Astronomers spot faint dwarf galaxy nearby. Why did they miss it before?

An international research team has discovered Virgo 1, an extremely faint dwarf satellite galaxy in the halo of the Milky Way. 

Astronomers in Hawaii have spotted – for the first time – a tiny galaxy not so far away, prompting some to wonder how many other nearby galaxies they might have missed.

An international research team discovered Virgo 1 about 280,000 light years away from us, which places it in an exclusive group of nearby galaxies known as our neighbors. Astronomers say that Virgo 1’s dimness made it difficult to spot before now.

"This discovery implies hundreds of faint dwarf satellites waiting to be discovered in the halo of the Milky Way," said lead astronomer Masashi Chiba of Tohuku University. "How many satellites are indeed there and what properties they have, will give us an important clue of understanding how the Milky Way formed and how dark matter contributed to it."

Astronomers at Tohuku University led the international team that discovered Virgo 1, using a giant telescope mounted camera called the Hyper-Suprime Cam. The discovery was part of the ongoing Subaru Strategic Survey, which uses the Subaru telescope to collect astronomical data.

Currently, researchers know of about 50 satellite galaxies in the halo of the Milky Way. About 40 of those galaxies are faint dwarf galaxies, yet this week’s announcement could mark the faintest galaxy ever discovered.

Virgo 1 has an absolute magnitude of -0.8, which makes it very dim and difficult to spot indeed. In contrast, bright stars such as Rigel and Betelgeuse have absolute magnitudes of -7 and -5.6 respectively.

The discovery of this tiny, dim galaxy in the halo of the Milky Way (within a neighborhood of 1.4 million light years) is also significant for another reason – it lends credence to a longstanding theory of galaxy formation.

Galaxies like the Milky Way are formed “through the hierarchical assembly of dark matter, forming dark halos, and through the subsequent infall of gas and star formation affected by gravity,” according to the Subaru Survey researchers.

Milky Way formation models predict hundreds of dim galaxies like Virgo 1 orbiting the much larger galaxy, but only a few dozen have been found. Researchers say that the discovery of Virgo 1 could breathe new life into theories that predict hundreds of orbiting galaxies – perhaps they have just not been discovered yet.

"We have carefully examined the early data of the Subaru Strategic Survey with HSC and found an apparent over density of stars in Virgo with very high statistical significance, showing a characteristic pattern of an ancient stellar system in the color-magnitude diagram," said graduate student Daisuke Homma of the planetary discovery.

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