INNER SPIRAL STRUCTURE OF THE GALAXY M51

MODELLING the structure and kinematics of spiral galaxies requires acc urate maps of the mass-tracing stellar population. But this has hither to been difficult because of dust obscuration and the presence of lumi nous young stars. To minimize the effects of dust and maximize sensiti vity to the dominant stellar population, we have obtained K-band (2.2- mum) images of the nearby 'grand-design' spiral galaxy NGC5194 (M51). Our observations reveal remarkable dynamical structures not visible in conventional optical images. We find that the spiral arms extend sign ificantly further towards the galaxy's centre than previously observed and can be traced continuously through almost three revolutions - rou ghly twice as far as with optical images. The coherence of the arms ov er this large radial range challenges current theories of spiral struc ture. We suggest that a combination of several mechanisms, such as the interaction of M51 with the neighbouring galaxy NGC5195, forcing by t he central 'bar', or distortions from density waves, is required to ge nerate the observed structure.