SPITZER/IRS 5-35 μm LOW-RESOLUTION SPECTROSCOPY OF THE 12 μm SEYFERT SAMPLE

Abstract

We present low-resolution 5.5-35 μm spectra for 103 galaxies from the 12 μm Seyfert sample, a complete unbiased 12 μm flux limited sample of local Seyfert galaxies selected from the IRAS Faint Source Catalog, obtained with the Infrared Spectrograph (IRS) on-board Spitzer Space Telescope. For 70 of the sources observed in the IRS mapping mode, uniformly extracted nuclear spectra are presented for the first time. We performed an analysis of the continuum emission, the strength of the polycyclic aromatic hydrocarbon (PAH) and astronomical silicate features of the sources. We find that on average, the 15-30 μm slope of the continuum is α 15−30 = −0.85 ± 0.61 for Seyfert 1s and −1.53 ± 0.84 for Seyfert 2s, and there is substantial scatter in each type. Moreover, nearly 32% of the Seyfert 1s, and 9% of the Seyfert 2s, display a peak in the mid-infrared spectrum at 20 μm, which is attributed to an additional hot dust component. The PAH equivalent width decreases with increasing dust temperature, as indicated by the global infrared color of the host galaxies. However, no statistical difference in PAH equivalent width is detected between the two Seyfert types, 1 and 2, of the same bolometric luminosity. The silicate features at 9.7 and 18 μm in Seyfert 1 galaxies are rather weak, while Seyfert 2s are more likely to display strong silicate absorption. Those Seyfert 2s with the highest silicate absorption also have high infrared luminosity and high absorption (hydrogen column density N H >10 23 cm −2 ) as measured from the X-rays. Finally, we propose a new method to estimate the active galactic nucleus contribution to the integrated 12 μm galaxy emission, by subtracting the "star formation" component in the Seyfert galaxies, making use of the tight correlation between PAH 11.2 μm luminosity and 12 μm luminosity for star-forming galaxies.

X-ray observations of highly obscuredτ9.7μm > 1 sources: an efficient method for selecting Compton-thick AGN?

“…We confirm that all of the sources selected with our primary technique also have τ 9.7 > 1 based on PAHFIT. The above re-analysis of the Wu et al (2009) Spitzer spectra shows that τ 9.7 > 1 for eleven AGN (see Table 1). The IRS spectra of many of these sources were also discussed in Armus et al (2007).…”

“…An ideal data set for this purpose is the Spitzer IRS spectroscopic sample of Wu et al (2009). These authors present low-resolution Spitzer 5.5−35 μm spectra for 103 galaxies from the 12 μm Seyfert sample (Rush et al 1993).…”

“…The structure of the paper is as follows: we first compile a local sample of high optical-depth (τ 9.7 μm > 1) sources from the Wu et al (2009) flux-limited Spitzer-IRS observations of the 12 μm Seyfert sample of Rush et al (1993). We investigate the X-ray spectra available in the literature, to deduce how many of these high-τ objects are heavily obscured or Compton-thick.…”

Nearby early-type galaxies with ionized gas

“…However we note that AGN activity is frequently associated with star-formation activity (see also Paper IV) so Seyfert galaxies will be characterized by spectra mixing Class-4 and Class-3 (see Wu et al 2009, for a collection of MIR spectra of Seyfert galaxies). Note that in some cases the AGN MIR spectra can be dominated by synchrotron emission, like in M 87 (Buson et al 2009).…”

“…In NGC 1052 and IC 5063, where the contribution from the old population is almost negligible (∼19% and ∼3%, respectively), and the spectrum is typical of an AGN (Wu et al 2009), we used suitable spline functions to represent the underlying continuum. Figure 3 show examples of the spectral decomposition in three typical galaxies with different degrees of activity.…”