The surface brightness fluctuations (SBF) method measures the intrinsic pixel-to-pixel intensity variance in a galaxy image resulting from statistical fluctuations in the numbers and luminosities of the stars within individual pixels. Since the SBF signal is convolved with the point spread function, one measures the Fourier space amplitude of the power spectrum on the scale of the PSF in the galaxy-subtracted image. The ratio of SBF variance to galaxy surface brightness has units of flux and scales inversely with the square of the galaxy distance. This ratio is usually converted to a magnitude called . The distance can be determined if the absolute , which depends on both the photometric bandpass and the stellar population, is known from empirical or theoretical calibration. SBF measurements in multiple bands can provide useful distance-independent information on the stellar content of a galaxy.
The SBF method was first quantified by Tonry & Schneider (1988). The Cefalú stellar populations workshop where this contribution was presented marked an interesting anniversary, being twenty years to the month since the publication of that seminal work. The first major application of the SBF method was by Tonry et al. (1990) for a sample of Virgo galaxies in the VRI bandpasses. They also made a first attempt to predict the behavior of as a function of galaxy color. Soon afterward, Tonry (1991) presented the first fully empirical SBF calibration, giving I as a function of (V - I). Following these early efforts, a large ground-based SBF survey (Tonry et al. 1997, 2001) presented a redetermination of the empirical I-band SBF calibration and measured distances for 300 early-type galaxies and spiral bulges within about 40 Mpc. For a comprehensive review of the first decade of SBF studies, see Blakeslee, Ajhar, & Tonry (1999).
Although the major part of SBF research has been concerned with the measurement of extragalactic distances, peculiar velocities, and three-dimensional structure in the local universe, recently there has been renewed interest in SBF as a stellar population indicator. This is because SBF is sensitive to the properties of the brightest stars in a galaxy in a given bandpass, and the detailed evolution of these brightest stars is usually not well constrained, especially for old, metal-rich stellar populations. There are few if any Galactic or Magellanic star clusters where such models can be tested directly against resolved stellar systems.
There have been several recent theoretical efforts to predict SBF magnitudes for various bandpasses and stellar populations (Liu et al. 2000; Blakeslee et al. 2001; Mei et al. 2001; Cantiello et al. 2003; Mouhcine et al. 2005; Raimondo et al. 2005; Marin-Franch & Aparicio 2006; Lee et al. 2009). Cerviño et al. (2008) have recently made a rigorous study of the theoretical underpinnings of the SBF method. Optical and near-IR SBF measurements for Magellanic Cloud star clusters of varying ages also provide important tests for stellar population models (González et al. 2004; González-Lópezlira et al. 2005; Raimondo et al. 2005). Although there is broad agreement in the predictions for the most common SBF bandpasses (especially I band), the agreement among different models, and between models and observations, worsens in the near-IR and UV/blue. We cannot be comprehensive in the limited space of this review, and we refer the interested reader to the original works for details. See also the contributions by M. Cantiello, R. Gonzalez-Lopezlira, and G. Raimondo in this volume. Here we simply highlight a few results from recent SBF work related to stellar population issues.