Largest galaxies grow up gradually like snowflakes

Space 2 March 2011
Formed from the inside out

THEY may be monsters of the universe, but elliptical galaxies can start life in the same way as snowflakes.

It has long been assumed that these most massive of galaxies form when two smaller spiral-shaped galaxies collide. But there is an alternative theory in which a cloud of gas collapses in on itself to form a dense core of stars which then grows larger by assimilating smaller galaxies over time. This is similar to how ice crystals build up around a microscopic dust grain as it falls to Earth, forming a snowflake.

Now there is evidence that a massive elliptical galaxy called NGC 1407 formed in this way. Duncan Forbes of Swinburne University of Technology in Hawthorn, Victoria, Australia, and colleagues used the colours of the star clusters in NGC 1407 to estimate its chemical composition. They found the concentration of heavy elements was highest at the core’s centre, decreasing towards its edges, which tallies with the gas cloud collapse theory. That’s because the gravity at the cloud’s centre would be stronger than at its edges, concentrating the heavy elements produced in stars there.

This heavy element gradient did not extend to the galaxy’s outer regions. This is consistent with a gradual assimilation of star clusters from already formed, smaller galaxies there, Forbes’s team report (

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