The Spitzer View of Low-Metallicity Star Formation: III. Fine Structure
  Lines, Aromatic Features, and Molecules
  We present low- and high-resolution Spitzer/IRS spectra, supplemented by IRAC
and MIPS measurements, of 22 blue compact dwarf (BCD) galaxies. The BCD sample
spans a wide range in oxygen abundance [12+Log(O/H) between 7.4 and 8.3], and
hardness of the interstellar radiation field (ISRF). The IRS spectra provide us
with a rich set of diagnostics to probe the physics of star and dust formation
in very low-metallicity environments. We find that metal-poor BCDs have harder
ionizing radiation than metal-rich galaxies: [OIV] emission is roughly 4 times
as common as [FeII] emission. They also have a more intense ISRF, as indicated
by the 71 to 160micron luminosity ratio. Two-thirds of the sample (15 BCDs)
show PAH features, although the fraction of PAH emission normalized to the
total infrared (IR) luminosity is considerably smaller in metal-poor BCDs
(~0.5%) than in metal-rich star-forming galaxies (~10%). We find several lines
of evidence for a deficit of small PAH carriers at low metallicity, and
attribute this to destruction by a hard, intense ISRF, only indirectly linked
to metal abundance. Our IRS spectra reveal a variety of H2 rotational lines,
and more than a third of the objects in our sample (8 BCDs) have >=3sigma
detections in one or more of the four lowest-order transitions. The warm gas
masses in the BCDs range from 10^3 to 10^8 Msun, and can be comparable to the
neutral hydrogen gas mass; relative to their total IR luminosities, some BCDs
contain more H2 than SINGS galaxies.
