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Hey guys. I'm trying to answer a few questions around radial velocity and Hubble constant for my studying. I know the rules around doing people's homework, so I'm just looking to be pointed in the right direction.

I have data for a Galaxy edge on. Hydrogen balmer line from two stars, one at each end of the Galaxy. So you would expect one to be moving towards us and one away, but both wavelengths have been stretched slightly, but I'm assuming this is because the Galaxy itself is moving away from us.

I've worked out their velocities but I am then asked to work out the galaxies recession velocity. This is where I'm stuck. I only have the wavelengths of light emitted from the stars, not the Galaxy itself. So how can I do this?

I haven't read anywhere in my text book that I can take the middle value of two stretched wavelengths, in effect cancelling out the rotation of the Galaxy and giving me a value I can use to then calculate the galaxies velocity. But this is the only way I can think of doing it.

Anyone have any ideas? Thanks!

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Would it not be as simple as add the 2 "edge" velocities together and then half the result ?

I would have thought that what we see is one edge is moving at Vg+Ve, while the other is moving at Vg-Ve.

So adding the 2 velocities and halving would give the velocity of the galaxy (core)

Vg = Velocity of Galaxy.

Ve = Velocity of edge.

Presumably both edges are rotaing at the same rate, otherwise there is breakup.

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Would it not be as simple as add the 2 "edge" velocities together and then half the result ?.

yes ronin

The average wavelength of the 2 edge stars should give you the overall wavelength shift, hence the galaxies relative velocity (from us).

All things being equal that is, ie the two stars are basically at an equal distance from the center, are on opposite sides of the galactic center (180 deg apart),their rotational speed around the galaxy is very much the same, they have very similar galactic orbital paths etc.

Edited by Cath
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Thanks for this guys. I didn't see the obvious. I've done this now and all the numbers look pretty good. Thanks!

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