Half-light Radius

Galaxy effective radius or half-light radius ($R_e$) is the radius at which half of the total light of a galaxy is emitted. This assumes the galaxy has either intrinsic spherical symmetry or is at least circularly symmetric as viewed in the plane of the sky. Alternatively, a half-light contour, or isophote, may be used for spherically and circularly asymmetric objects.

$R_e$ is an important length scale in \sqrt R term in de Vaucouleurs law, which characterizes a specific rate at which surface brightness decreases as a function of radius:

:$I(R) = I_e \cdot e^$

where $I_e$ is the surface brightness at $R = R_e$. At $R = 0$,

:$I(R=0) = I_e \cdot e^ \approx 2000 \cdot I_e$

Thus, the central surface brightness is approximately $2000 \cdot I_e$.

See also

References

Physical quantities

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