Australasian Science: Australia's authority on science since 1938

Swallowed Galaxy Coming Towards Us

By Stephen Luntz

The Siding Springs Observatory has been involved in the discovery of the closest remnant of a small galaxy that has been swallowed by the Milky Way.

Known as the Aquarius Stream, after its position in the sky, the evidence for the former galaxy comes in the form of a small number of giant stars coming in our direction at speeds of about 180 km/s. The closest of these stars is 1500 light years away, and they form a long, thin stream stretching out to a distance of 25,000 light years.

The galaxy was absorbed into our own around 700 million years ago, which is recent enough for the common motions of the stars to be detectable.

Dr Ken Freeman of the Mt Stromlo Observatory says that the stars’ origins were detected through a combination of their “anomalous motions” and their low iron concentrations. It is known that stars formed in small galaxies have much lower ratios of iron to hydrogen than similar-aged stars formed in the Milky Way, with the smallest galaxies having the lowest ratios.

“The ratio in this case suggests the galaxy was probably slightly smaller than the Small Magellanic Cloud,” Freeman says. However, with only 15–18 stars to work on, the estimate has a fair degree of error.

Freeman says that the Radial Velocity Experiment, which discovered the stream, only considers stars down to about 13th magnitude – visible with good backyard telescopes. One of the lines of research he is hoping to pursue is to look for fainter stars whose movement indicates that they are part of the same stream, providing a larger sample to study. He also hopes to do more detailed spectroscopy on the stars observed, giving an insight into the concentration of a wider range of elements.

Increasing evidence has appeared in recent years to suggest that the Milky Way was formed by cannibalising smaller galaxies while also absorbing the outer reaches of smaller dwarf galaxies whose cores remain as globular clusters. However, while indications have been found of stars that were once part of other galaxies, Freeman says the Aquarius Stream is the closest yet found. “We were able to detect it because it is pointed towards us. If it was perpendicular across the sky it would be much harder.”