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 What/where are the hydrocarbons?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 hydrocarbonsThe significance of hydrocarbons➢ Aliphatic hydrocarbons may contain 30% of Aliphatic hydrocarbons may contain 30% of

the interstellar Cthe interstellar C

➢ Widespread in space: seen in numerous diffuse Widespread in space: seen in numerous diffuse ISM lines of sightISM lines of sight

➢ Exists through much of the lifecycle of the ISM; Exists through much of the lifecycle of the ISM; appears to be available for incorporation into appears to be available for incorporation into (life-bearing?) planetary systems(life-bearing?) planetary systems

➢ Formation and evolution of the dust remain a Formation and evolution of the dust remain a little mysterious...little mysterious...

The lifecycle of hydrocarbonsThe lifecycle of hydrocarbons➢ Observed in circumstellar material of one protoplanetary Observed in circumstellar material of one protoplanetary

nebulanebula

➢ Carbonaceous material also hydrogenated in diffuse ISMCarbonaceous material also hydrogenated in diffuse ISM

➢ Competing model proposes that hydrocarbons result from Competing model proposes that hydrocarbons result from UV processing of icy grain mantles (test using mid-IR UV processing of icy grain mantles (test using mid-IR spectra)spectra)

➢ Hydrocarbons Hydrocarbons not detected not detected in molecular cloudsin molecular clouds

➢ Found in Murchison meteorite extract – but conflicting mid-Found in Murchison meteorite extract – but conflicting mid-IR spectra. Same material?IR spectra. Same material?

➢ --> Look at new and existing spectra of the 3.4--> Look at new and existing spectra of the 3.4µµm band in m band in galaxiesgalaxies

Hydrocarbon signatures – the 3.4 Hydrocarbon signatures – the 3.4 µµm m featurefeature

➢ 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 3.4The 3.4µµm band in galaxiesm 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 08572

IRAS 19254

Pendleton et al. (1994); Chiar et al. (1996); Mason et al. (2004); Risaliti et al. (2003)

Diffuse ISM ULIRG/ Sy2

ULIRG/ Sy2PPNe

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 Same dust composition in many galaxies?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 Should we have expected this?this?

➢ Laboratory spectra of Laboratory spectra of hydrocarbons very diversehydrocarbons very diverse

➢ In the lab, 3.4 In the lab, 3.4 µµm band profile is m band profile is sensitive to environment sensitive to environment (e.g. (e.g. radiation field, presence of radiation field, presence of elements other than C & H)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

WhyWhy are the profiles so are the profiles so uniform?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.