DETECTION OF THE TIP OF THE RED GIANT BRANCH IN NGC-3379 (M105) IN THE LEO-I GROUP USING THE HUBBLE-SPACE-TELESCOPE

We report the detection of individually resolved stars in the elliptic al galaxy NGC 3379, a luminous member of the Leo I Group. The bright e nd of the stellar luminosity function has a logarithmic slope that is consistent with these stars being Population II red giants. An abrupt discontinuity in the apparent luminosity function at I = 26.30 +/- 0.0 9 mag is identified with the tip of the first-ascent red giant branch (TRGB). Adopting M(I)(TRGB)= -4.0 +/- 0.1 mag gives a distance modulus of 30.30 mag +/- 0.14 (random errors) +/- 0.23 (systematic errors) co rresponding to a linear distance to NGC 3379 of 11.5 +/- 1.6 Mpc. The TRGB distance compares very well with the Cepheid distance of 11.9 +/- 0.9 Mpc (30.37 +/- 0.16 mag) to another group member M96 (= NGC 3368) . The distance to NGC 3379 can be used in turn to calibrate the zero p oints of four other distance indicators: surface brightness fluctuatio ns, planetary nebula luminosity functions, globular cluster luminosity functions and the D-n - sigma method. We apply two approaches to meas uring the Hubble constant: (1) using a simple Virgocentric infall mode l and (2) stepping out from Leo I to the Coma cluster using the previo usly measured relative distance between the two clusters. These give v alues of the Hubble constant in the range H-0 = 60-68 km s(-1) Mpc(-1) , each having a 20% uncertainty. The largest systematic errors could p otentially increase this value to H-0 = 74 +/- 14 km s(-1) Mpc(-1).