The global dust SED: Tracing the nature and evolution of dust with DustEM

Abstract: The Planck and Herschel missions are currently measuring the farIR-mm emission of dust, which combined with existing IR data, will for the first time provide the full SED of the galactic ISM dust emission with an unprecedented sensitivity and angular resolution. It will allow a systematic study of the dust evolution processes that affect the SED. Here we present a versatile numerical tool, DustEM, that predicts the emission and extinction of dust given their size distribution and their optical and thermal properties. In order to model dust evolution, DustEM has been designed to deal with a variety of grain types, structures and size distributions and to be able to easily include new dust physics. We use DustEM to model the dust SED and extinction in the diffuse interstellar medium at high-galactic latitude (DHGL), a natural reference SED. We present a coherent set of observations for the DHGL SED. The dust components in our DHGL model are (i) PAHs, (ii) amorphous carbon and (iii) amorphous silicates. We use amorphous carbon dust, rather than graphite, because it better explains the observed high abundances of gas-phase carbon in shocked regions of the interstellar medium. Using the DustEM model, we illustrate how, in the optically thin limit, the IRAS/Planck HFI (and likewise Spitzer/Herschel for smaller spatial scales) photometric band ratios of the dust SED can disentangle the influence of the exciting radiation field intensity and constrain the abundance of small grains relative to the larger grains. We also discuss the contributions of the different grain populations to the IRAS, Planck and Herschel channels. Such information is required to enable a study of the evolution of dust as well as to systematically extract the dust thermal emission from CMB data and to analyze the emission in the Planck polarized channels. The DustEM code described in this paper is publically available.

• The paper presents the DustEM model as a robust tool to simulate both dust emission and extinction profiles in the interstellar medium.
• It leverages far-infrared to millimeter data from missions like Planck and Herschel to constrain model parameters and study dust evolution.
• Key findings show that amorphous carbon better accounts for gas-phase carbon levels, improving our approach to CMB foreground removal.

Insights into Interstellar Dust with the {\tt DustEM} Model

Interstellar dust plays a crucial role in shaping the physics and chemistry of the interstellar medium (ISM), influencing a range of processes from the thermal equilibrium of interstellar gas to the formation of molecular hydrogen. The paper "The global dust SED: Tracing the nature and evolution of dust with {\tt DustEM}" introduces an extensively comprehensive numerical tool, {\tt DustEM}, designed to model the emission and extinction characteristics of dust grains of various types, structures, and size distributions within the ISM.

Overview of the Study

The authors leverage data from ongoing missions such as Planck and Herschel, which capture the far-infrared to millimeter emission of galactic interstellar dust. This data, in conjunction with existing infrared observations, provides a full spectral energy distribution (SED) of dust emission, enabling an elaborate study of dust evolution processes. These processes include grain growth and fragmentation, both of which cause redistribution of dust mass among different grain sizes, thereby affecting the SED's characteristics.

The paper introduces the {\tt DustEM} model, which can predict both the emission and extinction profiles of dust given specific properties like size distribution and thermal properties. Notably, the model's design allows it to incorporate new dust physics easily, thus making it a versatile tool for astrophysical research. The focus is on studying the diffuse high-galactic latitude (DHGL) region, which serves as a natural reference SED for dust evolution investigations. Key dust components incorporated into this model include polycyclic aromatic hydrocarbons (PAHs), amorphous carbon, and amorphous silicates.

Key Findings and Implications

The study presents a coherent set of observations for the DHGL SEDs obtained through correlating infrared and HI-21 cm data. Observational data have been used to constrain the model parameters, optimizing the fit to both the extinction and emission data. Results indicate that amorphous carbon, as opposed to graphite, better accounts for observed gas-phase carbon abundances in shocked ISM regions because of its susceptibility to alteration by shock-induced processes.

The {\tt DustEM} model also illustrates how variations in the exciting radiation field intensity and constraints on small-grain abundances influence the dust SED. This ability to delineate these influences allows researchers to parse the data for evidence of dust evolution processes occurring in various ISM environments.

Practical and Theoretical Implications

Practically, these findings have significant implications for removing the thermal dust emission from cosmological observations, such as those targeting the cosmic microwave background (CMB). Theoretical implications of the research underscore the need for a more nuanced understanding of dust grain physics, as these processes have direct implications for the evolution of the ISM and, subsequently, galaxy evolution.

Future Directions

Future developments in the field should build upon the compelling combination of observational data and enhanced modeling capabilities provided by {\tt DustEM}. Further observational campaigns, particularly those involving polarized dust emissions which {\tt DustEM} will eventually model, will provide additional constraints that will refine dust models even further. Moreover, continued improvements in computational modeling of the complex evolutionary processes affecting dust will offer deeper insights into the life cycle of dust in various galactic and extragalactic environments.

In summary, the {\tt DustEM} model represents a significant step forward in the analysis of interstellar dust properties, providing a robust tool for both tracking the evolution of dust and serving as a benchmark for theoretical and observational studies.