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Session 12 - Distance Scale.
Display session, Monday, January 13
We report HST--WFPC2 observations of the giant E2 galaxy IC 4051 located 14^\prime east of the center of the Coma Cluster. Eight full--orbit exposures were obtained with the F606W filter, and two with the F814W filter. Counts of globular clusters within the PC1 (f/28) frame, where faint GCs can be distinguished from noise spikes and background objects by PSF fitting, were made down to F606W = 28.2 mag (V = 28.5 mag), which is beyond the turnover of the globular cluster luminosity function (GCLF). The mean rest--frame color index of the IC 4051 GCs is (V-I)_0 = 1.06 mag, which implies [Fe/H] = -0.74. The intrinsic dispersion of color indices is \sigma_V-I = 0.16 mag, and the specific frequency of the GC system in IC 4051 is about 10. Using a maximum likelihood method, we have compared the observed GCLF of IC 4051 with model GCLFs transported to various strawman Coma Cluster distances, with photometric errors and incompleteness taken into account. For a model based on the GC systems of the Milky Way and M31, the inferred bolometric modulus of IC 4051 is 35.15 \pm 0.06 mag (random error only), and the corresponding luminosity distance (if q_0 = 0.5) is 107 \pm 3 Mpc. For a model based on the data of Whitmore et al. for the GC system of the giant elliptical M87 in the Virgo Cluster, the inferred bolometric modulus of IC 4051 is 35.07 \pm 0.17 mag (where the error includes an estimated uncertainty in the modulus of M87), and the corresponding luminosity distance is 103 \pm 8 Mpc. For Han amp; Mould's cosmological redshift of 7186 \pm 428 km/s, the weighted mean distance of IC 4051 implies a Hubble Constant H_0 = 68 \pm 5 km/s per Mpc.
Program listing for Monday