The Dust Content and Opacity of Actively Star-Forming Galaxies

TL;DR

This study uses ISO 150 μm and 205 μm photometry, combined with IRAS data, to model the far-infrared emission of eight nearby starburst galaxies. The authors test single- and two-component modified Planck fits with emissivity ε=2, finding that five galaxies require a warm (≈40–55 K) and a cool (≈20–23 K) dust component, while two galaxies are well described by a single warm component; the cool dust can dominate the thermal FIR and yields large cool-to-warm dust masses. The analysis links the UV–to–near-IR energy absorbed by dust to the observed FIR output, supporting a starburst obscuration curve with R_V' ≈4.05 and showing the curve predicts FIR emission within a factor of ~2 for individual galaxies on average. The results imply that high-redshift UV-bright galaxies may be faint in sub-mm surveys unless they are exceptionally luminous or harbor substantial cool dust, highlighting the importance of dust temperature distributions and bolometric corrections for interpreting early-universe star formation.

Abstract

(Abridged) We present far-infrared (FIR) photometry at 150 micron and 205 micron of eight low-redshift starburst galaxies obtained with the ISO Photometer. Five of the eight galaxies are detected in both wavebands and these data are used, in conjunction with IRAS archival photometry, to model the dust emission at lambda>40 micron. The FIR spectral energy distributions (SEDs) are best fitted by a combination of two modified Planck functions, with T~40-55 K (warm dust) and T~20-23 K (cool dust), and with a dust emissivity index epsilon=2. The cool dust can be a major contributor to the FIR emission of starburst galaxies, representing up to 60% of the total flux. This component is heated not only by the general interstellar radiation field, but also by the starburst itself. The cool dust mass is up to ~150 times larger than the warm dust mass, bringing the gas-to-dust ratios of the starbursts in our sample close to Milky Way values, once rescaled for the appropriate metallicity. The ratio between the total dust FIR emission in the range 1-1000 micron and the IRAS FIR emission in the range 40-120 micron is ~1.75, with small variations from galaxy to galaxy. The FIR emission predicted by the dust reddening of the UV-to-nearIR stellar emission is within a factor ~2 of the observed value in individual galaxies and within 20% when averaged over a large sample. If our sample of local starbursts is representative of high-redshift (z>1), UV-bright, star-forming galaxies, these galaxies' FIR emission will be generally undetected in sub-mm surveys, unless (1) their bolometric luminosity is comparable to or larger than that of ultraluminous FIR galaxies and (2) their FIR SED contains a cool dust component.