Detection of the Red Giant Branch Stars in M82 Using the Hubble Space Telescope

We present color-magnitude diagrams and luminosity functions of stars in two halo regions of the irregular galaxy in M82, based on F555W and F814W photometry taken with the Hubble Space Telescope and Wide Field Planetary Camera 2. The I-band luminosity function shows a sudden jump at I~23.95 mag, which is identified as the tip of the red giant branch (TRGB). Adopting the Lee et al. (1993) calibration of the TRGB based on the RR Lyrae distances to Galactic globular clusters, we obtain the distance modulus of (m-M)_0=27.95 +- 0.14 (random) +- 0.16 (systematic) mag. This corresponds to a linear distance of 3.9 +- 0.3 (random) +- 0.3 (systematicf) Mpc, which agrees well with the distance of M81 deteremined from the HST observations of the Cepheid variable stars. In addition, we observe a significant number of stars apparently brighter than the TRGB. However, with the current data, we cannot rule out whether these stars are blends of fainter stars, or are indeed intermediate-age asymptotic giant branch stars.