Astronomy & Astrophysics

Free access article

Issue		A&A Volume 407, Number 1, August III 2003
Page(s)		159 - 176
Section		Galactic structure and dynamics
DOI		10.1051/0004-6361:20030814

A&A 407, 159-176 (2003)
DOI: 10.1051/0004-6361:20030814

ISM properties in low-metallicity environments

II. The dust spectral energy distribution of NGC 1569

F. Galliano¹, S. C. Madden¹, A. P. Jones², C. D. Wilson³, J.-P. Bernard^{2, 4} and F. Le Peintre^{2, 5}

¹ Service d'Astrophysique, CEA/Saclay, L'Orme des Merisiers, 91191 Gif sur Yvette, France
² Institut d'Astrophysique Spatiale (IAS), Université de Paris XI, 91405 Orsay Cedex, France
³ Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1, Canada
⁴ Centre d'Étude Spatial des Rayonnements (CESR), 31028 Toulouse, France
⁵ Deceased October 2001

(Received 24 December 2002 / Accepted 28 May 2003)

Abstract
We present new 450 and 850 $\mu$ m SCUBA data of the dwarf galaxy NGC 1569. We construct the mid-infrared to millimeter SED of NGC 1569, using ISOCAM, ISOPHOT, IRAS, KAO, SCUBA and MAMBO data, and model the SED in order to explore the nature of the dust in low metallicity environments. The detailed modeling is performed in a self-consistent way, synthesizing the global ISRF of the galaxy using an evolutionary synthesis model with further constraints provided by the observed MIR ionic lines and a photoionisation model. Our results show that the dust properties are different in this low metallicity galaxy compared to other more metal rich galaxies. The results indicate a paucity of PAHs probably due to the destructive effects of the ISRF penetrating a clumpy environment and a size-segregation of grains where the emission is dominated by small grains of size ~ $3\;\rm nm$ , consistent with the idea of shocks having a dramatic effect on the dust properties in NGC 1569. A significant millimetre excess is present in the dust SED which can be explained by the presence of ubiquitous very cold dust ( T = 5-7 K). This dust component accounts for 40 to 70% of the total dust mass in the galaxy ( $1.6{-}3.4 \times 10^5$ $M_\odot$ ) and could be distributed in small clumps (size $\simeq$ a few pc) throughout the galaxy. We find a gas-to-dust mass ratio of 740-1600, larger than that of the Galaxy and a dust-to-metals ratio of 1/4 to 1/7. We generate an extinction curve for NGC 1569, consistent with the modeled dust size distribution. This extinction curve has relatively steep FUV rise and smaller 2175 Å bump, resembling the observed extinction curve of some regions in the Large Magellanic Cloud.

Key words: ISM: dust, extinction -- galaxies: dwarf -- galaxies: starburst -- infrared: galaxies -- submillimeter

Offprint request: F. Galliano, fredg@discovery.saclay.cea.fr

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