Low Metallicity ISM: excess submillimetre emission and CO-free H2 gas
  The low metallicity interstellar medium of dwarf galaxies gives a different
picture in the far infrared(FIR)/submillimetre(submm)wavelengths than the more
metal-rich galaxies. Excess emission is often found in the submm beginning at
or beyond 500 mu. Even without taking this excess emission into account as a
possible dust component, higher dust-to-gas mass ratios (DGR) are often
observed compared to that expected from their metallicity for moderately
metal-poor galaxies.
  The SEDs of the lowest metallicity galaxies, however, give very low dust
masses and excessively low values of DGR, inconsistent with the amount of
metals expected to be captured into dust if we presume the usual linear
relationship holding for all metallicities, including the more metal-rich
galaxies. This transition seems to appear near metalllicities of 12 + log(O/H)
~ 8.0 - 8.2. These results rely on accurately quantifying the total molecular
gas reservoir, which is uncertain in low metallicity galaxies due to the
difficulty in detecting CO(1-0) emission. Dwarf galaxies show an exceptionally
high [CII] 158 mu/CO (1-0) ratio which may be indicative of a significant
reservoir of 'CO-free' molecular gas residing in the photodissociated envelope,
and not traced by the small CO cores.
