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.
"Pretty much all of the compact massive galaxies were thought to be missing from the nearby universe," says Alister Graham of Swinburne University of Technology in Melbourne, Australia. "Very few compact massive galaxies had been found locally, just a handful."
Much like Earth's dinosaurs were killed by a cosmic collision, computer simulations showed that these dinosaur galaxies of the early universe could have been destroyed through mergers and collisions with each other.
Many astronomers thought this explained the discrepancy – but there was one problem: if there were that many mergers, we should see a lot more of those galaxies orbiting one another and heading towards collisions than we actually do.
"It was known that there are not enough mergers; this was an unexplained problem," says Graham.
Now, Graham and his colleagues think they have an explanation. When they took a closer look at surveys of galaxies in the local universe, they found many had been mischaracterised. 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.
Those bulges have "exactly the same physical mass and compact size as the galaxies in the early universe," Graham says. This suggests that the vast majority of compact spheroids that were thought to have disappeared aren't actually missing – they've just grown a disc, possibly by gathering hydrogen gas and stars from smaller galaxies but without major mergers.
"The original, compact spheroid of stars remains basically unchanged in their centres," says Graham. "They were hiding in plain sight." Astronomers were misled because unless those disc galaxies are facing edge-on to our line of sight, they can look like 3D clouds of stars.
The results suggest that there are 1000 times more of these galaxies in the local universe than previously thought – roughly as many as there were in the early universe.
Graham says at least part of our own galaxy's central bulge may once have been one of these compact galaxies. The disc that formed around it would have contributed some stars to the bulge, as could other processes such as mergers.
Emanuele Daddi at the French Alternative Energies and Atomic Energy Commission was one of the first researchers to notice the apparent excess of compact spherical galaxies in the early universe. "The idea did not occur to us that they could actually be bulges of local [disc galaxies] that had not yet grown their discs," says Daddi. "Neither did the few hundred papers that subsequently studied the problem consider this idea."
Daddi thinks there is a remaining mystery. The bulges in the nearby galaxies seem larger than those in the early universe, which leaves him with some doubt that this explanation will definitively solve the problem.