a) (Left panel) The HCN(4–3)/CO(3–2) velocity-integrated intensity ratio (R_{HCN / CO}, in logarithmic scale) versus the ratio of X-ray flux in the 6–8 keV band (in counts s^-1) to the CO(3–2) intensities (in K km s^-1) (X_hard/CO, in logarithmic scale) measured at a common spatial resolution of 0.5″ over the CND (open circles). Isodensity contours illustrate the distribution of values in this parameter space for 10%, 25%, and 50% of the data points. The straight line represents the bisector linear fit to the data in logarithmic scale [log(X_hard/CO), log(R_{HCN / CO})]. The yellow-shaded area shows the range allowed by the ordinary least squares fits of y as a function of x and viceversa. The parameters for the bisector fit are indicated. b) (Right panel) Same as a) but particularized for the HCO⁺(4–3)/CO(3–2) ratio (R_{HCO⁺/ CO}) versus X_hard/CO. The linear correlation coefficients of the regressions are r ~ 0.7