Calibration of the Fundamental Plane Zero-point in the Leo-I Group and an Estimate of the Hubble Constant

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We derive new effective radii and total magnitudes for 5 E and S0 galaxies in the Leo-I group from wide-field CCD images. These are used in conjunction with recent literature velocity data to construct the fundamental plane (FP) of the Leo-I group. The rms scatter that we find is only 6 % in distance. The zero point of this relation provides a calibration of the FP as a distance indicator and directly determines the angular diameter distance ratio between the Leo-I group and more distant clusters. In the language of Jerjen and Tammann (1993) we determine a cosmic velocity of the Leo-I group of 757+-68 km/s relative to the Coma cluster, or 796+-57 km/s relative to a frame of 9 clusters. Combining this velocity with the Cepheid distance to M96, a member of Leo-I, we find the Hubble constant to be H₀=67+-8 km/s/Mpc or H₀=70+-7 km/s/Mpc for each case. The distance we obtain for the Coma cluster itself (108+-12 Mpc) is in good agreement with a number of other recent estimates.