Left: Hubble Space Telescope (HST) image of the M 82 galaxy taken from http://hubblesite.org (Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)). The right panel shows the rotated HCO (2−1) interferometric map by García-Burillo et al. (2002) towards the central region of the galaxy. The dynamical centre is marked with a small black square. The north-east (NE) and south-west (SW) molecular lobes are shown with two dashed squares either side of the centre. The nominal position of our observations, at an offset (+13, +7.5), is marked with a cross. The beam sizes of the 2 mm survey for the lowest and highest frequencies are marked with two solid circles. The average beam size at 1.3 mm frequencies is marked with a dotted circle.