Cosmologists have long been occupied with trying to understand how galaxies formed and evolved into their intricate shapes and forms. With François Hammer and colleagues' results of a comparative census of galaxy types at two different epochs in the Universe's history, they will become even busier (R. Delgado-Serrano et al. Astron. Astrophys. 509, A78; 2010).

In his early-twentieth-century classification scheme, Edwin Hubble categorized the galaxies seen in today's Universe into three groups according to their visual appearance: ellipticals, spirals and lenticulars (the last being an intermediate type between the first two). Odd-shaped assemblies of gas, stars and dust that don't fall into any of these categories are termed peculiars.

But is the census of today's Hubble galaxies similar to that of moderately distant galaxies? The conventional view has been that it is. Surprisingly, Hammer and colleagues' scrutiny of two samples — 148 galaxies selected from the Great Observatories Origins Deep Survey, seen as they were 6 billion years ago, and 116 local galaxies from the Sloan Digital Sky Survey — finds that it isn't.

Credit: NASA/ESA/SDSS /F. HAMMER & R. DELGADO-SERRANO (OBS. PARIS)

The authors demonstrate that, in the distant sample, the fraction of peculiar galaxies (pictured) is about five times higher than that of their present-day counterparts, and that spirals were about 2.3 times less abundant. In striking contrast, the number of ellipticals and lenticulars is essentially the same.

But there's much more to Hammer and colleagues' study than counting galaxies and sorting them into classes. Their investigation suggests that many of the distant peculiar galaxies evolve to become today's spiral galaxies through gas-rich galactic collisions — a conclusion that is at odds with the belief that galaxy collisions lead to the formation of ellipticals. Such evidence needs to be taken into account in testing theories of galaxy formation and evolution.