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Michael C. Liu (UC Berkeley)
We present results from a program to use optical and near-infrared surface brightness fluctuations (SBFs) to measure distances and unresolved stellar populations of early-type galaxies.
(1) We develop new theoretical predictions for SBFs using models optimized for this purpose (Liu, Charlot & Graham 2000, ApJ, in press). With these models, we analyze the potential of SBFs for probing unresolved stellar populations. We find that optical/near-IR SBFs are much more sensitive to metallicity than to age. Therefore, SBF magnitudes and colors are a valuable tool to break the age/metallicity degeneracy.
(2) We derive a new calibration of K-band (2.2~\micron) SBFs based on high-quality observations of Fornax Cluster galaxies. This new sample is larger and covers a wider range in galaxy properties than past work. Combined with published data for galaxies in other nearby clusters, we calibrate K-band SBFs using HST Cepheid cluster distances and I-band SBF distances to individual galaxies. We establish that K-band SBFs have a significant dependence on the integrated galaxy color. The spread in the optical/IR SBFs, and also the correlations with integrated galaxy color, imply the stellar populations dominating the SBF signal have a significant age range. The results also suggest the lower mass cluster galaxies may have had more extended and more heterogenous star formation histories than those of the more massive galaxies.
(3) We have used the Keck~I Telescope to measure K-band SBFs of Coma Cluster ellipticals. Because the SBFs of early-type galaxies are dominated by giant stars, near-IR observations can reach to much larger distances than optical studies. We use improved SBF analysis techniques to derive accurate measurements, including the use of deep HST WFPC2 optical imaging to account for the contamination due to faint globular clusters. Using our new calibration of K-band SBFs, we measure the distance to the Coma cluster and use this to derive the Hubble Constant.
This work was supported by funding from NOAO, NSF, and STScI.