The Nature of the Low-Metallicity ISM in the Dwarf Galaxy NGC 1569

We are modeling the spectra of dwarf galaxies from infrared to submillimeter wavelengths to understand the nature of the various dust components in low-metallicity environments, which may be comparable to the ISM of galaxies in their early evolutionary state. The overall nature of the dust in these environments appears to differ from those of higher metallicity starbursting systems. Here, we present a study of one of our sample of dwarf galaxies, NGC 1569, which is a nearby, well-studied starbursting dwarf. Using ISOCAM, IRAS, ISOPHOT and SCUBA data with the Desert et al. (1990) model, we find consistency with little contribution from PAHs and Very Small Grains and a relative abundance of bigger colder grains, which dominate the FIR and submillimeter wavelengths. We are compelled to use 4 dust components, adding a very cold dust component, to reproduce the submillimetre excess of our observations.Comment: 4 pages, 4 postscript figures. Proceedings of "Infrared and Submillimeter Astronomy. An International Colloquium to Honor the Memory of Guy Serra" (2002

Photoelectric effect on dust grains across the L1721 cloud in the rho Ophiuchi molecular complex

We present ISO-LWS measurements of the main gas cooling lines, C+ 158 mum and O 63 mum towards a moderate opacity molecular cloud (Av=3), L1721, illuminated by the B2 star nu Sco (X = 5-10). These data are combined with an extinction map and IRAS dust emission images to test our understanding of gas heating and cooling in photo-dissociation regions (PDRs). This nearby PDR is spatially resolved in the IRAS images; variations in the IRAS colors across the cloud indicate an enhanced abundance of small dust grains within the PDR. A spatial correlation between the gas cooling lines and the infrared emission from small dust grains illustrates the dominant role of small dust grains in the gas heating through the photoelectric effect. The photoelectric efficiency, determined from the observations by ratioing the power radiated by gas and small dust grains, is in the range 2 to 3% in close agreement with recent theoretical estimates. The brightness profiles across the PDR in the C+ 158 mum and O 63 mum lines are compared with model calculations where the density profile is constrained by the extinction data and where the gas chemical and thermal balances are solved at each position. We show that abundance variations of small dust grains across the PDR must be considered to account for the LWS observations.Comment: 10 pages, 15 figure

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

We present new 450 and 850 microns 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 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 10^5 Msol) and could be distributed in small clumps (size 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 Angstroms bump, resembling the observed extinction curve of some regions in the Large Magellanic Cloud.Comment: 20 pages, 20 figures, accepted by A&

Le modèle TELEMAC-3D pour les écoulements tridimensionnels : De nouvelles perspectives pour les études d'environnement

Le modèle numérique est un outil qui a trouvé sa place en complément des modèles réduits et des mesures in-situ; il permet à l'ingénieur de mieux comprendre le fonctionnement d'un site et de prédire son évolution, en simulant éventuellement différents scénarios d'interventions humaines. Un nouveau pas vient d'être franchi grâce aux progrès informatiques et numériques. II est maintenant envisageable de simuler sur le long terme des phénomènes couplant l'écosystème et l'hydrodynamique. Le modèle tridimensionnel TELEMAC-3D est aujourd'hui opérationnel, d'un coût d'utilisation admissible pour des études courantes. Après une brève description de ses fonctionnalités, quelques applications sont évoquées

Prise de Saint-Omer, dans le grand escallier de Versailles : [estampe]

Référence bibliographique : Hennin, 492

Evolution of interstellar dust properties from diffuse medium to a dense cloud

The balloon borne experiment PRONAOS has measured the sub-millimetre emission from the diffuse interstellar medium to a quiescent filament in the Taurus molecular cloud. We model the output emission of this filament using a radiative transfer code and an independent tracer of the total column density (near infrared extinction). We show that inside the filament the transiently heated particles responsible of the 60 μm emission are removed and the dust sub-millimetre emissivity is increased by a significant factor, 3.4^(+0.3)_(-0.7) (for typically n_H > 3 ± 1 × 10^3 cm^(-3), A_V > 2.1 ± 0.5). This induces a deficit in the IRAS I_(60) μm/I_(100 μm) flux ratio in comparison with the diffuse interstellar medium and a decrease of the grain equilibrium temperature in excess to that predicted by extinction

Evolution of interstellar dust properties from diffuse medium to a dense cloud

The balloon borne experiment PRONAOS has measured the sub-millimetre emission from the diffuse interstellar medium to a quiescent filament in the Taurus molecular cloud. We model the output emission of this filament using a radiative transfer code and an independent tracer of the total column density (near infrared extinction). We show that inside the filament the transiently heated particles responsible of the 60 μm emission are removed and the dust sub-millimetre emissivity is increased by a significant factor, 3.4+0.3-0.7 (for typically nH > 3 ± 1 × 103 cm-3, AV > 2.1 ± 0.5). This induces a deficit in the IRAS I60 μm/I100 μm flux ratio in comparison with the diffuse interstellar medium and a decrease of the grain equilibrium temperature in excess to that predicted by extinction

Evolution of dust properties in an interstellar filament

We present submillimetre observations obtained using the balloon-borne experiment PRONAOS/SPM, from 200 to 600 μm with an angular resolution of 2–3.5′, of a quiescent dense filament (typically $A_{V} \sim 4$) in the Taurus molecular complex. This filament, like many other molecular clouds, presents a deficit in its IRAS $I_{\rm 60 \,\mu m}/I_{\rm 100 \,\mu m}$ flux ratio in comparison with the diffuse interstellar medium. We show, from the combination of the PRONAOS/SPM and IRAS data, that, inside the filament, there is no evidence for emission from the transiently heated small particles responsible for the 60 μm emission, and that the temperature of large grains in thermal equilibrium with the radiation field is reduced in the inner parts of the filament. The temperature is as low as 12.1$^{+0.2}_{-0.1}$ K with $\beta=1.9\pm 0.2$ (or 12.0$^{+0.2}_{-0.1}$ K using $\beta=2$) toward the filament centre. These phenomena are responsible for the IRAS colour ratio observed toward the filament. In order to explain this cold temperature, we have developed a model for the emission from the filament using star counts from the 2MASS catalog as an independent tracer of the total column density and a radiative transfer code. We first use the optical properties of the dust from the standard model of Désert et al. ([CITE]). The computed brightness profiles fail to reproduce the data inside the filament, showing that the dust properties change inside the filament. An agreement between data and model can be found by removing all the transiently heated particles from the densest parts of the filament, and multiplying the submillimetre emissivity by a significant factor, 3.4$^{+0.3}_{-0.7}$ (for typically $n_{H}> 3 \pm 1\times 10^3$ cm-3, $A_{V} > 2.1\pm 0.5$). We show that grain-grain coagulation into fluffy aggregates may occur inside the filament, explaining both the deficit of small grain abundance and the submillimetre emissivity enhancement of the large grains