Luminosity Profiles | CAS CMS

Luminosity Profiles

Galaxies generally exhibit exponential or Sersic (n=4 shown) profiles.

As a result of the decrease in the density of stars, the surface brightnesses of galaxies fall with increasing radius. The way in which the brightness falls is described by the luminosity (or surface brightness) profile. The two commonly observed luminosity profiles for galaxies - the exponential and Sersic profiles - are shown in the image to the right.

Exponential galaxies have luminosity profiles described by an equation of the form:

$\mu(r) = \mu_0 \cdot e^{- \beta \cdot \frac{r}{r_s}}$

Where μ(r) is the surface brightness at radius r, μ₀ is the central brightness, and r_e is the scale length. Both the disk and bulges of spiral galaxies generally exhibit exponential luminosity profiles.

Galaxies with Sersic profiles possess luminosity profiles described by an equation of the form:

$\mu(r) = \mu_0 \cdot e^{- \beta \cdot \frac{r}{r_s}^{1/n}}$

Where n is a number between approximately 2 and 6, and all other parameters are as above. Elliptical galaxies generally exhibit Sersic luminosity profiles with a typical n of 4. This is the famous de Vaucouleurs r^1/4 profile.