GASP. XXXII. Measuring the Diffuse Ionized Gas Fraction in Ram-pressure-stripped Galaxies

The diffuse ionized gas (DIG) is an important component of the interstellar medium, and it can be affected by many physical processes in galaxies. Measuring its distribution and contribution in emission allows us to properly study both its ionization and star formation in galaxies. Here, we measure for the first time the DIG emission in 38 gas-stripped galaxies in local clusters drawn from the GAs Stripping Phenomena in galaxies with the MUSE survey (GASP). These galaxies are at different stages of stripping. We also compare the DIG properties to those of 33 normal galaxies from the same survey. To estimate the DIG fraction (C_DIG) and derive its maps, we combine the attenuation-corrected Hα surface brightness with the [S II]/Hα line ratio. Our results indicate that we cannot use either a single Hα or [S II]/Hα value, or a threshold in equivalent width of Hα emission line to separate spaxels dominated by DIG and non-DIG emission. Assuming a constant surface brightness of the DIG across galaxies underestimates C_DIG. Contrasting stripped and nonstripped galaxies, we find no clear differences in C_DIG. The DIG emission contributes between 20% and 90% of the total integrated flux and does not correlate with the galactic stellar mass and star formation rate (SFR). The C_DIG anticorrelates with the specific SFR, which may indicate an older (>10⁸ yr) stellar population as the ionizing source of the DIG. The DIG fraction shows anticorrelations with the SFR surface density, which could be used for a robust estimation of integrated C_DIG in galaxies.