The Tully-Fisher relationship relies on broadening of the 21-cm radio line in distant spiral galaxies. What causes this broadening?
A) thermal motion of the gases in the galaxy
B) random motions of the stars in the galaxy
C) rotation of the galaxy
D) general expansion of the universe (Hubble flow)

Question

Quizplus · Accepted Answer

The broadening of the 21-cm radio line in distant spiral galaxies, as described by the Tully-Fisher relationship, is primarily caused by the rotation of the galaxy. This rotation leads to a Doppler shift in the frequency of the 21-cm line emitted by neutral hydrogen atoms, causing the line to broaden as different parts of the galaxy rotate at different speeds.

The Tully-Fisher Relationship Relies on Broadening of the 21-Cm Radio