Spitzer IRS High-Resolution Spectroscopy of the 12 μm Seyfert Galaxies. I. First Results

The first high resolution Spitzer IRS 9-37µm spectra of 29 Seyfert galaxies (about one quarter) of the 12µm Active Galaxy Sample are presented and discussed. The high resolution spectroscopy was obtained with corresponding off-source observations. This allows excellent background subtraction, so that the continuum levels and strengths of weak emission lines are accurately measured. The result is several new combinations of emission line ratios, line/continuum and continuum/continuum ratios that turn out to be effective diagnostics of the strength of the AGN component in the IR emission of these galaxies. The line ratios [NeV]/[NeII], [OIV]/[NeII], already known, but also [NeIII]/[NeII] and [NeV]/[SiII] can all be effectively used to measure the dominance of the AGN.We extend the analysis, already done using the 6.2µm PAH emission feature, to the equivalent width of the 11.25µm PAH feature, which also anti-correlates with the dominance of the AGN. We measure that the 11.25µm PAH feature has a constant ratio with the H2 S(1) irrespective of Seyfert type, approximately 10 to 1. Using the ratio of accurate flux measurements at about 19µm with the two spectrometer channels, having aperture areas differing by a factor 4, we measured the source extendness and correlated it with the emission line and PAH feature equivalent widths. The extendness of the source gives another measure of the AGN dominance and correlates both with the EWs of [NeII] and PAH emission. Using the rotational transitions of H2 we were able to estimate temperatures (200-300K) and masses (1-10 × 106 M⊙), or significant limits on them, for the warm molecular component in the galaxies observed. Finally we used the ratios of the doublets of [NeV] and of [SIII] to estimate the gas electron density, which appears to be of the order of ne ∼ 103−4 cm−3 for the highly ionized component and a factor 10 lower for the intermediate ionization gas.