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)

 

[tacman]

The Tully-Fisher relation is an empirical relationship between the luminosity (or brightness) and the rotation velocity of spiral galaxies. It was first discovered by astronomers Vera Rubin and Kent Ford in 1977 and has since been used as a tool to study the properties and evolution of galaxies.

The exact form of the Tully-Fisher relation does indeed depend on the wavelength by which the galaxy rotation is measured. This is because the rotation velocity of a galaxy is not a single, fixed value, but rather depends on the distribution and amount of matter within the galaxy. And this distribution and amount of matter can vary with different wavelengths of observation.

For example, when measuring the rotation velocity of a galaxy using visible light, we are primarily seeing the stars in the galaxy. However, when using radio waves, we are able to detect the distribution of gas and dust within the galaxy, which can contribute significantly to the overall rotation velocity. Therefore, the Tully-Fisher relation will appear different when measured at different wavelengths, as the different components of the galaxy (stars, gas, dust) contribute differently to the rotation velocity.

Additionally, the Tully-Fisher relation also takes into account the intrinsic properties of the galaxy, such as its mass and size. So, while the rotation velocity may be the same, the luminosity of the galaxy can vary depending on its other characteristics. This is why the Tully-Fisher relation is often expressed as a logarithmic relationship, rather than a simple linear one.

In short, the Tully-Fisher relation is not solely dependent on the rotation velocity, but also takes into account other factors that can vary with different wavelengths of observation. Therefore, the exact form of the relation will depend on how the rotation velocity is measured and what other properties of the galaxy are being considered.