TRACING ABUNDANCES IN GALAXIES WITH THE SPITZER ...

lines. For example, changing the adopted Te from 10000 K to 13000 K for the PN
IC 2448 decreases the abundance of Ne ++ derived from the IR [Ne III] line at 15.55
µm by 6% but decreases the abundance of Ne ++ derived from the optical [Ne III]
line at 3869 ˚A by 80%. (3) Some ions have lines in the infrared, but not in the
optical; for example, infrared spectra show the [Ne II] line which is not observable
in the optical, but this ion dominates the total elemental neon abundance in low
ionization nebulae.
1.3.4 Ionizing Radiation Field
Massive stars and active galactic nuclei produce a hard radiation field (a radiation
field which contains a large fraction of highly energetic photons) of high intensity.
The hardness of a radiation field gives information about the type of source creating
the field. Ratios of IR lines arising from ions which have ionization energies in the
ultraviolet indicate the radiation field hardness. For example, it takes 22 eV to
create Ne + from neutral neon and 41 eV to create Ne ++ from Ne + , so if the line flux
ratio F(Ne III at 15.6µm)/F(Ne II at 12.8 µm) is high, then the ionizing radiation
field is hard. Similarly it takes 23 eV to create S ++ from S + and 35 eV to create
S +3 from S ++ , so the line flux ratio F(S IV at 10.5 µm)/F(S III at 18.7 µm) also
probes the ionizing radiation field. Figure 1.4 shows the ionization energies for
these ions overplotted on synthetic spectra for a burst of star formation generated
with the code Starburst99 2 .
Several factors affect the hardness of the radiation field. Figure 1.4 shows four
synthetic spectra: one for each of two abundances (1 and 1/3 solar) and one for
each of two times (1 and 10 Myr) after the burst of star formation began. The
radiation field is harder, having higher energy photons (to the left in the figure)
2 Starburst99 (Leitherer et al., 1999) is available from the following website:
http://www.stsci.edu/science/starburst99/ .
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