SURFACE BRIGHTNESS FLUCTUATIONS IN NGC-5128 FROM WIDE-FIELD PHOTOGRAPHIC PLATES

Deep R- and B-band photographic plates of NGC 5128, obtained under photometric conditions at the f/3.3 prime focus of the AAT 3.9 m telescope, clearly show surface brightness fluctuations from the luminous, stellar population. The variance of these fluctuations has been used by Tonry and collaborators to determine distances to nearby galaxies. However, certain discrepancies have arisen with independent primary calibrators over comparable distance scales. We exploit the distinct statistical proper-ties of photographic plates when compared with CCDs to provide an independent analysis of the luminosity fluctuations. We demonstrate that the threshold and saturation levels of a detector skew the statistics at both extremes. Our model successfully accounts for the first three statistical moments of the photographic grain response measured from the density wedges on the photographic plates. We show that the statistical phases of the individual variance sources are uncorrelated if the analysis is restricted to a narrow photographic exposure range. After removing the grain noise component, the derived form of the power spectrum for the seeing-convolved luminosity fluctuations is in good agreement with those recently published by Tonry & Schechter [AJ, 100, 1794 (1990)] in the V and I bands. In the R band, we determine a value of 3.8 +/- 1.1 Mpc for the distance to NGC 5128. Although less accurate, our approach constitutes a statistically independent test and therefore provides strong support for Tonry's luminosity fluctuation distance method.