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IR Spectroscopic Tracers for Galaxy Evolution Studies: the Spitzer/Herschel database

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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.

This talk is part of the Institute of Astronomy Galaxies Discussion Group series.

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