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Ghostly galaxy discovered lurking on the edge of the Milky Way

A team of astronomers from the University of Cambridge have discovered a strange galaxy next door to the Milky Way. The dwarf galaxy, named Antlia 2, is dark and dim and gives out much less light than expected, hence being described as “ghostly.”

Dwarf galaxies were some of the earliest galaxies to form after the big bang, and because they are so ancient most of their stars are old and have low levels of metals. They have relatively few stars within them — from 100 million to several billion stars, far fewer than the 200 billion to 400 billion stars that are within our Milky Way galaxy. Dwarf galaxies are usually small as well, making them vulnerable to being pulled apart by neighboring galaxies which are much larger and more massive.

Antlia 2 is unusual for a dwarf galaxy, because it is much larger than is typical. It is as big as the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way, which is about one third the size of the Milky Way itself. Even more unusually, Antlia 2 gives out very little light. It is 10,000 times fainter than the LMC, which means that it is unexpectedly dim for such a large galaxy.

L-R: Large Magellanic Cloud, the Milky Way, Antlia 2 V. Belokurov based on the images by Marcus and Gail Davies and Robert Gendler

Researchers believe that Antlia 2 is dim because it is effected by the gravitational tides of the Milky Way. Although the ghost galaxy is far enough away from the Milky Way not to be torn apart by it — always remaining at least 40 kiloparsecs (130,000 light years) away — it is still affected by the huge mass of the larger galaxy. Antlia 2 has relatively very little mass to it, so it is highly susceptible to these gravitational forces. However, it would be expected that Antlia 2 would be small as well as low mass, and researchers cannot currently explain why Antlia 2 is so large. Normally the gravitational pull of a larger galaxy would cause a dwarf galaxy to shrink, not to grow.

Two possible explanations for Antlia 2’s low mass and large size have been offered: Firstly, it could have started off as a large galaxy and then grown even larger due to the vigorous birth of stars which expands gas around them, weakening the gravitational pull to the center of the galaxy. Alternatively, a different understanding of dark matter may be required. Previously it was believed that dark matter congregated in the center of galaxies, but it may be that dark matter is less prone to clustering than currently thought and could spread throughout a galaxy.

The scientific paper is available in pre-publication archive arXiv.

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