2001ApJ...555..280S

We report the detection of upper main-sequence stars and red giant branch stars in the halo of an amorphous galaxy, NGC 3077. The observations were made using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The red giant branch luminosity function in the I band shows a sudden discontinuity at I=24.0±0.1 mag. Identifying this with the tip of the red giant branch (TRGB) and adopting the calibration provided by Lee, Freedman, & Madore and a foreground extinction of A_B=0.21 mag, we obtain a distance modulus of (m-M)₀=27.93±0.14_rand±0.16_sys. This value agrees well with the distance estimates of four other galaxies in the M81 Group. In addition to the RGB stars, we observe a concentration of upper main-sequence stars in the halo of NGC 3077, which coincides partially with a feature known as the ``Garland.'' Using Padua isochrones, these stars are estimated to be less than 150 Myr old. Assuming that the nearest encounter between NGC 3077 and M81 occurred 280 Myr ago, as has been predicted by the numerical simulations of Yun, the observed upper main-sequence stars are likely the result of star formation triggered by the M81+NGC 3077 tidal interaction.