How Are Rotation Curves of Spiral Galaxies Determined for Distances

question 27

Multiple Choice

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

• A)by measuring the broadening of the galaxy's absorption lines
• B)by observations of the 21 cm line of atomic hydrogen
• C)by observations of spectral lines emitted by dark matter
• D)by watching the galaxies rotate over a period of decades
• E)by extrapolation of the measured rotation curve

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