Gas density maps, showing the identification and de-blending of ISM and CGM gas components in the PONOS simulation snapshot. The top panel shows the ISM components, which include the main galaxy disc (including all gas particles within twice the half light radius) and the discs of the merging satellites. The gas density of the “non-ISM” particles is shown by the purple contours. The bottom panel shows the CGM component, made up by all gas within the R_vir after masking out the galaxy discs.