The Warm Neutral Medium of Nearby Galaxies

Recent far-infrared observatories such as Herschel and SOFIA have amply demonstrated the importance of observing fine-structure lines in the far-infrared to study the warm neutral medium of galaxies. Lines such as [CII]158um and [OIII]88um act as natural cooler of molecular clouds in galaxies emitting up to 1% of the total infrared emission. Lines ratios such as [OIII]88/[OIII]52 and ([OIII]88+[OIII]52)/[NIII]57 can be used to estimate density and metallicity of the interstellar medium. Such estimates are, contrary to optical based estimates, unaffected by dust extinction. This is particularly important when studying, for instance, the evolution of ultra-luminous galaxies. Recent SOFIA observations have shown that, contrary to results based on optical observations, far-IR spectra show that ULIRGs lie on the fundamental metallicity relation radically changing the interpretation of the accretion of IR luminous galaxies. The interaction of the interstellar medium with the central active nucleus of the galaxy as well as with the medium surrounding the galaxy can be also put in evidence by studies of the [CII] emission. Recent SOFIA studies have revealed the effect of shocks in exciting the [CII] emission in galaxies. Further progress in the study of the ISM in nearby and more distant galaxies will happen only with instruments with higher sensitivity and resolution than those of PACS/Herschel and FIFI-LS/SOFIA. We discuss the limits of the past missions and what is needed to advance our knowledge with future missions such as the proposed "probe far-infrared mission for astrophysics" (PRIMA).