Left: density distribution of the gas with a sufficient CO abundance to be observable in a ¹²CO(4–3) excess component. CO is considered detectable if more than half of the carbon is locked in CO. Note that the density distribution is relatively bimodal, with the densest already shocked gas highlighted in red. Right: temperature distribution of all the CO gas in grey, with in red the temperature of the high density gas highlighted on the left. We find for the dense gas that almost all gas at T = 15–25 K has disappeared, leaving only the cold (<15 K) and warm gas (>25 K). This demonstrates that post-shock gas is either warm or cooled behind the shocked gas layer compared to the temperatureof the inflowing mass reservoir.