Galaxies Lecture 08

he Tully-Fisher relationship says that if we plot the luminosity of a (spiral) galaxy as a function of the circular velocity (${\displaystyle v_{c}R}$), which is the (constant) rotational velocity of a galaxy with effects of projection removed by assuming the galaxy is circular, we find that ${\displaystyle \log(L)}$ grows linearly with ${\displaystyle v_{c}}$, and that morphologically, increasing luminosities go like ${\displaystyle S_{c},S_{b},S_{a},S_{0}}$. This is very useful because it tells us the inherent luminosity of a galaxy based on measurements we can take. This tells us the distance to a galaxy, has can be used for cosmology. This relationship can be tightened by removing the effects of dust extinction–by measuring in infrared. Doing this, the relationship tightens to instrumental error. Therefore, something fundamental is being indicated in this relationship.\