Half-light Radius
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Galaxy effective radius or half-light radius (R_e) is the
radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...
at which half of the total
light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
of a
galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...
is emitted. This assumes the galaxy has either intrinsic
spherical symmetry In geometry, circular symmetry is a type of continuous symmetry for a planar object that can be rotated by any arbitrary angle and map onto itself. Rotational circular symmetry is isomorphic with the circle group in the complex plane, or the ...
or is at least circularly symmetric as viewed in the plane of the sky. Alternatively, a half-light
contour Contour may refer to: * Contour (linguistics), a phonetic sound * Pitch contour * Contour (camera system), a 3D digital camera system * Contour Airlines * Contour flying, a form of low level flight * Contour, the KDE Plasma 4 interface for tab ...
, or
isophote In geometry, an isophote is a curve on an illuminated surface that connects points of equal brightness. One supposes that the illumination is done by parallel light and the brightness is measured by the following scalar product: :b(P)= \vec n(P) ...
, may be used for spherically and circularly asymmetric objects. R_e is an important length scale in \sqrt R term in
de Vaucouleurs's law In astronomy, de Vaucouleurs's law, also known as the de Vaucouleurs profile or de Vaucouleurs model, describes how the surface brightness I of an elliptical galaxy varies as a function of apparent distance R from the center of the galaxy: \ln I( ...
, which characterizes a specific rate at which
surface brightness In astronomy, surface brightness (SB) quantifies the apparent brightness or flux density per unit angular area of a spatially extended object such as a galaxy or nebula, or of the night sky background. An object's surface brightness depends on ...
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 Radii Equations of astronomy {{astronomy-stub