Spitzer spectra of Seyfert galaxies

The first results of the Spitzer spectroscopic survey of the Seyfert galaxies of the 12µm sample (Tommasin et al 2008) show a clear inverse trend between the indicator of AGN dominance, the [NeV]14.3µm/[NeII]12.8µm line ratio, and the equivalent width of the 11.25µm PAH feature, which can be considered as an indicator of the star formation dominance, as shown in Figure 1a, where the sample objects increaed from 30 (Tommasin et al 2008) to 87 (Tommasin et al 2009). This result confirms an early finding of the ISO-SWS spectrometer (Genzel et al 1998) with a much better statistics. Here the Seyfert galaxies have been reclassified, following the results of spectropolarimetry (author?) (Tran 2001, Tran 2003), in type 1’s (including the classical Seyfert 1’s and the hidden Broad Line Region Seyfert 2’s, as discovered through spectropolarimetry) and “pure” type 2’s (for which a BLR was not detected). Most of the type 1 objects, including both Seyfert 1s and hidden Broad Line Region Seyfert 2s, are located at high values of the [NeV]14.3µm/[NeII]12.8µm line ratio and very low or absent PAH emission.

Another diagnostic diagram using both spectroscopic and photometric results is shown in Figure 1b: the spectral index between 25 and 60µm α (60µm-25µm) versus the [NeV]14.3µm/[NeII]12.8µm line ratio. A clear trend shows that when The two channels of the Spitzer high-resolution spectrometer: SH 9.6-19.5µm with slit size 4.7 ′′ × 11.3 ′′ and LH 19-39µm with slit size 11.1 ′′ × 22.3 ′′ allow multi-aperture photometry in the overlapping parts (19.0-19.5µm). The ratio of the flux measured in LH to that measured in SH gives the extendedness of the source. We used this measure of the extendedness of the source to estimate the line emitting regions (Tommasin et al 2008). In Figure 2a we plot the [NeII]12.8µm line equivalent width as a function of the source extendedness. We notice that those sources showing a significant mid-IR extendedness are mostly type 2 objects or non-Seyfert galaxies and have the highest [NeII]12.8µm line equivalent width, with only two exceptions of very nearby Seyfert 1’s. An high [NeII]12.8µm line equivalent width is a measure of a strong star formation component. This is not the case for the high excitation lines, originated from the AGN, such as [NeV], for which no apparent trend appears between source extendedness and line EWs , as can be seen in Figure 2b (Tommasin et al 2008).

To The sample of sources to which we apply this analysis is reduced because the models depend on the extendedness and the PAH equivalent width, which are not detected (for the PAH) or measurable (for the extendedness) for all the sources(from 86 to 59 objects). We found that the model can disentangle the AGN and the Starburst emission for 16 of 20 Seyfert 1’s, 14 of 16 HBLR’s, 13 of 14 nonHBLR’s, 7 of 9 objects re-classified as not Seyferts. The details of this approach will appear in Tommasin et al. (2009).

The 12MGS has been observed extensively from the radio to the X-rays and we can use the large set of data to search for correlations between different observed quantities. To show an example, we want to relate the X-ray luminosity, measuring the accretion, to the bolometric luminosity, as given by the 12µm luminosity. We plot in Figure 4 the unabsorbed 2-10keV luminosity and the 12µm luminosity.

Following the finding of Spinoglio & Malkan (1989) and Spinoglio et al (1995) that the 12µm luminosity is linearly proportional to the bolometric luminosity, at a given L bol in Figure 4a a sequence can be identified with decreasing accretion luminosity: from Seyfert 1’s → HBLR-Seyfert 2’s → pure Seyfert 2’s. Although these results are to be considered preliminary, as no statistical method has yet been applied, most Seyfert 1’s have: a:(left) The spectral index α (60µm-25µm) versus the [NeV]14.3µm/[NeII]12.8µm line ratio: the solid line fitting the data is the model and the dashed lines separate the diagram in 3 regions with different AGN contribution, (100% -70%),(70% -50%), (50% -0%). b:(right) EW [NeII]12.8µm vs EW PAH 11.25µm: same notation as in Fig. 3a.

We preminilarily suggest that black hole accretion, as measured by X-rays, is the dominant mechanism determining the observational nature of a galaxy: when accretion is not an important energy source, we have galaxies without Seyfert nuclei, dominated by stellar evolution processes (called here non-Seyfert’s), then when accretion increases we have a sequence from the pure Seyfert 2’s, to the HBLR-Seyfert 2’s and finally when accretion dominates the bolometric luminosity, we have the Seyfert 1’s.

In an analogous way, we try to correlate the IRAS 60µm luminosity (measuring the integrated star formation activity) and the H 2 S(1) line luminosity (typical star formation indicator) in Fi

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