Hydrocarbon Dust Absorption in Seyfert Galaxies and Ultraluminous Infrared Galaxies

Abstract

We present new spectroscopic observations of the 3.4 m absorption feature in the Seyfert galaxies NGC 1068 and NGC 7674 and the ultraluminous infrared galaxy IRAS 08572+3915. A signature of CÀH bonds in aliphatic hydrocarbons, the 3.4 m feature indicates the presence of organic material in Galactic and extragalactic dust. Here we compare the 3.4 m feature in all the galaxies in which it has been detected. In several cases, the signalto-noise ratio and spectral resolution permit a detailed examination of the feature profile, something which has rarely been attempted in extragalactic lines of sight. The 3.4 m band in these galaxies closely resembles that seen in the Galactic diffuse interstellar medium (ISM) and in newly formed dust in a protoplanetary nebula. The similarity implies a common carrier for the carbonaceous component of dust, and one which is resistant to processing in the interstellar and /or circumnuclear medium. We also examine the mid-IR spectrum of NGC 1068, because absorption bands in the 5-8 m region further constrain the chemistry of the 3.4 m band carrier. While weak features like those present in the mid-IR spectrum of diffuse dust toward the Galactic center would be undetectable in NGC 1068, the strong bands found in the spectra of many proposed dust analog materials are clearly absent, eliminating certain candidates and production mechanisms for the carrier. The absence of strong absorption features at 5-8 m is also consistent with the interpretation that the similarity in the 3.4 m feature in NGC 1068 to that in Galactic lines of sight reflects real chemical similarity in the carbonaceous dust.

On the Optical-to-silicate Extinction Ratio as a Probe of the Dust Size in Active Galactic Nuclei

“…Indeed, the 3.4 µm absorption feature, commonly attributed to the C-H stretching mode in saturated aliphatic hydrocarbon dust, is seen in AGNs (Wright et al 1996, Imanishi et al 1997. Mason et al (2004) argued that the 3.4 µm absorption feature at least in face-on Seyfert 2 galaxies arises in dust local to the active nucleus rather than in the diffuse ISM of the galaxy. Since carbon dust contributes to A V but not to ∆τ 9.7 , with carbon dust included one expects a higher ratio of A V /∆τ 9.7 for a given dust size, and therefore, the observed AGN ratio of A V /∆τ 9.7 ≈ 5.5 would require the dust size to be even larger than that derived in §2 based on a pure silicate composition.…”

Observations of Interstellar Carbon Compounds

“…In the last years, several extragalactic obscured active galactic nuclei (AGN) sources displaying a-C:H absorptions have been observed, mainly via the stretching modes (Pendleton et al 1994;Mason et al 2004;Dartois et al 2004;Imanishi 2006;Risaliti et al 2006;Imanishi et al 2008). In NGC 1068, the hydrocarbon distribution has even been imaged to better constrain its spatial distribution, and foresee its nature (Mason et al 2006;Geballe et al 2009).…”

The Interstellar Medium of IRAS 08572+3915 NW: \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagesty

“…This spectrum is shown in Figure 1, because it is a better quality spectrum of the redshifted 3.4 m hydrocarbon absorption feature in IRAS 08572+3915 NW than those published to date (Imanishi & Dudley 2000;Mason et al 2004;Imanishi et al 2006). The spectrum also illustrates the difficulty of detecting weak lines from the ground at medium spectral resolution in the thermal infrared region.…”