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Dust in External Galaxies

1991; Springer Nature (Netherlands); Linguagem: Inglês

10.1007/978-94-011-3402-6_10

2214-7985

F. Hoyle, N. C. Wickramasinghe,

Astrophysics and Star Formation Studies

Interstellar dust is not a phenomenon in any way peculiar to our own galaxy. Photographs of external galaxies show striking evidence for dust, particularly in spiral and irregular systems (Sandage, 1961). Dust lanes often serve to delineate spiral arms, young stars and HII regions that are present in these galaxies. One of the most dramatic examples of extragalactic dust is to be seen in NGC 4594 (the Sombrero Hat, Fig. 10.1) where the galaxy is divided through its central plane by an opaque dust layer. Scarrott et al. (1987) dicovered linear polarisations of a few percent perpendicular to the dust layer near the extremities of the disk, and attributed this to scattering by grains. A galaxy such as ours viewed edge on would look like this, with a dust layer some 150 pc or so thick along its central plane. Fig. 10.2 shows the dust lanes in M51, which is a spiral galaxy viewed almost face-on. The integrated light from such galaxies shows linear polarisation, indicating the presence of aligned grains. The galaxy NGC 891 shown in Fig. 10.3a is interesting in having protruberences in the dust layer extending normal to its plane, to angular distances of about 30 sec of arc, corresponding to heights of ~ 100 pc. It can be argued that such filaments are evidence of dust being lifted by radiation pressure against the gravitational potential energy of the galaxy. There is other evidence of entire galaxies being shrouded in dust, showing that dust generated within the galaxy is somehow expelled.