Surface brightness fluctuation
Surface brightness fluctuation (SBF) is a secondary distance indicator used to estimate distances to galaxies. The technique uses the fact that galaxies are made up of a finite number of stars. The number of stars in any small patch of the galaxy will vary from point to point, creating a noise-like fluctuation in the surface brightness distribution. While the various stars present in a galaxy will cover an enormous range in luminosity, the SBF can be characterized as if all stars had the same brightness, which is the luminosity-weighted integral over the luminosity distribution of stars. Older elliptical galaxies have fairly consistent stellar populations, thus the typical ``fluctuation star" closely approximates a standard candle. In practice, corrections are required to account for variations in age or metallicity from galaxy to galaxy.
The SBF pattern is measured from the power spectrum of the residuals left behind from a deep galaxy image after a smooth model of the galaxy has been subtracted. The SBF pattern is evident as the transform of the point spread function in the Fourier domain. The amplitude of the spectrum gives the luminosity of the fluctuation star. Because the technique depends on a precise understanding of the image structure of the galaxy, extraneous sources such as globular clusters and background galaxies must be excluded from the analysis. Areas of interstellar dust absorption must also be excluded. In practice this means that SBF works best for elliptical galaxies or the bulges of S0 galaxies, as spiral galaxies generally have complex morphologies and extensive dust features.
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