The Dust Content and Opacity of Actively Star-Forming Galaxies

TLDR: In this article, far-infrared (FIR) photometry at 150 and 205 micron(s) of eight low-redshift starburst galaxies obtained with the Infrared Space Observatory (ISO) ISOPHOT is presented.

Abstract: We present far-infrared (FIR) photometry at 150 and 205 micron(s) of eight low-redshift starburst galaxies obtained with the Infrared Space Observatory (ISO) ISOPHOT. Five of the eight galaxies are detected in both wave bands, and these data are used, in conjunction with IRAS archival photometry, to model the dust emission at lambda approximately greater than 40 microns. The FIR spectral energy distributions (SEDs) are best fitted by a combination of two modified Planck functions, with T approx. 40 - 55 K (warm dust) and T approx. 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 approx. 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 resealed for the appropriate metallicity. The ratio between the total dust FIR emission in the range 1-1000 microns and the IRAS FIR emission in the range 40 - 120 microns is approx. 1.75, with small variations from galaxy to galaxy. This ratio is about 40% larger than previously inferred from data at millimeter wavelengths. Although the galaxies in our sample are generally classified as "UV bright," for four of them the UV energy emerging shortward of 0.2 microns is less than 15% of the FIR energy. On average, about 30% of the bolometric flux is coming out in the UV-to-near-IR wavelength range; the rest is emitted in the FIR. Energy balance calculations show that the FIR emission predicted by the dust reddening of the UV-to-near-IR stellar emission is within a factor of approx. 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 approx. greater than 1), UV - bright star-forming galaxies, these galaxies' FIR emission will be generally undetected in submillimeter 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.

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FIG. 3.ÈOpacity at 0.16 km, A(0.16), plotted as a function of the color excess E(B[V ) for a sample of 30 starburst galaxies (Calzetti et al. 1994). E(B[V ) is measured from the ionized gas emission. Symbols are as in Fig. 2. A representative 1 p error bar is given at the bottom right corner of the diagram. The continuous line shows the trend predicted by the starburst obscuration curve k@(j) (eqs. [4] and [5]), after requiring that the opacity of the stellar continuum be zero [A(0.16)\ 0] if the reddening of the ionized gas is zero [E(B[V )\ 0]. The predicted line marks the lower envelope of the correlation between A(0.16) and E(B[V ).

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FIG. 2.ÈOpacity at 0.16 km, A(0.16), plotted as a function of the UV spectral slope b for a sample of 47 starburst galaxies from Kinney et al.Ïs IUE Atlas. The Ðlled triangles mark the position of our Ðve galaxies ; upper limits are indicated as downward arrows. A representative 1 p error bar is given at the bottom right corner of the diagram. A(0.16) is derived from the ratio to UV luminosity. b is a measure of the dust reddeningFIR IRASa†ecting the galaxy. The tight correlation between the two quantities demonstrates that the same amount of dust responsible for the reddening is also responsible for the total obscuration (see Meurer et al. 1999). The continuous line is the trend predicted by the starburst obscuration curve k@(j) (eqs. [4] and [5]). The slope of the line is dictated by the functional shape of k@(j), and the intercept by the UV characteristics of the intrinsic stellar SED (see text).

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