Observation of the Halo of NGC 3077 Near the “Garland” Region Using the Hubble Space Telescope

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 Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The red giant branch luminosity function in 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 (1993) and the foreground extinction of AB = 0.21 mag, we obtain a distance modulus of (m −M)0 = 27.93 ± 0.14r ± 0.16s. 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 <150 Myrs old. Assuming that the nearest encounter between NGC 3077 and M81 occurred 280 Myrs ago as predicted by the numerical simulations (Yun 1997), the observed upper main sequence stars are likely the results of the star formation triggered by the M81-NGC 3077 tidal interaction. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, operated by AURA, Inc. under NASA contract No. NAS5-26555. Kitt Peak National Observatory, National Optical Astronomy Observatories, Tucson AZ 85726 Division of Astronomy and Astrophysics, University of California, Los Angeles, Los Angeles, CA 90095 NASA/IPAC Extragalactic Database, California Institute of Technology, Pasadena CA 91125 Carnegie Observatories, Pasadena CA 91101