MODELLING the structure and kinematics of spiral galaxies requires accurate maps of the mass-tracing stellar population. But this has hitherto been difficult because of dust obscuration and the presence of luminous young stars. To minimize the effects of dust and maximize sensitivity to the dominant stellar population, we have obtained K-band (2.2-μm) 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 significantly further towards the galaxy's centre than previously observed and can be traced continuously through almost three revolutions - roughly twice as far as with optical images. The coherence of the arms over this large radial range challenges current theories of spiral structure. We suggest that a combination of several mechanisms, such as the interaction of M51 with the neighbouring galaxy NGC5195, forcing by the central 'bar', or distortions from density waves, is required to generate the observed structure.
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