Analysis of the diffuse ionized gas database: DIGEDA
  Studies of the Diffuse Ionized Gas (DIG) have progressed without providing so
far any strict criterion to distinguish DIGs from HII regions. In this work, we
compile the emission line measurements of 29 galaxies that are available in the
scientific literature, thereby setting up the first DIG database (DIGEDA).
Making use of this database, we proceed to analyze the global properties of the
DIG using the [NII]/Ha, [OI]/Ha, [OIII]/Hb and [SII]/Ha lines ratios, including
the H alpha emission measure. This analysis leads us to conclude that the
[NII]/Ha ratio provides an objective criterion for distinguishing whether an
emission region is a DIG or an HII region, while the EM(Ha) is a useful
quantity only when the galaxies are considered individually. Finally, we find
that the emission regions of Irr galaxies classified as DIG in the literature
appear in fact to be much more similar to HII regions than to the DIGs of
spiral galaxies.
