Detection of the Tip of the Red Giant Branch in NGC 3379 (M105) in the Leo I Group Using theHubble Space Telescope

Abstract

We report the detection of individually resolved stars in the elliptical galaxy NGC 3379, a luminous member of the Leo I Group. The bright end 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 ± 0.09 mag is identified with the tip of the first-ascent red giant branch (TRGB). Adopting MI(TRGB) = -4.0 ± 0.1 mag gives a distance modulus of 30.30 mag ± 0.14 (random errors) ± 0.23 (systematic errors) corresponding 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 points of four other distance indicators: surface brightness fluctuations, planetary nebula luminosity functions, globular cluster luminosity functions and the Dn - σ method. We apply two approaches to measuring the Hubble constant: (1) using a simple Virgocentric infall model and (2) stepping out from Leo I to the Coma cluster using the previously measured relative distance between the two clusters. These give values of the Hubble constant in the range H0 = 60-68 km s-1 Mpc-1, each having a 20% uncertainty. The largest systematic errors could potentially increase this value to H0 = 74 ± 14 km s-1 Mpc-1.