Comparison between the results obtained at small scales using PACS (left) with those derived in Lin et al. (2011) (right). Left: excitation temperature (T_ex; red colors) and column density (log N_CO; blue colors) values derived using PACS data for the inner spaxels. The highest temperatures are found in correspondence of the two spaxels closed to the nucleus, in the northern and southern directions, while the peak in the column density is found in the central spaxel. Right (back): simulated spiral density waves overplotted to the velocity structure (Lin et al. 2011) in a FoV of 20″ × 20″. High density values are in red while lower values are in blue. Right (front): corresponding PACS spaxels overplotted onto the same FoV to show the respective excitation temperature distribution derived from PACS data. Higher values of the simulated density map reveal a pair of tightly wound spirals in the center which correspond to the highest N_CO and lower T_ex in PACS data. On the other hand, lower values in the surface density correspond to higher T_ex and lower N_CO according to PACS. North is at the top and East to the left.