Did small galaxies come together to make big galaxies?

Astronomers believe that it’s from humble “building blocks” that mighty galaxies like our own Milky Way grew into giants.

Astronomers think giant galaxies like our own Milky Way formed when smaller galaxies smashed together. The little ones are thought to be torn to pieces. Their material – mostly stars, gas, and dust – formed into new structures in the new big galaxies. So, it’s hard for astronomers to know what the small ‘building block’ galaxies once looked like.

But in 1998 astronomers discovered a dwarf galaxy that now appears to be a leftover ‘building block.’ The Cetus dwarf galaxy is thought to be like a brick which might have gone into building a house – but instead was never used.

It lies far from two big nearby galaxies – 2.5 million light-years from our Milky Way galaxy and nearly as far from the Andromeda galaxy.

If this dwarf galaxy had passed close to another larger galaxy, the encounter would have torn stars out of the dwarf. Astronomers don’t see these stars so they assume no such encounter has taken place. And that means the Cetus dwarf galaxy might be a pristine building block of big galaxies like our own.

It’s like a brick that might have gone into building a house, but was never used. This galaxy’s stars number in the millions – a far cry from the hundreds of billions of stars that throng the Milky Way. Yet astronomers believe that it’s from such humble building blocks that mighty galaxies like our own grew into giants.

EarthSky