Abstract

The rest-frame mid- to far-IR range contains a unique suite of spectral lines and dust features that allow us to determine the physical conditions in galaxies (e.g. density, ionisation, heavy element abundances) using tracers with a feeble response to both extinction and temperature.

In this study we present a spectroscopic database of mid- to far-IR fine-structure lines observed with Spitzer/IRS and Herschel/PACS for a sample of 170 local AGN , 20 starburst galaxies, and 43 dwarf galaxies. The observations are compared to a set of Cloudy photoionisation models to estimate the physical quantities through the different diagnostic diagrams. The new [OIV]25.9um/[OIII]88um vs [NeIII]15.6um/[NeII]12.8um diagram is proposed as new IR BPT diagnostic diagram, able to separate: 1) AGN activity from any kind of star formation; and 2) low-metallicity dwarf galaxies from starburst galaxies. Current stellar atmosphere models fail to reproduce the observed [OIV]25.9um/[OIII]88um ratios, which are much higher when compared to the predicted values. Finally, the ([NeIII]15.6um 12.8um)/([SIV]10.5um [SIII]18.7um) ratio is proposed as a promising metallicity tracer to be used in obscured objects, where optical lines fail to accurately measure the metallicity.