2017年IOAA理论第3题-银河系遥远的外围新悬臂

英文原题

The Milky Way’s Distant Outer Arm [10 marks]

In 2011, Dame and Thaddeus found a new part of the outer arm of the Milky Way by studying the CO line using the CfA 1.2m telescope. They found that the CO line was detected at galactic longitude l=13.25^。 (marked A in the figure) where it had a radial velocity of 20.9 $$km s^-1$$ towards the Sun. Assume that the galactic rotation curve is flat beyond 5 kpc from the Galactic centre. The distance between the Sun and the Galactic centre is 8.5 kpc. The velocity of the Sun around the Galactic centre is 220 $$km s^-1$$.

a) Find the distance from the start of the arm (point A) to the Galactic centre. [7]

b) Find the distance from the start of the arm (point A) to the Sun. [3]