How do we measure the masses of far away galaxies?
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Measuring the mass of galaxies :-
1) Find a spiral galaxy that we view neither face-on nor edge-on, but somewhere in between.
2) Collect the light from the stars in a band across the galaxy at each distance R from the center of the galaxy.
3) For each R, measure the doppler shift of the starlight.
1) Find a spiral galaxy that we view neither face-on nor edge-on, but somewhere in between.
2) Collect the light from the stars in a band across the galaxy at each distance R from the center of the galaxy.
3) For each R, measure the doppler shift of the starlight.
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✔️✔️Traditionally people have used the virial theorem, most famously first used by Fritz Zwicky in 1933 to measure the mass of the Coma cluster of galaxies.
In this approach, under some assumptions, one can relate the mass of the cluster to velocity dispersion of galaxies along the line of sight, and you can measure the latter as described in the earlier post to this question.
(This brilliant logic, using existing measurements at the time, led Zwicky to announce that Coma must be dominated by invisible, dark, matter).
There are also other methods to get masses, but they are messy.
Typically, one infers the mass-to-light ratio of the object (M/L), and then measures L to get M.
These approaches have significant uncertainties.
Therefore, as mentioned in the post above, gravitational lensing is in principle the cleanest method.
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