Analysis of the diffuse ionized gas database: DIGEDA

Abstract

Studies of the Diuse Ionized Gas (DIG) have progressed without providing so far any strict criterion to distinguish DIGs from H II regions. In this work, we compile the emission line measurements of 29 galaxies that are available in the scientic literature, thereby setting up the rst DIG database (DIGEDA). Making use of this database, we proceed to analyze the global properties of the DIG using the [N II] λ6583/Hα, [O I] λ6300/Hα, [O III] λ5007/Hβ and [S II] λ6716/Hα lines ratios, including the Hα emission measure. This analysis leads us to conclude that the [N II]/Hα ratio provides an objective criterion for distinguishing whether an emission region is a DIG or an H II region, while the EM(H) is a useful quantity only when the galaxies are considered individually. Finally, we find that the emission regions of Irr galaxies classied as DIG in the literature appear in fact to be much more similar to H II regions than to the DIGs of spiral galaxies.