TRACING ABUNDANCES IN GALAXIES WITH THE SPITZER ...

at the younger time (1 Myr) than at the older time (10 Myr). This shows up in
infrared ionization line ratios because at early times stars produce photons hard
enough to form Ne ++ and S +3 and thus produce Ne III and S IV lines in the
infrared spectra, but as the stars evolve off the main sequence they cool and do
not produce such hard photons, and the Ne III and S IV lines become weaker
and finally disappear in the infrared spectra. Additionally, the radiation field is
harder at lower abundances: at higher photon energies (left in Figure 1.4) the
lower abundance 1/3 Z⊙ synthetic spectra lie above and to the left of the higher
abundance 1 Z⊙ spectra. Thus a hard radiation field may be caused by radiation
from a group of stars that are either young or have low abundances.
1.4 Dust: Gemstones and Carcinogens in Space
Dust dramatically alters the spectra of galaxies: typically �30% of starlight is
absorbed by dust and then reradiated in the infrared (Bernstein et al., 2002).
Additionally, dust grains efficiently radiate away heat, which helps stars form by
removing the energy from a collapsing cloud (Mathis, 1990). After star formation,
dust is necessary to create planets and life. While elemental abundances can
give information about the evolution of the Galaxy, studying the dust can give
information about the evolution of stars.
RGB and AGB stars produce the bulk of the dust observed in the interstellar
medium, although novae, Wolf-Rayet stars, and supernovae may contribute some
as well. The most abundant elements in the atmospheres of these giant stars after
hydrogen and helium are oxygen and carbon. Due to the stability of the CO
molecule, CO forms in these stars until the supply of either carbon or oxygen is
exhausted. This leads to a dichotomy in the type of dust produced: if the star has
C/O < 1 then oxygen-rich dust such as oxides, quartz (SiO2) and silicates forms; if
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