And the mystery is that these galaxies--nine of them, each only 5,000 light years in diameter and with as many stars as normal, much larger galaxies--have been measured to be 11 billion light years away. That means we're supposedly seeing them as they appeared 11 billion years ago. Now, according to big bang cosmology, the universe is about 14 billion years old. The compact galaxies we see today should appear to be only 3 billions years old. So what's the problem?

The problem is that galaxies so young shouldn't be so compact. The apparent engine of galaxy formation is gravity, and the longer gravity can work on a system, the more compact the system should get. But most of the "old" galaxies we observe today are not as compact, comparatively speaking. If galaxies do indeed change from one type into another, what phenomenon caused these young compact galaxies to become less compact with age?

In a Hubble news release today, astronomers pondered the issue: 

 "Seeing the compact sizes of these galaxies is a puzzle," said Pieter G. van Dokkum of Yale University in New Haven, Conn., who led the study. "No massive galaxy at this distance has ever been observed to be so compact. It is not yet clear how they would build themselves up to become the large galaxies we see today. They would have to change a lot over 11 billion years, growing five times bigger. They
could get larger by colliding with other galaxies, but such collisions may not be the complete answer."

. . . .

How did these small, crowded galaxies form? One way, suggested van Dokkum, involves the interaction of dark matter and hydrogen gas in the nascent universe. Dark matter is an invisible form of matter that accounts for most of the universe's mass. Shortly after the Big Bang, the universe contained an uneven landscape of dark matter. Hydrogen gas became trapped in puddles of the invisible material and began spinning rapidly in dark matter's gravitational whirlpool, forming stars at a furious rate.

*Tirade warning*
I hate it when scientists have to invoke dark matter to explain conundrums. No one knows what dark matter is. Using dark matter to explain a mystery seems to be like labeling a major engine part a "thingamajig" in hopes that other mechanics will have no qualms installing them, although none of them know what the "thingamajig" actually is. I'll look forward to the day when we have more information to work with than the simple label, "dark matter." Then maybe we'll know with more certainty whether dark matter and hydrogen gas ever had such a furious relationship.
*Tirade warning ended* 

 In the meantime, we can enjoy the discovery of compact galaxies, and puzzle along with astronomers as they observe and study these "small" exhibits of creation.

Image courtesy NASA, ESA, A. Feild (STScI), and P. van Dokkum (Yale University)