HERschel Observations of Edge-on Spirals (HEROES). IV. Dust energy balance problem (1804.09071v1)

Abstract: We present results of the detailed dust energy balance study for the seven large edge-on galaxies in the HEROES sample using 3D radiative transfer (RT) modelling. Based on available optical and near-infrared observations of the HEROES galaxies, we derive the 3D distribution of stars and dust in these galaxies. For the sake of uniformity, we apply the same technique to retrieve galaxy properties for the entire sample: we use a stellar model consisting of a S\'ersic bulge and three double-exponential discs (a superthin disc for a young stellar population and thin and thick discs for old populations). For the dust component, we adopt a double-exponential disc with the new THEMIS dust-grain model. We fit oligochromatic radiative transfer (RT) models to the optical and near-infrared images with the fitting algorithm FitSKIRT and do panchromatic simulations with the SKIRT code at wavelengths ranging from ultraviolet to submillimeter. We confirm the previously stated dust energy balance problem in galaxies: for the HEROES galaxies, the dust emission derived from our RT calculations underestimates the real observations by a factor 1.5-4 for all galaxies except NGC 973 and NGC 5907 (apparently, the latter galaxy has a more complex geometry than we used). The comparison between our RT simulations and the observations at mid-infrared-submillimeter wavelengths shows that most of our galaxies exhibit complex dust morphologies (possible spiral arms, star-forming regions, more extended dust structure in the radial and vertical directions). We suggest that, in agreement with the results from Saftly et al. (2015), the large- and small-scale structure is the most probable explanation for the dust energy balance problem.

• The paper presents a detailed 3D radiative transfer analysis addressing the dust energy balance problem in seven edge-on spiral galaxies.
• It employs a unified oligochromatic model combining optical, near-infrared, and Herschel far-infrared data to map stellar and dust distributions.
• The study finds that dust emission is underestimated by factors of 1.5 to 4, underscoring the need for models that account for complex dust structures.

Examining the Dust Energy Balance Problem in Edge-on Spiral Galaxies: Insights from HEROES Observations

The paper presented in the paper titled "HERschel Observations of Edge-on Spirals (HEROES): IV. Dust Energy Balance Problem" delivers an in-depth analysis of the dust energy balance in seven edge-on spiral galaxies, utilizing advanced 3D radiative transfer modeling. This analysis aims to address the persistent discrepancy between predicted and observed dust emissions in such galactic structures. Known as the dust energy balance problem, this discrepancy often arises when the radiative transfer models, calibrated with optical and near-infrared wavelengths, fail to reproduce the far-infrared and submillimeter fluxes as detected by instruments like the Herschel Space Observatory.

Methodology and Findings

The research employs a uniform technique for deriving the properties of these spiral galaxies, including both stellar and dust components. The methodology hinges on utilizing oligochromatic radiative transfer models fit to existing optical and near-infrared data to ascertain the three-dimensional distribution of stars and dust. Specifically, the paper utilizes a defined stellar model incorporating a S\ bulge supplemented by three double-exponential discs. For the dust component, the state-of-the-art THEMIS dust-grain model is employed.

Significant discrepancies in dust emissions were noted in the majority of the galaxies studied. Models typically underestimated the observed dust emissions at longer wavelengths by factors ranging from 1.5 to 4, most notably excluding NGC 973 and NGC 5907. It was suggested that the clumpy and complex dust morphologies, which include spiral arms and star-forming regions, could underlie the discrepancies observed. Notably, these structures were corroborated with results from Saftly et al. (2015), indicating that large- and small-scale structural complexities are viable explanations for the dust energy balance problem.

Implications and Future Considerations

The findings of this paper have profound implications for both theoretical and practical aspects of astrophysics. From a theoretical perspective, the research highlights the limitations of current modeling techniques in predicting dust emissions and suggests a necessity for enhanced models that can incorporate the complexity and inhomogeneity of galactic dust structures. Practically, it underscores the importance of high-resolution and multi-wavelength observations in astrophysical research, especially in exploring the interplay between galactic components.

The unresolved issue of the dust energy balance problem prompts questions about the accuracy of current estimates of dust mass and distribution in galaxies, impacting our understanding of galaxy formation and evolution. Further studies should focus on refining radiative transfer models by integrating more sophisticated representations of dust clumping and large-scale structures. Additionally, expanding the sample size and diversity of galaxies can aid in validating these models across different galactic environments.

In conclusion, the insights garnered from the HEROES observations contribute significantly to our understanding of galactic dust components, even as they illuminate the complexities that remain unsolved in modeling these celestial phenomena. Continued exploration and refinement in this field will be crucial for advancing our grasp of the universe's fundamental structures.