Dwarf galaxies may not be the most impressive members of the cosmic menagerie, but they sometimes have hidden qualities. A team of astronomers headed by Helmut Jerjen from the Australian National University (Canberra) has made detailed observations with the ESO Very Large Telescope (VLT) which show that the seemingly normal dwarf galaxy IC 3328 has an unexpected spiral structure. This structure is the faintest and smallest spiral ever seen in any galaxy.

The deep R-band CCD image of IC3328 (left; rendered in "negative" with dark objects and a bright background) illustrates the overall morphology of this galaxy that was classified as a dwarf elliptical galaxy. After removal of the axis-symmetrical part of the light from the galaxy by a special image processing algorithm, the "residual" image reveals a remarkable 2-armed spiral structure (right). Photo: ESO

The discovery was made during a survey of dwarf members of the Virgo Cluster of galaxies, some 50 million light years away. The primary goal of the observations with the FORS1 instrument on the 8.2 metre ANTU telescope was to obtain carefully calibrated images of the galaxies in different colours in order to find out more about their distances. To the amazement of the observing team, the processed image of IC 3328 revealed a regular two-armed spiral structure - something never seen before in a dwarf elliptical galaxy.

Spiral patterns in large galaxies such as the Milky Way are attributed to density waves - oscillations in the gravitational field that cause the stars and gas clouds to shift position within the galaxy. The presence of the spiral structure in IC 3328 implies that it has a thin disk. Whether it is a pure disk galaxy or a hybrid with a disk embedded in a spheroidal mass of stars is not yet known.

It may be that the spiral is the result of past tidal interactions with two nearby dwarf galaxies. Another possibility is the presence of cold clumps of gas in the disk region which caused a shearing effect on the stellar orbits in the disk to produce the pattern we now see.

Dwarf galaxies are present in all major clusters of galaxies and are the most numerous type of galaxy in the Universe. They may contain a few million stars, compared with the hundreds of billions of stars in galaxies such as the Milky Way. Dwarf ellipticals contain very little gas or dust, and are almost pure congregations of stars. Random motions of these stars allow them to keep their shape.

An article on this discovery will appear in the journal Astronomy & Astrophysics.