INFRARED DIAGNOSTICS FOR THE EXTENDED 12 μm SAMPLE OF SEYFERTS

Abstract

We present an analysis of Spitzer IRS spectroscopy of 83 active galaxies from the extended 12 μm sample. We find rank correlations between several tracers of star formation which suggest that (1) the polycyclic aromatic hydrocarbon feature is a reliable tracer of star formation, (2) there is a significant contribution to the heating of the cool dust by stars, and (3) the H 2 emission is also primarily excited by star formation. The 55-90 versus 20-30 spectral index plot is also a diagnostic of the relative contribution of starburst to active galactic nuclei (AGNs). We see there is a large change in spectral index across the sample: Δα ∼ 3 for both indices. Thus, the contribution to the IR spectrum from the AGN and starburst components can be comparable in magnitude but the relative contribution also varies widely across the sample. We find rank correlations between several AGN tracers. We find correlations of the ratios14 μm with the silicate strength which we adopt as an orientation indicator. This suggests that some of the [O iii]λ5007 emission in these Seyferts is subject to orientation dependent obscuration as found by Haas et al. for radio galaxies and quasars. There is no correlation of [Ne v] equivalent width with the silicate 10 μm strength, indicating that the [Ne v] emission is not strongly orientation dependent. This suggests that the obscuring material (e.g., torus) is not very optically thick at 14 μm consistent with the results of Buchanan et al. We search for correlations between AGN and starburst tracers and we conclude that the AGN and starburst tracers are not correlated. This is consistent with our conclusion that the relative strength of the AGN and starburst components varies widely across the sample. Thus, there is no simple link between AGN fueling and black hole growth and star formation in these galaxies. The density diagnostic [Ne v] 14/24 μm and [S iii] 18/33 μm line ratios are consistent with the gas being near the low density limit, i.e., ∼10 3 cm −3 for [Ne v] and n e ∼ few hundred cm −3 for [S iii]. The distribution of silicate 10 μm and 18 μm strengths is consistent with the clumpy torus models of Sirocky et al. We find a rank correlation between the [Ne v] 14 μm line and the 6.7 μm continuum which may be due to an extended component of hot dust. The Sy 2's with a hidden broad-line region (HBLR) have a higher ratio of AGN-to-starburst contribution to the spectral energy distribution than Sy 2's without an HBLR. This may contribute to the detection of the HBLR in polarized light. The Sy 2's with an HBLR are more similar to the Sy 1's than they are to the Sy 2's without an HBLR.

Nuclear obscuration in LINERs

“…These diagnostics are based on the mid-IR continuum slope, the EW of the PAH features, the ratio of [NeV] (or [OIV]) over [NeII], and the EW of the PAH at 6.2 or 11.3 µm versus the 9.7 µm optical depth τ 9.7 µm Lutz et al 1998;Dale et al 2006;Sturm et al 2006;Spoon et al 2007;Baum et al 2010;Hernán-Caballero & Hatziminaoglou 2011). In this section we use several diagnostics to separate those Spitzer/IRS spectra that are strongly contaminated by non-AGN emission; however, the SB contribution to the mid-IR spectra does not exclude the presence of an AGN.…”

“…The steepness of the mid-IR spectra characterises the relative contribution of warm and cool dust to the mid-IR (Baum et al 2010). We refer here to the steepness at the spatial resolutions of the Spitzer/IRS spectra (i.e.…”

“…This steepness has been defined in several bands by different authors; e.g. 20−30 µm (Baum et al 2010;Weedman et al 2005) or 15−30 µm (Brandl et al 2006;Wu et al 2009;Nardini et al 2008). We present the 20 and 30 µm luminosities to produce an estimate of the steepness of the mid-IR spectra in our sample.…”

The deeply obscured AGN of NGC 4945

“…The [Cl II] 14.37 μm fine-structure emission line was clearly detected (>3σ) in the whole region mapped with the IRS/SH module, and it presents a similar (although more spreadout) distribution to the [Ne II] 12.81 μm line. Because its ionization potential (97.1 eV) is too much to allow production by main-sequence stars, the fine-structure line [Ne V] 14.32 μm is commonly used to probe the narrow line region of AGNs (e.g., Moorwood et al 1996a;Genzel et al 1998;Sturm et al 2002;Armus et al 2007;Alonso-Herrero et al 2009;Baum et al 2010;Willett et al 2010, and references therein). It has recently been used as a diagnostic tool for unambiguously identifying AGN in galaxies that have not been identified as such using optical spectroscopy (Goulding & Alexander 2009).…”

THE RELATION BETWEEN LUMINOUS AGNs AND STAR FORMATION IN THEIR HOST GALAXIES

“…Because the sources in our study are emitting close to the Eddington limit and hence uniformly represent approximately the maximum states of their outputs, our conclusions are not hostage to modeling the AGN variability. There have been a number of suggestions that Type 2 AGNs may lie in galaxies with relatively high rates of star formation, made both from a theoretical perspective (Wada & Norman 2002;Ballantyne et al 2006) and from observations (e.g., Maiolino et al 1995;Mouri & Taniguchi 2002;Buchanan et al 2006;Deo et al 2007;Meléndez et al 2008;Baum et al 2010;Castro et al 2014;Villarroel & Korn 2014). This result is controversial (e.g., Pereira-Santaella et al 2010;Diamond-Stanic & Rieke 2012;Merloni et al 2014); nonetheless, the Type-2 members of our sample test whether our conclusions are valid for such systems.…”