Analysis of the diffuse ionized gas database: DIGEDA

TL;DR

This paper compiles a comprehensive DIG database from literature, analyzes its properties using emission line ratios, and proposes an objective criterion to distinguish DIGs from HII regions, revealing differences across galaxy types.

Contribution

It introduces the first DIG database (DIGEDA) and identifies [NII]/Ha as an objective criterion for distinguishing DIGs from HII regions.

Findings

[NII]/Ha ratio effectively differentiates DIGs from HII regions.

EM(Ha) is useful only when analyzing individual galaxies.

Irr galaxy DIGs resemble HII regions more than spiral DIGs.

Abstract

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.