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

F. Galliano, S. C. Madden+4 more|ArXiv.org|Jun 10, 2003

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy55 references116 citations

TL;DR

This study models the dust spectral energy distribution (SED) of the low-metallicity dwarf galaxy NGC 1569 using multi-wavelength data from IR to submillimeter wavelengths. It reveals a paucity of PAHs, dominance of small grains (~3 nm), and a significant millimeter excess attributed to very cold dust (5–7 K), accounting for 40–70% of the total dust mass, with a high gas-to-dust ratio (740–1600) and a dust-to-metals ratio of 1/4 to 1/7, indicating distinct dust properties in low-metallicity environments compared to the Milky Way.

ABSTRACT

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.

Motivation & Objective

• To understand the physical properties of dust in low-metallicity environments using a comprehensive SED model.
• To investigate how the interstellar radiation field (ISRF) and clumpy ISM affect dust emission and grain evolution.
• To determine the dust mass, size distribution, and composition in NGC 1569, a nearby dwarf galaxy with 1/4 solar metallicity.
• To assess the validity of Galactic dust models in low-metallicity galaxies and quantify deviations in dust properties.
• To derive a dust extinction curve and assess the impact of stochastic heating and cold dust components on SED modeling.

Proposed method

• Constructed a complete dust SED from 12 to 850 µm using data from IRAS, ISO, KAO, SCUBA, MAMBO, and ISOPHOT.
• Employed a self-consistent modeling approach combining the PÉGASE stellar population synthesis model with the CLOUDY photoionization code to derive the global ISRF.
• Used the DBP90 dust model to explore a wide range of dust parameters, including grain size distribution, composition, and emissivity.
• Incorporated constraints from observed MIR ionic lines (e.g., [Ne II], [O III]) to refine the ISRF and dust properties.
• Modeled stochastic heating of small grains (≤3 nm) due to their low heat capacity, critical for interpreting FIR/submm emission.
• Fitted the SED with a two-component dust model: warm dust (thermal equilibrium) and cold dust (T = 5–7 K) with β = 1, accounting for submillimeter excess.

Experimental results

Research questions

• RQ1 How do dust properties in low-metallicity dwarf galaxies like NGC 1569 differ from those in the Milky Way?
• RQ2 What is the contribution of very cold dust (T ≈ 5–7 K) to the total dust mass and submillimeter emission in NGC 1569?
• RQ3 Why is there a significant millimeter excess in the SED, and can it be explained by cold, clumpy dust components?
• RQ4 To what extent do PAHs contribute to the MIR emission in NGC 1569, and what causes their apparent deficiency?
• RQ5 How does the clumpy, inhomogeneous ISM in NGC 1569 affect dust heating and the validity of standard Galactic dust models?

Key findings

• The dust SED of NGC 1569 exhibits a significant millimeter excess, best explained by a population of very cold dust at 5–7 K, contributing 40–70% of the total dust mass.
• The total dust mass is estimated at (1.6–3.4) × 10⁵ M☉, with the cold component dominating the submillimeter emission and likely distributed in small clumps (size ~ a few pc).
• PAHs are severely depleted in NGC 1569, likely due to destructive effects of the intense ISRF in a clumpy medium, with grain sizes dominated by small particles (~3 nm) emitting in stochastic heating mode.
• The gas-to-dust mass ratio is 740–1600, significantly higher than the Galactic value, and the dust-to-metals mass ratio is 1/4 to 1/7, indicating inefficient dust production or retention in low-metallicity environments.
• The derived extinction curve for NGC 1569 shows a steep FUV rise and a reduced 2175 Å bump, resembling the Large Magellanic Cloud’s extinction law, consistent with the modeled dust size distribution.
• The bulk of the cold dust is distributed across star-forming regions rather than the outer galactic disk, supporting a clumpy ISM with a low filling factor (φ ≈ 3–6 × 10⁻⁶), consistent with high [C II] 158 µm emission.