Mid-Infrared line diagnostics of active galaxies^*

A spectroscopic AGN survey with ISO-SWS

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
² School of Physics and Astronomy and Wise Observatory, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁴ Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy, and Telescopio Nazionale Galileo & Centro Galileo Galilei, calle Alvarez de Abreu 70, 38700 Santa Cruz de La Palma, TF, Spain

Corresponding author: E. Sturm, sturm@mpe.mpg.de

Received: 1 July 2002
Accepted: 11 July 2002

Abstract

We present medium resolution () ISO-SWS 2.4–45 μm spectra of a sample of 29 galaxies with active nuclei. This data set is rich in fine structure emission lines tracing the narrow line regions and nuclear star formation regions, and it provides a coherent spectroscopic reference for future extragalactic studies in the mid-infrared.
 We use the data set to briefly discuss the physical conditions in the narrow line regions (density, temperature, excitation, line profiles) and to test for possible differences between AGN sub-types. Our main focus is on new tools for determining the properties of dusty galaxies and on the AGN-starburst connection. We present mid-IR line ratio diagrams which can be used to identify composite (starburst + AGN) sources and to distinguish between emission excited by active nuclei and emission from (circum-nuclear) star forming regions. For instance, line ratios of high to low excitation lines like [O IV]25.9 μm/[Ne II]12.8 μm, that have been used to probe for AGNs in dusty objects, can be examined in more detail and with better statistics now. In addition, we present two-dimensional diagnostic diagrams that are fully analogous to classical optical diagnostic diagrams, but better suited for objects with high extinction. Finally, we discuss correlations of mid-infrared line fluxes to the mid- and far-infrared continuum. We compare these relations to similar relations in starburst galaxies in order to examine the contribution of AGNs to the bolometric luminosities of their host galaxies. The spectra are available in electronic form from the authors.