----- Submission 1 (Mon May 15 19:31:24 GMT 2006) ----- Assigned ID: 699 ----- Submission 1 (Sat Jan 28 00:55:58 GMT 2006) ----- Calibration of ACS F814W Surface Brightness Fluctuations The surface brightness fluctuations (SBF) method has emerged as the primary distance indicator for mapping local large-scale structures (Virgo, Fornax), as well as the velocity field out to nearly 15,000 km/s (z < 0.05). This is because other precision distance indicators either lack the requisite depth (Cepheids, TRGB) or are too rare for adequate sampling (supernovae), while more traditional methods (Tully-Fisher, fundamental plane) lack the necessary precision. The SBF method is now being used with great success in several major ACS Wide Field Camera programs. However, whereas the band of choice for the nearby structure studies has been F850LP, for the distant large-scale flow studies it is F814W because of its much greater throughput. As a result, the current calibration for the more distant studies is inadequate. We propose to establish the first systematic calibration of the SBF method in the important F814W ACS WFC bandpass. We will do this by measuring SBF in an optimized sample of galaxies in the nearby compact Fornax cluster. Given the large amount of effort and HST time being dedicated to F814W SBF measurements, it is imperative that we correct this outstanding calibration problem while time remains. For an extremely modest expenditure of orbits, we will remove a significant systematic error and vastly improve the overall accuracy of the ongoing ACS F814W SBF work. These data will also greatly enhance the legacy value of the HST archive for future SBF studies.
We will observe each target galaxy for one orbit with the ACS/WFC. We take three dithered 408-second exposures in F814W using a 3-point dither line pattern, followed by two dithered 340-second exposures in F475W using a 2-point dither pattern. The orientation of target 1 is constrained so as to include both NGC1374 and its close neighbor NGC1375, which will also be useful for our calibration. Several of the other targets have their orientations constrained so that bright nearby stars do not land at the edge of (or just off) the field-of-view, since we have found that bright stars in these locations can create an enormous amount of scattered light, and thus greatly diminish the data quality. 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