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

We present color-magnitude diagrams and luminosity functions of stars in twohalo regions of the irregular galaxy in M82, based on F555W and F814Wphotometry taken with the Hubble Space Telescope and Wide Field PlanetaryCamera 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 Leeet al. (1993) calibration of the TRGB based on the RR Lyrae distances toGalactic 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 distanceof 3.9 +- 0.3 (random) +- 0.3 (systematicf) Mpc, which agrees well with thedistance of M81 deteremined from the HST observations of the Cepheid variablestars. In addition, we observe a significant number of stars apparentlybrighter than the TRGB. However, with the current data, we cannot rule outwhether these stars are blends of fainter stars, or are indeed intermediate-ageasymptotic giant branch stars.Comment: 8 figure