The Dust Content and Opacity of Actively Star‐forming Galaxies

TLDR

Starburst galaxies contain a cool dust component heated by both the interstellar radiation field and the starburst itself. The authors obtained 150 µm and 205 µm far‑infrared photometry of eight nearby starburst galaxies with the ISO Photometer and combined these measurements with archival IRAS data to model dust emission at wavelengths longer than 40 µm. The FIR spectral energy distributions are best described by two modified Planck components (warm 40–55 K and cool 20–23 K, ε = 2), with the cool dust contributing up to 60 % of the total flux, having a mass up to 150 times that of the warm dust, and bringing the gas‑to‑dust ratios close to Milky Way values; the total 1–1000 µm FIR emission is about 1.75 times the IRAS 40–120 µm range, and dust‑reddening predictions agree with observations within a factor of two per galaxy and 20 % on average.

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.