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Session 2 - Spiral, Irregular and Elliptical Galaxies.
Display session, Monday, June 09
South Main Hall,
Eleven early-type galaxies with relatively strong IRAS emission were observed at submillimeter wavewlengths to measure the amount and properties of the dust. Four of the eleven galaxies were detected at at least one wavelength.
The grain emissivity index at submillimeter wavelengths is \beta\sim 1.5-2, with a formal value (assuming the dust properties to be identical for all galaxies in the sample) of \beta=1.6\pm0.2. The total dust masses range from about 10^3 to 10^7M_ødot, and the gas-to-dust ratios range from about 200 to 1200.
The dust colour temperatures are usually higher than those found in normal spiral galaxies; however, this does not require the presence of star formation. The stellar density in the inner regions of early-type galaxies is much higher than in the disks of spirals, and simple models of dust heating by ambient starlight show that the dust in these central regions can easily be heated to the observed temperatures. This model is probably reasonable for the galaxies which emit only a small fraction of their total luminousity in the infrared, but for several of these galaxies L_FIR\sim L_B, and this together with their blue colours suggests the importance of star formation. There is no obvious correspondence between optical colour and total dust mass.
Both the dust temperatures found from the IRAS 60\mu m and 100\mu m data, and the size of the emitting regions deduced from comparison of the small-beam submillimeter with the large-beam IRAS measurements, show that the dust is concentrated towards the inner regions of the galaxies, well within the half-light radii. The dust must still be less centrally concentrated than the stars, suggesting that some of the dust, at least, has its origin from sources other than stellar mass loss. We find no direct evidence for cold dust (T_d\leq 20 K) in any galaxy. However, neither IRAS nor extant submillimeter measurements can rule out the existence of very extended, cold dust, distributed for instance as the known HI envelopes.
Program listing for Monday