Dust outflows from starburst galaxies

We present submillimeter SCUBA images of the nearby, starburst galaxies NGC 253, NGC 4631 and M82 (primarily at wavelengths of 450 and 850 μm). The edge-on orientation of our targets, in conjunction with the resolution of the instrument (15′′ FWHM at 850 μm), allows us to probe the optically-obscured centre and resolve the minor-axis superwind from the main disk. For NGC 253 and M82, we detect a bright source, a few hundred parsec in diameter, which corresponds to the central starburst. In the case of M82, the bi-lobal nature of the central source is suggestive of a torus or ring. We confirm the existence of a dust outflow along the minor axis of M82 and make a similar (but somewhat more tentative report) for the other two galaxies NGC 253 and NGC 4631. The scale-size of the ‘vertical’ features is 0.7–1.2 kpc. A mass of 106−7M is infered for the outflowing grains in M82. We conclude that this amount of grain material could either have accrued from an inflow along the disk (e.g. a bar) or, if the lower mass limit applies, have been synthesized by massive stars in the starburst. The ejected grains are probably travelling close to the escape velocity of the host galaxy and assuming, hypothetically, that they do manage to breach the halo, we expect superwinds to expel up to 10% of the dust residing in interstellar disks into the intergalactic medium.