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8.1.1.4 Galaxies ¶

Today, most practitioners agree that the flux of galaxies can be modeled with one or a few generalized de Vaucouleur’s (or Sérsic) profiles.

$$I(r) = I_e \exp \left ( -b_n \left[ \left( r \over r_e \right)^{1/n} -1 \right] \right )$$

Gérard de Vaucouleurs (1918-1995) was first to show in 1948 that this function resembles the galaxy light profiles, with the only difference that he held $n$ fixed to a value of 4. Twenty years later in 1968, J. L. Sérsic showed that $n$ can have a variety of values and does not necessarily need to be 4. This profile depends on the effective radius ($r_e$) which is defined as the radius which contains half of the profile’s 2-dimensional integral to infinity (see Profile magnitude). $I_e$ is the flux at the effective radius. The Sérsic index $n$ is used to define the concentration of the profile within $r_e$ and $b_n$ is a constant dependent on $n$. MacArthur et al.²²⁹ show that for $n>0.35$, $b_n$ can be accurately approximated using this equation:

$$b_n=2n - {1\over 3} + {4\over 405n} + {46\over 25515n^2} + {131\over 1148175n^3}-{2194697\over 30690717750n^4}$$