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Unveiling the Secret of a Virgo Dwarf Galaxy

3 maj 2000

Dwarf galaxies may not be as impressive in appearance as their larger brethren, but they are at least as interesting from a scientific point of view. And sometimes they may have hidden properties that will only be found by means of careful observations, probing the signals of their stars at the faintest level. Such as the entirely unexpected, well developed spiral structure within an otherwise seemingly normal dwarf elliptical galaxy! This is the surprise result of a new study by a team of astronomers [1], headed by Helmut Jerjen from the Australian National University (Canberra) who obtained detailed observations with the ESO Very Large Telescope (VLT) of the dwarf galaxy IC 3328 in the Virgo Cluster of Galaxies, some 50 million light-years away.

Dwarf galaxies

Dwarf galaxies are present in all major clusters of galaxies and dominate by numbers in the universe. They may contain a few (tens of) millions of stars, as compared to galaxies of normal size with hundreds of billions of stars.

About two dozen dwarf galaxies are known in the "Local Group" of galaxies of which the Milky Way galaxy in which we live is also a member. The Large and Small Magellanic Clouds are some of the best known dwarf galaxies - they are of the irregular type - while NGC 147 and NGC 205, two companions to the great Andromeda Galaxy, are of the elliptical type.

Dwarf elliptical galaxies are characterized by their smooth appearance. From various studies, it is known that they are tri-axial ellipsoids of different degrees of elongation. Some are almost spherical while others are more pancake- or cigar-shaped.

Like the elliptical galaxies of normal size, dwarf ellipticals are almost pure aggregates of stars. In contrast, spiral galaxies also contain clouds of gas and dust. The visible mass of spiral galaxies is in a rotating disk. Dwarf ellipticals generally keep their form because of the random motions of their stars.

VLT observations of dwarf elliptical galaxies

Using the FORS1 multi-mode instrument mounted at the first 8.2-m VLT Unit telescope, ANTU, the astronomers observed a series of dwarf elliptical galaxies in the Virgo Cluster of Galaxies, in the constellation of that name (The Virgin).

The primary goal of the observations was to obtain carefully calibrated images of the galaxies in different colours. They can be used to study the distribution of light over the galaxy and thus its content of stars. The galaxies that are found to have smooth light distributions are of special interest, because it is then possible to measure their approximate distance by means of the so-called Surface Brightness Fluctuation method [2].

The distance to the Virgo Cluster is still not known with high accuracy, although it constitutes a most important step towards the universal distance ladder. Any additional determination of this distance would therefore be most valuable.

A matter of a small difference

The central task of the Surface Brightness Fluctuation method is to determine the pixel-to-pixel fluctuations in the light distribution of the galaxy that is due to the finite number of unresolved stars. These fluctuations are obtained by subtracting a suitably smoothed galaxy model from the CCD image.

In the case of the seemingly inconspicuous dwarf galaxy IC 3382, the astronomers made an amazing discovery. When the best fitting model was removed from the observed image, a neat and regular spiral structure appeared in the residual image!

Nothing like this has ever been seen before in a dwarf elliptical galaxy. The light associated with the spiral constitutes a 3% modulation of the surface brightness. To see this effect at all, requires the excellent image quality of FORS1 and ANTU.

The origin of the spiral structure

What is the cause for this faintest and smallest spiral ever discovered in a galaxy? Two possible explanations have been proposed by the astronomers.

It has been known for several decades that the spiral patterns seen in disk galaxies, like for instance in the Milky Way galaxy, are "density waves". The patterns are due to collective oscillations in the gravitational field that moves the stars and gas back and forth.

The presence of a spiral pattern in IC3328 implies that it harbours a thin disk. The available data do not allow to distinguish between a pure disk galaxy, or a disk embedded in a spheroidal mass distribution. Both configurations are known to exist.

Transient spiral patterns, as that seen in the well-known, nearby galaxy Messier 51, can be generated by tidal interactions. In the present case, there are two close and faint dwarf galaxies which may have disturbed IC3328 in the past and thereby produced the spiral pattern we now see.

If what we see is a pure disk galaxy, the exceptionally small amplitude of the spiral pattern suggests another possibility: it could be swing-amplified noise . A modest amount of cold lumpy gas in the disk may have provided some initial "graininess" which was then gradually amplified by a shearing effect of the stellar orbits in the disk to produce the striking spiral pattern we now see.

Noter

[1] The team consists of Helmut Jerjen and Agris Kalnajs (Research School of Astronomy and Astrophysics, Australian National University, Canberra, Australia) and Bruno Binggeli (Astronomical Institute, University of Basel, Switzerland). This work was financially supported by the Swiss National Science Foundation via a fellowship for advanced researchers.

[2] The Surface Brightness Fluctuation method is used to determine the distance to an elliptical galaxy. It is based on the discrete sampling of a galaxy with a CCD detector and the resulting pixel-to-pixel luminosity fluctuations arising from unresolved stars. The amplitude of such fluctuations is inversely proportional to the distance of the galaxy.

Mer information

A research article about this discovery is being published in the European journal Astronomy & Astrophysics; the preprint is now available on the web at astro-ph/0004248.

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