Herschel Spectroscopic Observations of LITTLE THINGS Dwarf Galaxies
  We present far-infrared spectral line observations of five galaxies from the
LITTLE THINGS sample: DDO 69, DDO 70, DDO 75, DDO 155, and WLM. While most
studies of dwarfs focus on bright systems or starbursts due to observational
constraints, our data extend the observed parameter space into the regime of
low surface brightness dwarf galaxies with low metallicities and moderate star
formation rates. Our targets were observed with Herschel at the [CII] 158um,
[OI] 63um, [OIII] 88um, and NII 122um emission lines using the PACS
Spectrometer. These high-resolution maps allow us for the first time to study
the far-infrared properties of these systems on the scales of larger
star-forming complexes. The spatial resolution in our maps, in combination with
star formation tracers, allows us to identify separate PDRs in some of the
regions we observed. Our systems have widespread [CII] emission that is bright
relative to continuum, averaging near 0.5% of the total infrared budget -
higher than in solar-metallicity galaxies of other types. [NII] is weak,
suggesting that the [CII] emission in our galaxies comes mostly from PDRs
instead of the diffuse ionized ISM. These systems exhibit efficient cooling at
low dust temperatures, as shown by ([OI]+[CII])/TIR in relation to 60um/100um,
and low [OI]/[CII] ratios which indicate that [CII] is the dominant coolant of
the ISM. We observe [OIII]/[CII] ratios in our galaxies that are lower than
those published for other dwarfs, but similar to levels noted in spirals.
