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 Ám 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 Ám spectrum shows several distinct types of infrared emission:
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.
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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 Ám 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 Ám (rest wavelength) and a flat emission between 16 and 22 Ám (the regions between 9 - 10 and 13 - 16 Ám 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.
|In the second figure we compare
the measured infrared energy distributions of IRAS 08572+3915 and two other luminous
infrared galaxies with models that were initially calculated to explain the spectra of
infrared protostars. The models consist of a central compact source of power, surrounded
by a spherical shell of dust whose density and temperature decrease away from the center.
The models have to be greatly scaled up to match the luminosity of these infrared
galaxies, but the agreement between model and data strongly suggests to us that the
geometry of a spherical protostar can be successfully applied to galaxies with deep
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.
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Gareth Wynn-Williams (firstname.lastname@example.org)
Last modified December 19, 2001