Early galaxies are strangely stuffed with stars

(Image: NASA/ESA/Anderson/van der Marel)

In this image of the Milky Way star cluster Omega Centauri, bright stars have been coloured blue, faint ones red. For more distant galaxies, though, faint stars are impossible to see. Now it turns out some of the most distant galaxies in the universe are more packed with stars than astronomers expected.

The only information galaxies give astronomers comes from the light of their stars. But stars don't carry all of a galaxy's mass. Some of the mass is tied up in invisible dark matter, which can't be measured directly.

To get around that, astronomers generally estimate a galaxy's mass by charting how its stars move with respect to each other. They compare the amount of visible mass to the total mass, and assume the rest can be made up with dark matter.

"To estimate exactly what is the mass that they have, we always use this conversion factor, to convert light into mass," says University of Oxford astronomer Michele Cappellari. "The conversion we used for many decades was wrong."

Cappellari found that the relationship between visible light and stellar mass is not the same for all galaxies -- it varies from galaxy type to galaxy type. The mismatch was worst for the most distant galaxies, which are three times more star-filled than expected; astronomers had not been counting faint stars - like the red ones above - in these distant galaxies.

That raises a new cosmic conundrum: when we see distant galaxies, we see them as they were when they were very young, but how did these star-stuffed galaxies get to be so big so early on in their lives? "They need to grow faster than people thought," Cappellari says.