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Session 124 - Elliptical Galaxies.
Oral session, Saturday, January 10

[124.06] Robust Relative Distances between Elliptical Galaxies

E. L. Gates, J. T. McGraw (UNM)

Measuring accurate distances between galaxies is crucial to understanding galaxy cluster dynamics and the structure of the universe. Using a modified surface brightness fluctuation (SBF) technique (based on Tonry and Schneider 1988) and additional self-diagnostic analyses, we can estimate robust relative distances between elliptical galaxies and spiral bulges to within a few percent. The basis of the SBF technique is that the pixel-to-pixel brightness mottling of a galaxy image is caused by the Poisson statistics of stars. Thus, the RMS of mottling decreases linearly with increasing distance. This is measured by the spatial power spectrum. The power is inversely proportional to the distance squared. The additional analysis consists of plotting the power spectra ratios of different regions of a galaxy versus the surface brightness ratio of those regions. Regions of differing surface brightness will be differently affected by read-out noise, photon noise, and incorrect background subtraction. Each effect introduces characteristic trends to the spectral data. This self-diagnostic procedure allows better analysis and correction of the effects of noise and sky background flux on the distance estimates.

Realistic simulations were performed demonstrating the feasibility of these analysis tools. These techniques were then applied to HST WF/PC2 archive data of various elliptical galaxies in the V- and I-band. Galaxies in the sample included M32, Virgo and Coma Cluster ellipticals, and other intermediate distance galaxies. We used published V-I colors and Mg_2 measurements of the galaxies to correct for errors in the calculated relative distances caused by differences in the stellar luminosity function. We compared our distance ratios to distances published elsewhere using the SBF method, D_n-\sigma method, and other available distance data. Our results demonstrate a fully diagnostic method for finding relative distances between galaxies, which we compare to other distance finding techniques.

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