Intensity ratio and optical depth estimates. Top: Ratio map , t¹³, and t¹⁸ (from top to bottom). The ratio map is derived from the peak emission at the spatial resolution of 30″. Only pixels with T¹⁸ > 3s have been considered, where s = 0.4 K. Middle: Distribution of the values of the ratio map. Bottom: Distribution of the optical depth of respectively. The C¹⁸O(2–1) emission is optically thin over the entire studied region.