Dust in the Cores of Early-Type Galaxies

Abstract

The dust properties of all early-type galaxies imaged with the Planetary Camera of the HST in cycles I-III are examined. Dust is detected in 31 out of 64 galaxies, although the sample is biased against the detection of dust. From the distribution of observed axis ratios of the dust features it is inferred that 78 \pm 16% of early-type galaxies contain nuclear dust. From the dependence of the angle between dust major axis and galaxy major axis on size and appearance of the dust, it is found that dust features with semi-major axis size a_d > 250 pc are generally not settled. If it is assumed that dust lanes with a_d < 250 pc are in equilibrium in the galaxy potential, the observed distribution of misalignment angles can only be explained if elliptical galaxies have a mean triaxiality >= 0.4. The best fit is obtained for = 0.8, higher than values derived through other methods. This difference cannot be explained by a dependence of $\langle T \rangle$ on radius. Therefore, it seems dust is not always completely relaxed, even on the smallest scales. Furthermore, the rotation axis of the dust does not generally coincide with that of the stars. This seems to suggest that the dust is acquired from outside, and has some way of losing its angular momentum efficiently.