Question

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

A) by extrapolation
B) through observations of the 21 cm line of atomic hydrogen
C) through observations of spectral lines of dark matter
D) by watching the galaxies rotate over a period of years
E) by measuring the broadening of absorption lines

Answer 1

Rotation curves of spiral galaxies beyond radii where starlight can be detected are determined through observations of the 21 cm line of atomic hydrogen.

Step-by-step explanation:

The rotation curves of spiral galaxies beyond radii where starlight can be detected are determined through observations of the 21 cm line of atomic hydrogen (option B).

Astronomers use the 21 cm radiation emitted by rotating hydrogen gas in the galaxies. The width of the 21 cm line indicates the range of orbital velocities of the gas, with broader lines suggesting faster rotation and more massive galaxies.