Credit: B.G. ELMEGREEN ET AL.& HST/NASA

These may be pictures of sound waves, say B. G. Elmegreen et al. in the 20 August issue of Astrophysical Journal Letters (503, L119-L122; 1998).

Both images show the inner 3,000 light years of the spiral galaxy NGC2207. The one on the right has been processed to highlight the dark dust lanes, which form a tangled spiral pattern. Nothing odd about finding spirals in a spiral galaxy, you might think — but these are different.

There is no sign of rapid star formation, which occurs in the huge spiral arms that define such galaxies. And the conventional spiral-arm-making mechanism — in which correlated stellar orbits produce spiral density waves, compressing interstellar gas and so triggering the formation of bright new stars — doesn't work so near to the nucleus.

Instead, Elmegreen et al. propose that these spirals are amplified sound waves. Gentle acoustic waves are generated further out in the galaxy, perhaps by supernova explosions going off in star-forming regions. As they travel inwards, they should be focused and amplified by the disk-like geometry, to become strong disturbances such as these. If so, they would constitute an impressively loud bass sound, about 56 octaves below middle C.