  The far-infrared (FIR) lines are key tracers of the physical conditions of
the interstellar medium (ISM) and are becoming workhorse diagnostics for
galaxies throughout the universe. Our goal is to explain the differences and
trends observed in the FIR line emission of dwarf galaxies compared to more
metal-rich galaxies. We present Herschel PACS spectroscopic observations of the
CII157um, OI63 and 145um, OIII88um, NII122 and 205um, and NIII57um
fine-structure cooling lines in a sample of 48 low-metallicity star-forming
galaxies of the guaranteed time key program Dwarf Galaxy Survey. We correlate
PACS line ratios and line-to-LTIR ratios with LTIR, LTIR/LB, metallicity, and
FIR color, and interpret the observed trends in terms of ISM conditions and
phase filling factors with Cloudy radiative transfer models. We find that the
FIR lines together account for up to 3 percent of LTIR and that star-forming
regions dominate the overall emission in dwarf galaxies. Compared to metal-rich
galaxies, the ratios of OIII/NII122 and NIII/NII122 are high, indicative of
hard radiation fields. In the photodissociation region (PDR), the CII/OI63
ratio is slightly higher than in metal-rich galaxies, with a small increase
with metallicity, and the OI145/OI63 ratio is generally lower than 0.1,
demonstrating that optical depth effects should be small on the scales probed.
The OIII/OI63 ratio can be used as an indicator of the ionized gas/PDR filling
factor, and is found ~4 times higher in the dwarfs than in metal-rich galaxies.
The high CII/LTIR, OI/LTIR, and OIII/LTIR ratios, which decrease with
increasing LTIR and LTIR/LB, are interpreted as a combination of moderate FUV
fields and low PDR covering factor. Harboring compact phases of low filling
factor and a large volume filling factor of diffuse gas, the ISM of
low-metallicity dwarf galaxies has a more porous structure than that in
metal-rich galaxies.
