Interstellar matter in elliptical galaxies - III. Properties of dust extinction

Abstract

We have used multicolour CCD surface photometry of 10 elliptical galaxies with largescaledust lanes or rings to investigate the wavelength dependence of the dust extinction. In general, the extinction curves are found to run parallel to the Galactic curve of Rieke & Lebofsky. The ratio of total to selective extinction Rv is found to range between 2.1 and 3.3, and is lower on average than the canonical Galactic value of 3.1, implying that the ‘large’ dust grains – which are responsible for the extinction of optical light – are generally smaller than in our Galaxy. We show that systematic effects concerning the location of the dust within the galaxy body, which may modify the observed extinction curves, do not change this conclusion. We have estimated the dust content of the lanes directly from total extinction values (e.g., Av ), using the observed grain sizes. Extinction efficiencies of refractory grains and a grain size distribution are taken from the literature. The dust masses calculated using the total extinction values turn out to be up to 35 per cent lower than those calculated using the Galactic ratio of neutral hydrogen column density to E(B – V) colour excess, showing that the effect of smaller ‘large’ grain sizes on the total dust mass is significant. We find that the galaxies with Rv values (and, hence, characteristic grain sizes) that are smaller than in our Galaxy have smooth, regularly distributed dust lanes, whereas the dust in galaxies with ‘normal’ grain sizes is typically much more irregularly distributed, e.g., in patches. Assuming that the dust in these galaxies has an external origin, this suggests that the observed characteristic dust grain size is determined by the time elapsed since the dust lane was accreted from outside. Following the current theoretical model calculations of formation time-scales of regular dust lanes and rings in early-type galaxies, we predict that elliptical galaxies containing regular dust lanes with typical dust masses of 106 – 107 Mʘ do not contain hot, X-ray-emitting gas.