Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.mrao.cam.ac.uk/~bn204/mk2/publications/2005/IREvolvePasadenaConfPoster.pdf Дата изменения: Wed Nov 25 23:11:02 2009 Дата индексирования: Thu Apr 8 14:07:13 2010 Кодировка: Поисковые слова: iapetus

Toward a Quantitative Interpretation of MidIR Spectra of Galaxies
B. Nikolic (NRAO), P. Alexander (Cambridge), M. Clemens (Padova), D. Ford (Cambridge), G. Cotter (Oxford), M.S. Longair (Cambridge)
The Method

Starburst99

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Comprehensive dust model

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Simple radiative transfer

Allows investigation of the effects of starburst age, metallicity, IMF etc. Also, can directly derive other parameters such as ionising photon flux, S/N rate.

Takes fully into account stochastic heating for a full size distribution of carbonaceous and silicate grains.

Appropriate for midIR models since only UV/optical photons are important in exciting the emission.

Figure B: As Figure A but showing the expected and observed correlations between 24micron luminosity and Halpha luminosity. Figure A: The model correlation between 8micron luminosity (convolved with the Spitzer filter response) and Halpha luminosity (broken line styles) for dust shell radii of 100pc, 2kpc and 5kpc (dashed, dotted and dashdotted respectively). Also shown is bestfitting correlation by Wu et al (2005, solid line and inset).

Case study: NGC 520
ISOCAM/CVS spectra (solid line)

Kband + ISO Dust particles 3.56 A destroyed


Summary
We have constructed a framework for modelling midIR emission of galaxies which makes it possible to directly relate observed spectra or photometry to the starformation rate, dust properties and/or geometry. The models naturally reproduce the observed correlation between midIR luminosity and starformation rate. They indicate that dust responsible for the bulk of midIR emission is not directly associated with the young stars. More detailed studies of a moderate (NGC 520) and very intense (Arp 220) starburst are presented. Although there is a rough agreement between models and observations, it is clear properties of the dust must be changing. The effect of destruction of the smallest grains is investigated and shown to be consistent with the observations. A reasonable fit to the observed spectrum of Arp 220 requires destruction of a large fraction of the small grain population. Figure C: Strengths of the 3.3 (dotted line), 7.7 (solid line) and 11.3 (dashed line) micron emission features as functions of shell gas column density. The exciting spectrum is from a continuously star forming starburst.



7.9 micron MICHELLE image

Galactic dust model


The model spectra were produced for a starformation rate of 7 solar masses per year (derived from Brackett line measurements, radio measurements), and with dust shell radius of 400pc (derived from midIR imaging above).



Case study: Arp 220
Spitzer/IRS spectra (solid line)

HST/ Thompson 2005

Dust particles 3.535 A destroyed


Acknowledgements & References
We have made use of observations obtained from the ISO and Spitzer archives. Additionally, the results here are based on observations with UKIRT/MICHELLE. We would also like to thank Dr. Kotilainen for providing us with the Kband image of NGC 520. The dust model is based on the work of Draine, Li and Weingartner (published in a series of three articles in ApJ in 2001). The Starburst99 stellar population synthesis models are described by Leitherer et al, 1999, ApJS. The midIR/SFR correlations are from Wu et al, 2005, ApJ.



11 micron MICHELLE image

Galactic dust model



Assumed star formation rate 120 solar masses per year (Bracket lines, FIR luminosity), dust shell radius 150pc (midIR imaging), column density 1.21022 cm (Brackett lines).

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Figure D: Evolution of midIR emission with starburst age, shown at ages of 5, 100, 475 and 1000 million years. Top is absolute midIR luminosity, while in the bottom the traces have been normalised by the stellar luminosity at that age.