Cloudy cooling curves

The following plots show cooling / heating curves generated with Cloudy. Spefically, they are the result of modifying the hazy_coolingcurve program to take into account the Haardt & Madau background.

The plot below shows the cooling and heating functions for solar metallicty gas with three average densities: \(n_h = 0.1\), \(n_h = 1\), and \(n_h = 10\). The default cloudy background is used, which includes the CMB and radio to x-ray background (see Hazy). A galactic cosmic ray background is also included, as it dominates the chemistry of the molecular gas.

The following plot modifies the background described above. Instead of the default background, it uses the Haardt & Madau background (HM05). The CMB and cosmic rays are still included.

So, the question is - do we need to include cooling for gas below \(10^4\) K? At low densities it won’t be able to cool, so it is probably okay to neglect it (should I include a heating term at these densities?). At densities greater than \(n_h \approx 0.1\) the gas is clearly still able to cool, but the efficiency has dropped by 2-3 orders of magnitude, so perhaps the cooling times are long enough that we can still safely ignore cooling in this regime?