expected moving away from the central bulge
  • outside the bulge, it increases and eventually stagnates at $220-250$ km s${}^{-1}$ at $R_{\odot }$
  • additional oscillations are signature of the spiral arms

• if mass is left as a variable in the centrifugal-gravitational acceleration balance:

• $M(r)\propto r$ is needed to make the RHS constant (outside the bulge)
• this implies a large amount of mass at larger radii that does not interact with EM radiation (cannot be seen)
• this component of matter is called dark matter