Surface Brightness Fluctuations

Transcript: A clever technique for measuring the distance of galaxies that applies to galaxies with smooth light distributions like ellipticals is called surface brightness fluctuations. The idea of surface brightness is that it is the flux per unit area in a galaxy. The important thing to realize is that the observed flux of a galaxy goes down as the distance squared, inverse square law, but in any fixed area of a detector like a CCD detector the number of stars enclosed by a pixel of the detector increases as the distance squared. The product of these two, flux and number of stars enclosed, is a constant. That means the surface brightness of galaxies is constant from one distance to another. But as the galaxy becomes more distant the variation in the flux from one pixel to the next goes down because the number of stars enclosed in a pixel goes up, and the RMS variation or the square root of the number is a smaller fraction. Thus the distance indicator involves the fact that the fluctuations in surface brightness from one pixel to another of a CCD detector are proportional to the distance, and this can be used to measure the relative distance of elliptical galaxies.