Millions of missing galaxies found hiding in plain sight

Millions of ancient galaxies thought to be all but extinct today seem to have been hiding in plain sight, concealed by discs of stars stolen from other galaxies. Even our own Milky Way may be hiding one in its centre.

In 2005, astronomers found a mysterious excess of compact spherical galaxies in the early, distant universe. These galaxies, which appeared about a third the size of similarly shaped ones in our own backyard with a comparable mass, were abundant about 11 billion years ago but seemed to be scarce today. The local universe is dominated by large “elliptical” galaxies – giant clouds of stars with little structure – and disc galaxies like our own Milky Way.

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> “the vast majority of compact spheroids that were thought to have disappeared aren’t actually missing – they’ve just grown a disc”

That’s a weird and possibly correct interpretation of galaxy evolution. Look at it in these terms – where did spiral and barred-spiral galaxies come from? I have seen studies in which spiral galaxies become barred spirals and the other way around. I’ve seen studies of small irregular galaxies merging into bigger ones. This is the first time I’ve seen a realistic suggestion (other than as an interpretation of Hubble’s fork) that spiral galaxies evolved from compact elliptical ones.

> More careful analysis of images revealed that 21 galaxies that originally looked like big 3D clouds of stars – “giant elliptical galaxies” – were actually flat 2D disc galaxies with bulges in the middle.

Not a huge number, it’d be interesting to see the analysis.