Section 4.5.3 Active Galaxy: NGC 3031/Messier 81:
NGC 3031 or Messier 81 is an Sb(r)I-II galaxy at 1.4 Mpc. It has been an important galaxy in the study of star formation and density wave theory. It has an inclination of 58°.
Optical and HI observations (Yun, Ho and Lo 1994) of NGC 3031/M 81 and NGC 3034/M 82 (the dominant members of the M 81 Group) are shown in Figure 4.120.
NGC 3031/M 81 is given a secondary classification of I.
Kendall et al. (2008) derive the density wave structure via residual mass maps using Spitzer, optical and 2MASS data. The offset between the density wave and gas shocks (seen via 8 µm dust emission) implies a long-lived spiral structure. Gordon et al. (2004) investigate star formation and dust content using Spitzer, UV, H and 20 cm continuum images. HI observations (Yun, Ho and Lo 1994) of NGC 3031/M 81 and NGC 3034/M 82 (the dominant members of the M 81 Group) (Figure 4.120) show dramatic evidence of tidal interactions (filaments) between group members that is not seen in the optical.