A GRADIENT IN THE SURFACE BRIGHTNESS FLUCTUATIONS OF NGC-3379

We present an investigation of the radial behavior of the surface brightness fluctuations of the standard elliptical galaxy NGC 3379, and we demonstrate that it is possible to measure radial changes accurately. Our results are based on a thorough procedure, which takes into account that the unwanted point sources, such as foreground stars, background galaxies, and globular clusters are located in a background of varying amounts of luminosity and photon fluctuations. Our reduction procedure leaves no systematic effects due to the varying detection threshold of point sources, and we arrive at a significant radial gradient in the surface brightness fluctuations Δm̄I/Δ log R = -0.29±0.08. If our broadband surface brightness profile of NGC 3379 in I is combined with a corresponding profile in B taken from the literature the radial behavior of m̄I may be expressed as Δm̄I/Δ(B - I) = 1.9±0.5. The weighted mean of the measured fluctuation magnitudes is m̄I= 28.67±0.04. Our results have implications on the distance measurements based on surface brightness fluctuations and the discussion of the degenerate effects of age and metallicity on stellar populations of elliptical galaxies. The linear relationship between m̄I and B - I establishes an empirical calibration, which is needed for the distance measurements based on surface brightness fluctuations. The slope of the calibration obtained for NGC 3379 seems to be in agreement with the slope achieved earlier by others for Local Group galaxies and the Fornax and Eridanus clusters. The fluctuation magnitude m̄I proves to be a sensitive indicator for radial changes in the underlying stellar population. However, the correlation between m̄I and B - I together with state-of-the-art synthesized models indicates that these two observables do not give independent information on the underlying stellar population. © 1995 American Astronomical Society.