Seven elusive dwarf galaxy groups revealed

The discovery forms an early piece of the galactic evolution puzzle. 

Four dwarf galaxies identified by astronomers. These tiny galaxies can offer insight into the formation of larger ones, such as the Milky Way.
Kelsey E Johnson, Sandra E Liss and Sabrina Stierwalt

A piece of the galactic growth chart has been revealed with seven gangs of tiny galaxies, long-sought by astronomers, confirmed in Nature Astronomy.

The finds provide insights into how mid-sized galaxies, such as our own Milky Way, formed.

Astronomers think most medium-to-large galaxies grew through collisions. You can see evidence for such mergers – streams of stars and gas can be flung out as two galaxies combine.

The Milky Way and our nearest major galactic neighbour Andromeda are on a collision course, tipped to combine into a larger galaxy in around four billion years.

Of course, that's a long wait. So researchers find and examine groups of dwarf galaxies, 10 to 1,000 times smaller than the Milky Way, to see if they might show signs of such mergers.

The problem is dwarf galaxies are hard to find, let alone groups of them. Given the universe is 13.8 billion years old, it's harder still to find galactic groups out on their own in space consisting only of dwarfs.

Systems previously identified were quite close to a large galaxy, or the galaxies in the group were very far from each other – conditions that could affect their behaviour.

So to hunt down tightly bound dwarf galaxy congregations that were far enough from massive galaxies, Sabrina Stierwalt from the National Radio Astronomy Observatory in the US and colleagues searched the Sloan Digital Sky Survey for dwarf galaxy pairs. They turned up 60 candidates.

Confirmation came from observations from telescopes such as the 3.5-metre telescope at Apache Point Observatory in the US and Magellan Telescope in Chile. And given time, it's thought they'll merge into intermediate-mass galaxies.

They even found one dwarf galaxy, dubbed DDO68, which appeared to be the product of a collision of two even tinier galaxies. It too had star streams indicating a merger.

This example, the researchers write, will be "fertile ground" for future telescopes and deeper surveys such as the planned Large Synoptic Survey Telescope.

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Belinda Smith is a science and technology journalist in Melbourne, Australia.
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