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 \pm 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 A _ { B } = 0.21 mag , we obtain a distance modulus of ( m - M ) _ { 0 } = 27.93 \pm 0.14 _ { r } \pm 0.16 _ { s } .
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 .