TRACING ABUNDANCES IN GALAXIES WITH THE SPITZER ...

etween recapture and photoionization sets the degree of ionization in each part
of the nebula. The ion recaptures the electron to an excited level in the process
called recombination, and the excited atom decays to lower levels by radiating
photons. When a hydrogen ion (H + ) recombines with an electron (leading to an
excited H 0 atom), the excited H 0 atom decays to the ground state by emitting a
series of photons which generate the H I recombination lines, such as the H I(6-5)
Pfund-α line at 7.46 µm and the H I(7-6) Humphreys-α line at 12.37 µm observed
in Spitzer spectra of photoionization regions. Figure 1.2 shows a close-up of the
Humphreys-α line observed in the spectrum of PNG000.7+04.7. Other elements
such as helium also emit recombination lines.
1.2.1 Planetary Nebulae
The name “planetary” is a misnomer and was originally derived from the fact that
when PNe were studied through small telescopes over two hundred years ago they
looked like planets. The name “nebula”, Latin for “cloud” is, however, more appro-
priate. Huggins & Miller (1864) were the first to look at planetary nebulae through
a spectrograph, finding that the spectra of these nebulae differed significantly from
those of stars. The planetary nebulae showed only three bright emission lines with
little continuum emission in their spectra, and Huggins & Miller (1864) remark
“In place of an incandescent solid or liquid body transmitting light of all [wave-
lengths] through an atmosphere which intercepts by absorption a certain number of
them, such as our sun appears to be, we must probably regard these objects, or at
least their photo-surfaces, as enormous masses of luminous gas or vapour. For it
is alone from matter in the gaseous state that light consisting of certain definite
[wavelengths] only, as is the case with the light of these nebulae, is known to be
emitted.” This marked the beginning of our understanding of the physical nature
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