Hydrocarbon dust in Seyfert galaxies and ULIRGs R. Mason (NOAO Gemini Science Center) G. Wright (Astronomy Technology Centre) Y. Pendleton (NASA Ames) A. Adamson (Joint Astronomy Centre)

What/where are the hydrocarbons? Aromatic skeleton – but not PAHs Traces of other elements other than C & H (affect spectral features) Aliphatic hydrocarbon chains and side groups

The significance of hydrocarbons Aliphatic hydrocarbons may contain 30% of the interstellar C Aliphatic hydrocarbons may contain 30% of the interstellar C Widespread in space: seen in numerous diffuse ISM lines of sight Widespread in space: seen in numerous diffuse ISM lines of sight Exists through much of the lifecycle of the ISM; appears to be available for incorporation into (life-bearing?) planetary systems Exists through much of the lifecycle of the ISM; appears to be available for incorporation into (life-bearing?) planetary systems Formation and evolution of the dust remain a little mysterious... Formation and evolution of the dust remain a little mysterious...

The lifecycle of hydrocarbons Observed in circumstellar material of one protoplanetary nebula Observed in circumstellar material of one protoplanetary nebula Carbonaceous material also hydrogenated in diffuse ISM Carbonaceous material also hydrogenated in diffuse ISM Competing model proposes that hydrocarbons result from UV processing of icy grain mantles (test using mid-IR spectra) Competing model proposes that hydrocarbons result from UV processing of icy grain mantles (test using mid-IR spectra) Hydrocarbons not detected in molecular clouds Hydrocarbons not detected in molecular clouds Found in Murchison meteorite extract – but conflicting mid-IR spectra. Same material? Found in Murchison meteorite extract – but conflicting mid-IR spectra. Same material? --> Look at new and existing spectra of the 3.4µm band in galaxies --> Look at new and existing spectra of the 3.4µm band in galaxies

Hydrocarbon signatures – the 3.4 µm feature Stretching of (sp3) C-H bonds absorbs light at about 3.4µm Can have asymmetric and symmetric stretches of CH2 and CH3 groups --> complex feature Shape of band reflects chain lengths, presence of electronegative groups etc. Lots of feature shapes observed in the lab. The 3.4µm feature in 2 different materials

The 3.4µm band in galaxies Spectra of 14 galaxies available (Mason et al. 2004; Dartois et al. 2004; see also work by M. Imanishi, G. Risaliti) Sample contains bright Sy2 galaxies and ULIRGs Most of dust within few hundred pc of nucleus Various X-ray luminosities and obscuring columns With/without circumnuclear starbursts/icy regions

NGC1068 (Sy2) compared with... IRS7, Galactic Centre CRL 618 IRAS IRAS Pendleton et al. (1994); Chiar et al. (1996); Mason et al. (2004); Risaliti et al. (2003) Diffuse ISM ULIRG/ Sy2 PPNe

And the rest: All the spectra with good S/N look startlingly similar New VLT spectra of NGC7172 and NGC5506 also bear a remarkable resemblance to NGC1068 and Galactic lines of sight. (Dartois et al. 2004) Many spectra with lower S/N also appear to bear a good resemblance Imanishi, Mason

Same dust composition in many galaxies? Different formation mechanisms can produce very similar 3.4µm bands but surely not just a coincidence in this many lines of sight? Mid-IR spectrum of NGC1068 agrees with this – as in our galaxy, strong absorptions at 5-9 µm are absent

Should we have expected this? Laboratory spectra of hydrocarbons very diverse Laboratory spectra of hydrocarbons very diverse In the lab, 3.4 µm band profile is sensitive to environment (e.g. radiation field, presence of elements other than C & H) In the lab, 3.4 µm band profile is sensitive to environment (e.g. radiation field, presence of elements other than C & H) Hydrocarbons with the right 3.4 µm and mid-IR spectra have been made in the lab (Dartois et al. 2004). But slight alteration of conditions results in spectral features that no longer match the observations

Why are the profiles so uniform? Conditions in regions of dust formation very uniform in these galaxies? Or is uniformity a result of rapid and thorough dust processing after its formation? Heating? (Grishko & Duley 2002) Would be nice to see more lab work on response of e.g. HAC to X-rays, UV, heating etc...

In Summary... Despite environmental differences, the chemical make- up of the hydrocarbons in the nuclei of Seyferts and ULIRGs appears startlingly similar to Galactic dust Seems to suggest rapid and thorough post-formation processing of dust Low-S/N spectra of galaxies exist in which the band profile appears different; are the hydrocarbons really as uniform as they seem?? Will follow up promising targets on Gemini North this semester.