Why Does the Tully-Fisher Relation Change with Different Wavelengths?

[notknowing]

Is there an accepted explanation as to why the exact form of the Tully-Fisher relation depends on the wavelength by which the galaxy rotation is measured? I would think that rotation = rotation independent on how you observed it.


Rudi Van Nieuwenhove

 

[Leopold]

Hi notknowing,

Blue stars contribute much of a galaxy's light; a galaxy that's recently gone through a burst of star formation will appear disproportionately bright in blue wavelengths while its rotation speed will remain unchanged. Consequently, data points (i.e. galaxies) in a plot of observed luminosity versus peak rotation speed would show lots of scatter if observation were done toward the blue end of the spectrum. AFAIK, the best choice is to measure galaxy brightness in the red or near-infrared.

(sorry about the lateness in this reply)