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, andWLM.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 [C II] 158μm, [O I] 63μm, [O III] 88μm, and [N II] 122μm 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 [C II] 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. [N II] is weak, suggesting that the [C II] emission in our galaxies comes mostly from PDRs instead of the di↵use ionized ISM. These systems exhibit e cient cooling at low dust temperatures, as shown by ([O I]+[C II])/TIR in relation to 60μm/100μm, and low [O I]/[C II] ratios which indicate that [C II] is the dominant coolant of the ISM. We observe [O III]/[C II] ratios in our galaxies that are lower than those published for other dwarfs, but similar to levels noted in spirals. Subject headings: galaxies: dwarf — galaxies: ISM — galaxies: individual (DDO 69, DDO 70, DDO 75, DDO 155, WLM)