Article

# Dust temperature tracing the ISRF intensity in the Galaxy

http://dx.doi.org/10.1051/0004-6361/201014540 01/2010; DOI:10.1051/0004-6361/201014540

ABSTRACT New observations with ensuremath Herschelensuremath<?iensuremath> allow accurate measurement of the equilibrium temperature of large dust grains heated by the interstellar radiation field (ISRF), which is critical in deriving dust column density and masses. We present temperature maps derived from the ensuremath Herschelensuremath<?iensuremath> SPIRE and PACS data in two fields along the Galactic plane, obtained as part of the Hi-GAL survey during the ensuremath Herschelensuremath<?iensuremath> science demonstration phase (SDP). We analyze the distribution of the dust temperature spatially, as well as along the two lines-of-sight (LOS) through the Galaxy. The zero-level offsets in the ensuremath Herschelensuremath<?iensuremath> maps were established by comparison with the IRAS and ensuremath Planckensuremath<?iensuremath> data at comparable wavelengths. We derive maps of the dust temperature and optical depth by adjusting a detailed model for dust emission at each pixel. The dust temperature maps show variations in the ISRF intensity and reveal the intricate mixture of the warm dust heated by massive stars and the cold filamentary structures of embedded molecular clouds. The dust optical depth at 250 ensuremath Î?ensuremath<?iensuremath>m is well correlated with the gas column density, but with a significantly higher dust emissivity than in the solar neighborhood. We correlate the optical depth with 3-D cubes of the dust extinction to investigate variations in the ISRF strength and dust abundance along the line of sight through the spiral structure of the Galaxy. We show that the warmest dust along the LOS is located in the spiral arms of the Galaxy, and we quantify their respective IR contribution.

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##### Article: Dust emissivity variations in the Milky Way
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ABSTRACT: Aims. Dust properties appear to vary according to the environment in which the dust evolves. Previous observational indications of these variations in the far-infrared (FIR) and submillimeter (submm) spectral range are scarce and limited to specific regions of the sky. To determine whether these results can be generalised to larger scales, we study the evolution in dust emissivities from the FIR to millimeter (mm) wavelengths, in the atomic and molecular interstellar medium (ISM), along the Galactic plane towards the outer Galaxy.Methods. We correlate the dust FIR to mm emission with the HI and CO emission, which are taken to trace the atomic and molecular phases, respectively. The study is carried out using the DIRBE data from 100 to 240 $\mu$m, the Archeops data from 550 $\mu$m to 2.1 mm, and the WMAP data at 3.2 mm (W band), in regions with Galactic latitude |$b$| $\le$ 30°, over the Galactic longitude range (75° < l < 198°) observed with Archeops. We estimate the average dust temperature in each phase and divide the emission spectral energy distribution (SED) by a black body at this temperature to derive the emissivity profile. A detailed verification of the impact of the implied simplification, such as temperature mixing along the line of sight, is provided.Results. In all regions studied, the emissivity spectra in both the atomic and molecular phases are steeper in the FIR ($\beta$ = 2.4) than in the submm and mm ($\beta$ = 1.5). We find significant variations in the spectral shape of the dust emissivity as a function of the dust temperature in the molecular phase. Regions of similar dust temperature in the molecular and atomic gas exhibit similar emissivity spectra. Regions where the dust is significantly colder in the molecular phase show a significant increase in emissivity for the range 100–550 $\mu$m. We exclude the possibility of this effect being an artifact of our temperature determination or the assumptions made. This result supports the hypothesis of grain coagulation in these regions, confirming results obtained over small fractions of the sky in previous studies and allowing us to expand these results to the cold molecular environments in general of the outer MW. Possible reasons for the observed emissivity increase in the molecular phase that vanishes in the mm range are discussed by comparison with dust models, involving dust aggregation and solid state physics processes specific to amorphous material. We note that it is the first time that these effects have been demonstrated by direct measurement of the emissivity, while previous studies were based only on thermal arguments.
http://dx.doi.org/10.1051/0004-6361/200811246. 01/2009;
• ##### Article: Evolution of dust properties in an interstellar filament
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ABSTRACT: We present submillimetre observations obtained using the balloon-borne experiment PRONAOS/SPM, from 200 to 600 $\mu$m with an angular resolution of 2–3.5$^{\prime}$, 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 $\mu$m 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.
http://dx.doi.org/10.1051/0004-6361:20021309. 01/2003;
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##### Article: Far-infrared spectral observations of the Galaxy by COBE
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ABSTRACT: We derive Galactic continuum spectra from 5-96/cm from COBE/FIRAS observations. The spectra are dominated by warm dust emission, which may be fit with a single temperature in the range 16-21 K (for nu^2 emissivity) along each line of sight. Dust heated by the attenuated radiation field in molecular clouds gives rise to intermediate temperature (10-14 K) emission in the inner Galaxy only. A widespread, very cold component (4-7 K) with optical depth that is spatially correlated with the warm component is also detected. The cold component is unlikely to be due to very cold dust shielded from starlight, because it is present at high latitude. We consider hypotheses that the cold component is due to enhanced submillimeter emissivity of the dust that gives rise to the warm component, or that it may be due to very small, large, or fractal particles. Lack of substantial power above the emission from warm dust places strong constraints on the amount of cold gas in the Galaxy. The microwave sky brightness due to interstellar dust is dominated by the cold component, and its angular variation could limit our ability to discern primordial fluctuations in the cosmic microwave background radiation.
The Astrophysical Journal 09/1995; 451:188-199. · 6.73 Impact Factor