Comparison between the observed (black symbols) and modelled (coloured symbols and lines) H₂O fluxes in Mrk 273 within panel a: PACS and panel b: SPIRE. The model includes two components: the core (in blue), which accounts for most absorption lines observed within PACS, and the inner disc (in green), which dominates the emission of the sub-mm lines with E_upper < 400 K observed with SPIRE (see Table 4). Red colours and symbols indicate total modelled fluxes. The numbers at the bottom of panels a and b indicate rounded-up transition wavelengths in μm. Panel c: the SED of Mrk 273, including the Spitzer/IRS spectrum, Herschel/PACS and SPIRE continuum data from observations of both H₂O and OH lines, sub-mm data at 800 and 880 μm (Rigopoulou et al. 1996; Wilson 2008), and our measured flux density at 1 mm (starred-red symbol), is compared with the prediction of our composite model.