Spitzer irs high-resolution spectroscopy of the 12 mu m Seyfert galaxies. I. First results

The first high-resolution Spitzer IRS 9-37 mu m spectra of 29 Seyfert galaxies of the 12 mu m Active Galaxy Sample are presented. The spectroscopy was obtained with off-source observations, allowing excellent background subtraction and thus accurate measurements of continuum levels and strengths of weak emission lines. Several new combinations of emission-line ratios and line/continuum ratios were identified as effective diagnostics of the strength of the AGN component in these galaxies. The line ratios [Ne V]/[Ne II], [O IV]/[Ne II], already known, but also [Ne III]/[Ne II] and [Ne V]/[Si II] can be used to measure the dominance of the AGN. The equivalent width of the 11.25 mu m PAH feature, like that of the 6.2 mu m PAH feature, already studied, also anticorrelates with the dominance of the AGN. Our measurements indicate that the 11.25 mu m PAH feature has a constant ratio with H-2 S( 1) irrespective of Seyfert type, approximately 10 to 1. The ratio of the 19 mu m fluxes observed with the two spectrometer channels, with aperture areas differing by a factor of 4, measures the source extendedness, giving another estimate of the AGN dominance, which correlates with both the EWs of [Ne II] and PAH emission. Using the rotational transitions of H-2, we estimated temperatures (200-300K) and masses (1-10; 10(6) M-circle dot) for the warm molecular component. Finally, we used the ratios of the doublets of [Ne V] and of [S III] to estimate the gas electron density, which appears to be of the order of n(e) similar to 10(3) - 104 cm(-3) for the highly ionized component and a factor 10 lower for the intermediate-ionization gas.