Tiny Young Galaxies "Full of Stars" Discovered

Brian Handwerk
for National Geographic News

April 30, 2008

A newly discovered type of young galaxy has astronomers echoing David Bowman's famous last words in the novel 2001: A Space Odyssey: "My God, it's full of stars."

While these galaxies are small enough to fit within the central hub of our own Milky Way, they each contain as many stars as larger, more mature galaxies.

The light we see from the densely packed galaxies dates to a time when the universe was relatively young, less than three billion years old.

Previously observed tiny galaxies from this time period had correspondingly small numbers of stars.

But the newfound galaxies—each only about 5,000 light-years across—weigh in at about 200 billion times the mass of the sun.

"The general trend in the nearby universe is that if you have more stars, you tend to have a bigger galaxy," said Yale University's Pieter G. van Dokkum, lead author of a new study on the galaxies.

"Apparently in these early times you could have these small galaxies with a huge number of stars."

Van Dokkum and colleagues now suspect that the small, dense systems could account for half of all the galaxies of similar mass that existed 11 billion years ago.

Young and Rich

Using the Hubble Space Telescope and the W.M. Keck Observatory on Mauna Kea, Hawaii, the team was able to measure the sizes of the distant, compact galaxies.

Further observations with the Gemini South Telescope Near-Infrared Spectrograph in Chile showed that even though the galaxies were young, they had already finished the phase of intense star formation.

"That in itself was a surprise," van Dokkum said.

That leaves scientists wondering how the tiny, star-rich galaxies formed in the first place.

It's possible that in the uneven landscape of the early universe, pockets of dark matter trapped pools of hydrogen gas, which began spinning rapidly and forming stars at a breakneck pace.

Such events "could be quite spectacular," van Dokkum said. "They had to form a lot of stars in a very small [volume] during a very short time."

The find also raises questions about whether the tiny young galaxies contributed to the growth of larger galaxies that exist today.

One theory suggests that the dense galaxies eventually puffed up through collisions and mergers. (See images of galaxies colliding.)

But it's unclear whether this kind of growth could have swelled the objects by a factor of five or six to reach the size of present-day galaxies.

"Could galaxies really decrease that much in density by adding new gas and stars?" asked Eric Gawiser, a cosmologist at Rutgers University who was not involved with the research.

"Or could it be that this team has overestimated the densities in the past due to the difficulty of the measurement?

"One sign that the [team's] result is correct is that the average density of matter in the universe was ten times greater when these ultradense galaxies formed," Gawiser said.

"So everything was denser when the universe was younger."