How are rotation curves of spiral galaxies determined for distances beyond where starlight can be detected?

Question

Quizplus · Accepted Answer

Observations of the 21 cm line of atomic hydrogen can be used to determine the rotation curve of spiral galaxies for distances beyond where starlight can be detected. This is because atomic hydrogen can be found throughout the galaxy, including in the dark matter halo, and emits radio waves at a wavelength of 21 cm when its spin is flipped. By observing the Doppler shift of these emissions as the galaxy rotates, astronomers can determine the velocity of the hydrogen gas and thus map out the rotation curve of the entire galaxy.

How Are Rotation Curves of Spiral Galaxies Determined for Distances