Heating and cooling rates depend on density via the ionization balance and
via the rates for the heating and cooling per ion.
In the previous subsection we have shown that at the highest densities we
consider the recombination rates can be enhanced by 3 body recombination, or
reduced, by continuum lowering and collisional ionization. The former
process is dominant at high densities for H I and He I, while the latter
dominates for moderate densities for other ions. Since the thermal
balance in a photoionized gas is dominated by H and He when the ionization
parameter is low (i.e. log(), and by more highly charged ions
at higher
, we expect the heating and cooling to be affected differently
at high densities in the two different regimes. Although the
dependence of cooling rate on ionization balance at low densities
is not generally monotonic (c.f. figure 6), for many ions the heating rate
is greater at lower ionization parameter. The per ion heating rate
depends on the photon flux rather than the gas density, while the
per ion cooling rate is suppressed by collisional deexcitation.
Figure 11 shows the dependence of heating and cooling rates on density and
temperature, in a form analogous to that of figure 10. Curves show
cooling (dashed) and heating (solid) rates at 5 temperatures spaced
logarithmically between between 10
K and 10
K, for log(
)=2
and a
power law ionizing spectrum. For highly ionized species
heating rates are decrease slightly with density, while cooling rates increase.
H and He I behave in the opposite way, owing to the increase in recombination
(which increases the neutral fraction and hence the photoionization heating)
and to the collisional suppression of radiative decays (which
decreases the net radiative cooling).