Yes it would, If the spiral arms were say in the process of expanding that would have an effect on the reading of the doppler shift. The problem making measurements with the spiral galaxies is they aren't symetrical and uniform so what is happening on one side isn't necessaraily happening on the other to balance it out. So when you compare the rotating away side with the rotating towards you side without knowing what else is going on the results will be off.
However, remember that we have access to millions of spirals in the universe through optical observations. Statistically, if a spiral galaxy is behaving in some odd way, we would should see it, unless the timescale of the odd behaviour is extremely short.
Not exactly because in order to get correct measurements the galaxy has to be perfectly oriented toward us so there is not that many.
Statistically, if a spiral galaxy is behaving in some odd way, we would should see it, unless the timescale of the odd behaviour is extremely short.
Actually, It would be the opposite being that the timescales are extremely long. If a spiral arm was expanding, contracting, or there are stellar eddies or whatever else could be happening that we don't/wouldn't know about due to the extremely long timespans we wouldn't be able to percieve it for millions of years.
Spiral Galaxies are very complex and there is nothing anywhere near comparable to them for a reference. So until we can get a really good computer model we don't know. Meanwhile Elipitical Galaxies are more uniform and they behave in an understandable "Kepler" fashion so there is no need for scientist to invent this mysterious Dark Matter with them. Spiral galaxies evolve into elipitical galaxies so where does all this matter go? Are we to believe that 90% of the mass that some scientist claim dark matter makes up in spiral galaxies just goes *poof* and vanishes into the void of space?
Also it should be noted that in elipitical galaxies they use planatary nebula instead of Hydrogen clouds to measure rotation. Planatary nebula might be a better choice because also do we really know how exactly a Hydrogen cloud behaves over time?
Using Occam's Razor which is more plausible.
A) There is this mysterious, invisable, undetectable Dark Non-Periodic table matter with multiple properties in spiral galaxies that magically vanishes when the galaxy evolves into an eliptical one.
or
B) Our measurements of spiral galaxies are wrong