[Paper Review] Modelling the spectral energy distribution of galaxies. III. Attenuation of stellar light in spiral galaxies

Richard J. Tuffs, C. C. Popescu+3 more|ArXiv.org|Jan 30, 2004

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy40 references139 citations

TL;DR

This paper presents a physically motivated radiative transfer model for spiral galaxies that separately calculates attenuation for stellar bulges, old disks, and young thin disks, incorporating diffuse and clumpy dust components. It derives polynomial fits for attenuation across UV to near-infrared wavelengths as a function of inclination and optical depth, showing that bulge-to-disk ratio significantly affects observed attenuation, with neglecting bulges leading to overestimation of disk opacity.

ABSTRACT

We present new calculations of the attenuation of stellar light from spiral galaxies using geometries for stars and dust which can reproduce the entire spectral energy distribution from the UV to the FIR/submm and can also account for the surface brightness distribution in both the optical/NIR and FIR/submm. The calculations are based on the model of Popescu et al. (2000), which incorporates a dustless stellar bulge, a disk of old stars with associated diffuse dust, a thin disk of young stars with associated diffuse dust, and a clumpy dust component associated with star-forming regions in the thin disk. The attenuations, which incorporate the effects of multiple anisotropic scattering, are derived separately for each stellar component, and presented in the form of easily accessible polynomial fits as a function of inclination, for a grid in optical depth and wavelength. The wavelength range considered is between 912 AA and 2.2 micron, sampled such that attenuation can be conveniently calculated both for the standard optical bands and for the bands covered by GALEX. The attenuation characteristics of the individual stellar components show marked differences between each other. A general formula is given for the calculation of composite attenuation, valid for any combination of the bulge-to-disk ratio and amount of clumpiness. As an example, we show how the optical depth derived from the variation of attenuation with inclination depends on the bulge-to-disk ratio. Finally, a recipe is given for a self-consistent determination of the optical depth from the Halpha/Hbeta line ratio.

Motivation & Objective

• To develop a physically consistent model for the attenuation of stellar light in spiral galaxies by combining UV, optical, and FIR/submm SED constraints.
• To resolve discrepancies in previous studies by accounting for multiple dust components: diffuse dust in disks, clumpy dust in star-forming regions, and diffuse dust in spiral arms.
• To quantify how inclination and bulge-to-disk ratio affect the observed attenuation of different stellar populations.
• To provide a self-consistent method for deriving optical depth from Hα/Hβ line ratios, especially for galaxies without FIR/submm data.

Proposed method

• Uses the radiative transfer code of Kylafis & Bahcall (1987), including anisotropic multiple scattering, to compute attenuation for three stellar components: bulge, old disk (disk), and young thin disk.
• Models dust geometry with three components: a diffuse dust disk (same scale height as old stars), a second diffuse dust disk co-located with the young stellar disk (for submm emission), and clumpy dust in star-forming regions (for FIR colors).
• Calculates attenuation separately for each stellar component across a grid of face-on B-band optical depth (τ_B^f,disk) and inclination (i), from 912 Å to 2.2 μm.
• Fits the resulting attenuation curves with polynomial functions in τ_B^f,disk and wavelength, enabling interpolation for any combination.
• Analytically models local attenuation in clumpy regions based on geometry, assuming strong heating and spatial correlation with UV-emitting stars.
• Combines global attenuation (from diffuse dust) and local attenuation (from clumps) into a general formula for composite attenuation of integrated light.

Experimental results

Research questions

• RQ1 How does the attenuation of light from different stellar components (bulge, disk, thin disk) vary with inclination and optical depth?
• RQ2 To what extent does the presence of a bulge affect the observed attenuation and the derived optical depth from inclination trends?
• RQ3 Can the Hα/Hβ line ratio be reliably used to infer the face-on optical depth in galaxies lacking FIR/submm photometry?
• RQ4 How do the contributions of diffuse and clumpy dust components affect the overall SED and observed attenuation curves?

Key findings

• The bulge experiences the strongest attenuation at low and intermediate inclinations, with the steepest wavelength dependence, while the thin disk dominates attenuation at edge-on orientations.
• At face-on orientations, the thin disk has the lowest attenuation, but this reverses at high inclinations, where its pathlength through dust becomes dominant.
• Increasing the bulge-to-disk ratio mimics the effect of increasing disk opacity, leading to systematic overestimation of disk optical depth if bulges are ignored.
• The model successfully predicts the UV magnitude of NGC 891 at i = 89.8°, matching observations by Marcum et al. (2001) within 0.1 mag.
• Polynomial fits for Hα/Hβ ratio as a function of inclination and τ_B^f,disk are provided in Table 7, enabling optical depth derivation from emission-line ratios.
• The inclusion of a second diffuse dust disk (coincident with spiral arms) is essential to reproduce the observed submillimeter emission amplitudes.