ISM Properties in Low-Metallicity Environments II.
The Dust Spectral Energy Distribution of NGC 1569

The absorption of stellar radiation by dust and its subsequent reemission in the infrared (IR) to the submillimeter (submm) is a fundamental process controlling the heating and cooling of the interstellar medium (ISM). The dust IR spectral energy distribution (SED) of a galaxy is its footprint reflecting fundamental physical parameters such as initial mass function (IMF), age, stellar population and metallicity. Thus, knowledge of the physical characteristics of dust in galaxies opens the door to understanding the star formation history and the evolution of galaxies. Dwarf galaxies in our local universe are ideal laboratories for studying the interplay between the ISM and star formation in low-metallicity environments.

In this paper, we present the complete modeling for NGC1569, a nearby (D = 2.2 ± 0.6 Mpc) dwarf galaxy with an average metallicity of 1/4 solar, currently in the aftermath of a massive burst of star formation.

In order to constrain the dust emission, the global observed SED of NGC 1569 was constructed, as completely as possible, from observations we obtained with SCUBA (850 and 450µm) and ISOCAM (4-16 µm spectrum; Madden et al. 2003). We incorporated additional data from the literature (IRAS, KAO, ISOPHOT, MAMBO). The image presented on the left is the 850µm SCUBA map, tracing the cold dust continuum. Its morphology is similar to the Hα emission.

The modelling of the dust properties was done in a self-consistent manner, including the link between the stellar evolution and photoionization. We fit the IR-to-mm SED with the Désert et al. (1990) dust model and deduced the dust emission and extinction properties. Several interstellar radiation fields (ISRF), heating the dust, were synthesized with the stellar evolutionary synthesis model, PÉGASE (Fioc & Rocca-Volmerange 1997) and constrained by the UV-to-optical SED.

The MIR ionic line ratios from the ISOCAM spectra were used to constrain the photoionization model CLOUDY (Ferland 1996) in order to remove the degeneracy on the ISRFs found with PÉGASE. The figure on the right represents the synthesized ISRF of NGC 1569 compared to the Galactic one. The extinction curve, deduced from the dust properties, was used to deredden the UV-to-optical data and this procedure was iterated on until we reached a consistent solution.

The figure presented on the left is the final modeled dust SED of NGC 1569. The data are indicated by crosses. The lines are the dust model and its different components: PAHs are the carriers of the unidentified IR bands, VSGs (Very Small Grains) are small carbonaceous grains, BGs (Big Grains) are silicates and VCGs (Very Cold Grains) are grains modeled with a modified black body used to explain a submillimetre excess. Some new results emerge from this model:

• There is lack of PAHs which are likely destroyed by the hard radiation field combined with the low screening effects of the dust, due to the low-metallicity ISM.

The dust emission is dominated by small grains (of radius ~3 nm). The redistribution of large dust grains into smaller sizes is supported by the shock model of Jones et al. (1996) and is consistent with an ISM heavily influenced by supernova activity.