de Vaucouleurs Law

• Many elliptical galaxies have a projected luminosity distribution that follows a de Vaucouleurs (or R^1/4) law:

  

  or, on a magnitude scale,

  

• So, a plot of surface brightness in magnitudes versus radius to the quarter power for such a galaxy will appear as a straight line.

• Note that this law is a purely empirical fit with no physical basis.
  • However, any theory of elliptical galaxy formation must reproduce it.