[Paper Review] The dust distribution in edge-on galaxies. Radiative transfer fits of V and K'-band images

S. Bianchi|ArXiv.org|May 10, 2007

Galaxies: Formation, Evolution, Phenomena36 references57 citations

TL;DR

This study uses radiative transfer modeling with scattering to analyze V and K′-band images of seven edge-on spiral galaxies, finding that dust disks are radially extended (h_d/h_s ≈ 1.5) and vertically thinner (z_d/z_s ≈ 1/3) than stellar disks, with face-on optical depth τ₀ ≈ 0.5–1.5. No evidence is found for a second, massive, thin dust disk in the K′-band, suggesting that submillimeter dust emission may instead arise from clumpy or optically thick dust clouds with enhanced emissivity.

ABSTRACT

Aims: I have analyzed a sample of seven nearby edge-on galaxies observed in the V and K'-band, in order to infer the properties of the dust distribution. Methods: A radiative transfer model, including scattering, have been used to decompose each image into a stellar disk, a bulge, and a dust disk. The parameters describing the distributions have been obtained through standard X^2 minimization techniques. Results: The dust disks fitted to the V-band images are consistent with previous work in literature: the radial scalelength of dust is larger than that for stars (h_d/h_s ~ 1.5); the dust disk has a smaller vertical scalelength than the stellar (z_d/z_s ~ 1/3); the dust disk is almost transparent when seen face-on (central, face-on, optical depth tau_0 =0.5-1.5). Faster radiative transfer models which neglect scattering can produce equivalent fits, with changes in the derived parameters within the accuracy of full fits including scattering. In the K'-band, no trace is found of a second, massive, dust disk which has been invoked to explain observations of dust emission in the submillimeter. I discuss the model degeneracies and the effect of complex structures on the fitted distributions. In particular, most bulges in the sample show a box/peanuts morphology with large residuals; two lower-inclination galaxies show a dust ring distribution, which could be the cause for the large fitted dust scalelengths.

Motivation & Objective

• To determine the 3D dust distribution in edge-on spiral galaxies using multi-band imaging and radiative transfer modeling.
• To test whether a second, thin, massive dust disk—previously invoked to explain submillimeter emission—is detectable in near-infrared (K′-band) observations.
• To assess the impact of structural complexities (e.g., box/peanut bulges, dust rings) on radiative transfer fitting results.
• To evaluate whether scattering effects significantly alter derived dust parameters compared to models that neglect scattering.

Proposed method

• A 3D radiative transfer model incorporating dust scattering and extinction was applied to V and K′-band images of seven edge-on galaxies.
• The model decomposed each galaxy image into three components: a stellar disk, a spheroidal bulge, and a dust disk, each with radial and vertical exponential distributions.
• Parameters (scalelengths, scaleheights, optical depth) were derived via χ² minimization to fit the observed surface brightness profiles.
• The model was tested both with and without scattering to assess its impact on fitted parameters.
• Fits were performed on high-resolution, deep K′-band images to detect potential signatures of a second, thinner dust disk.
• Model degeneracies and the influence of complex structures (e.g., dust rings, box/peanut bulges) were systematically evaluated.

Experimental results

Research questions

• RQ1 Is a second, thin, massive dust disk—previously proposed to explain submillimeter dust emission—detectable in K′-band images of edge-on galaxies?
• RQ2 How do the radial and vertical scalelengths of the dust disk compare to those of the stellar disk in edge-on spirals?
• RQ3 To what extent do structural complexities such as box/peanut bulges or dust rings affect the derived dust disk parameters?
• RQ4 Does including scattering in the radiative transfer model significantly alter the fitted dust parameters compared to models that neglect scattering?
• RQ5 Can the observed energy balance deficit between absorbed stellar radiation and re-emitted infrared emission be resolved by dust clumpiness or enhanced emissivity rather than an additional dust disk?

Key findings

• The dust disk in the V-band has a radial scalelength about 1.5 times larger than the stellar disk (h_d/h_s ≈ 1.5), indicating a more extended radial distribution.
• The dust disk is vertically thinner than the stellar disk, with a vertical scaleheight ratio of approximately 1/3 (z_d/z_s ≈ 1/3).
• The face-on optical depth of the dust disk is low (τ₀ ≈ 0.5–1.5), confirming it is nearly transparent when viewed face-on.
• No evidence was found for a second, thin, massive dust disk in the K′-band, contradicting earlier models that invoked such a component to explain submillimeter emission.
• Box/peanut bulges were detected in all but one galaxy, causing significant residuals and potentially biasing dust disk parameter estimates.
• Dust ring structures in two lower-inclination galaxies may explain anomalously large fitted dust scalelengths, as the ring’s extinction peak is offset from the galactic center.