[Paper Review] The Herschel Exploitation of Local Galaxy Andromeda (HELGA) VII: A SKIRT radiative transfer model and insights on dust heating

S. Viaene, M. Baes+18 more|arXiv (Cornell University)|Sep 27, 2016

Galaxies: Formation, Evolution, Phenomena143 references31 citations

TL;DR

This study presents a 3D radiative transfer model of M31 using the SKIRT code to simulate dust heating by stellar populations. It finds that evolved stars dominate dust heating (91% of absorbed radiation), challenging assumptions linking IR emission directly to young stars, and shows sSFR/NUV−r as a promising tracer of dust heating fractions across galaxies.

ABSTRACT

The radiation of stars heats dust grains in the diffuse interstellar medium and in star-forming regions in galaxies. Modelling this interaction provides information on dust in galaxies, a vital ingredient for their evolution. It is not straightforward to identify the stellar populations heating the dust, and to link attenuation to emission on a sub-galactic scale. Radiative transfer models are able to simulate this dust-starlight interaction in a realistic, three-dimensional setting. We investigate the dust heating mechanisms on a local and global galactic scale, using the Andromeda galaxy (M31) as our laboratory. We perform a series of panchromatic radiative transfer simulations of Andromeda with our code SKIRT. The high inclination angle of M31 complicates the 3D modelling and causes projection effects. However, the observed morphology and flux density are reproduced fairly well from UV to sub-millimeter wavelengths. Our model reveals a realistic attenuation curve, compatible with previous, observational estimates. We find that the dust in M31 is mainly (91 % of the absorbed luminosity) heated by the evolved stellar populations. The bright bulge produces a strong radiation field and induces non-local heating up to the main star-forming ring at 10 kpc. The relative contribution of unevolved stellar populations to the dust heating varies strongly with wavelength and with galactocentric distance.The dust heating fraction of unevolved stellar populations correlates strongly with NUV-r colour and specific star formation rate. These two related parameters are promising probes for the dust heating sources at a local scale.

Motivation & Objective

• To construct a detailed 3D radiative transfer model of M31 based on multi-wavelength observations from the Herschel space telescope.
• To investigate the relative contributions of evolved and young stellar populations to dust heating in a resolved, moderately inclined spiral galaxy.
• To assess the reliability of 2D observational tracers like sSFR and NUV−r color in estimating dust heating fractions at local scales.
• To evaluate the limitations of deprojected models due to geometry, dust grain properties, and star-forming region subgrid treatment.

Proposed method

• Employed the SKIRT radiative transfer code to simulate 3D dust re-emission and stellar radiation fields in M31.
• Used a parametric 3D geometry with distinct components: a thin disk, thick disk, and spheroidal bulge for stars and dust.
• Calibrated the model using observed SEDs and morphologies from far-UV to submillimeter wavelengths.
• Constrained free parameters including dust mass, luminosity of young stellar populations, and intrinsic ionizing luminosity.
• Performed a 3D analysis of the radiation field to quantify contributions from different stellar populations to dust heating.
• Compared model outputs with observed fluxes and morphologies across multiple bands, quantifying deviations via median absolute deviation.

Experimental results

Research questions

• RQ1 What is the relative contribution of evolved versus young stellar populations to dust heating in M31’s interstellar medium?
• RQ2 To what extent can 2D observational proxies like sSFR and NUV−r color reliably trace the true dust heating fraction in galaxies?
• RQ3 How do geometric effects, dust grain properties, and star-forming region resolution impact the accuracy of deprojected radiative transfer models?
• RQ4 Can the observed SED and morphological structure of M31 be consistently reproduced by a 3D radiative transfer model with minimal free parameters?

Key findings

• The model reproduces the observed SED of M31 well, with a broader UV bump in the derived attenuation curve compared to standard templates.
• The median absolute deviation between model and observations across all bands is 22%, with systematic underestimation in star-forming rings and overestimation in inter-ring regions.
• Evolved stellar populations account for 91% of the total absorbed stellar radiation, primarily due to the bright bulge dominating the radiation field out to 10 kpc.
• Young stellar populations contribute 10–30% of the radiation field in most regions, increasing inside and beyond the main star-forming ring.
• The sSFR and NUV−r color show a smooth, continuous relation with dust heating fractions across M31 and M51, suggesting their potential as general tracers.
• Non-local heating and 3D geometry significantly complicate 2D analyses, undermining direct links between IR emission and young stellar populations.