TRACING ABUNDANCES IN GALAXIES WITH THE SPITZER ...

We extract spectra processed through version S15.3 of the Spitzer Science Cen-
ter’s pipeline with CUBISM 3 . Figure 4.2 displays IRS spectra of the M51 H II
regions. The spectrum of the H II region in the nucleus (Region A) shows lines
from Ne II, Ne III, Ne V, S III, S IV, and molecular hydrogen as well as features
from Polycyclic Aromatic Hydrocarbon (PAH) at 11.2 and 12.7 µm. The extranu-
clear H II regions have weaker lines and do not show the high ionization Ne V or
S IV lines.
Table 4.1 gives the area extracted for each region in steradians. Each extracted
area was a rectangle approximately 15 ′′ × 26 ′′ , or ∼600 × 1000 pc at the adopted
distance of 8.4 Mpc to M51. Once we have the spectra we measure the fluxes in
the spectral lines with SMART (Higdon et al., 2004) by fitting a gaussian to each
line. Table 4.2 lists the observed line fluxes; most have uncertainties