Hot dust in galaxies

Gareth Wynn-Williams, University of Hawaii

To many astronomers interstellar dust is just a nuisance, since it hides many of the most interesting regions of galaxies from our gaze. The infrared spectrum that is emitted by the interstellar matter in a galaxy, however, contains important clues about how the dust was heated. By examining such spectra we may be able to identify the energy sources that lie deep inside molecular clouds or nuclear regions. Graduate student Chris Dudley and I have been measuring the 8 - 22 µm spectra of a number of galaxies using the CGS3 spectrometer on UKIRT with a view to discovering how infrared galaxies of different types are powered.

NGC 253 is a nearby spiral galaxy which displays strong infrared emission from its central region. The 8 - 13 µm spectrum shows several distinct types of infrared emission:

• The 12.8 µm emission line from singly ionized helium. [NeII]. This spectrum line comes from gas that has been ionized by hot young stars.
• The 8.6 µm and 11.3 µm PAH emission features. These are produced when polycyclic aromatic hydrocarbon molecules are heated by ultraviolet radiation in photo-dissociation regions in molecular clouds
• Strong emission from the long wavelength side of the 7.7 µm PAH emission feature (the short wavelength side of the emission is cut off by the Earth's atmosphere). The gray band shows the predicted strength of the 7.7 µm feature based on the more easily observed 11.3 µm feature and a ratio of 7.7/11.3 µm feature strengths found in parts of our Galaxy.
• Continuum emission from about 10 - 14 µm that can be fitted by a 145 K black body (solid line). At least part of this emission comes from dust heated by Lyman-alpha radiation from the ionized gas. This continuum emission continues to get stronger all the way to about 60 µm.
• A broad "plateau" of emission, probably also from PAH molecules, between 11.1 and 12.9 µm. This is the first galaxy (apart from the Milky Way) that this feature has been seen.

All these types of emission are clues that the infrared radiation from NGC253 comes from a burst of star formation within the central 100 pc region. 

For more information see 

Dudley, C. C. and Wynn-Williams, C. G.,
Mon. Not. R. Astron  Soc.304 549 (1999)

IRAS 08572+3915 is one of the most luminous galaxies known, emitting about 2 x 10¹² times the power of the Sun. Its 8 - 22 µm spectrum is quite unlike that of NGC 253 (above); there are no signs of the neon line nor any of the PAH emission features. The spectrum is dominated by a very deep silicate absorption feature near 10 µm (rest wavelength) and a flat emission between 16 and 22 µm (the regions between 9 - 10 and 13 - 16 µm are absorbed by the Earth's atmosphere.) A silicate absorption this deep would correspond to 75-90 mag if is were caused by a layer of cold dust in front of a star formation region. In our paper we present arguments against this interpretation and note that the only other known objects that show such deep silicate absorption features are infrared "protostars". In a "protostar" the silicate absorption feature is formed as radiation is transported outwards from a central energy-generating core through many layers of optically-thick dust. No external extinction is required.

When the scaling correction is applied we find that the central energy source of IRAS 08572+3915 must be no more than 3 parsecs across. This small size effectively rules out the possibility that the power comes from a starburst. We conclude that the only reasonable power source for these galaxies is some kind of obscured active galaxy nucleus (AGN) such as an accretion disk around a massive black hole.

For more information see
Dudley, C. C., & Wynn-Williams, C. G.
Astrophys. J. 488: 720-729. (1997)