[mmaimcal] do lines get brighter at high z?

[Harvey Liszt]

Some of you may recall that when I extended my heating and cooling
calculations to z > 5, some of the CO lines started getting brighter
again.  I have traced this effect to an anomaly in the excitation of
the O I fine structure levels whereby they are heating the gas instead
of cooling it (ie their excitation falls below 2.735 * (1+z) K.)  This
behaviour seems to be embedded in the cross-sections I'm using but this
doesn't mean it is "real".

regards, Harvey