Galaxy Diversity Reveals Clues to Cosmic Evolution

Astronomers peering out into our cosmic backyard have long understood that the Milky Way's galactic neighbors only seem similar on the surface. Now a detailed survey from NASA's Hubble Space Telescope has revealed the diversity of those galaxies as they evolve over time.

The ACS Nearby Galaxy Survey Treasury (ANGST) program zeroed in on 14 million stars in 69 nearby galaxies. Such galaxies sit close enough so that Hubble's sharp eyes could single out the brightest stars instead of seeing a giant smear of light, and may help settle raging debates over how galaxies and their stars form in the first place.

"Instead of picking and choosing particular galaxies to study, our survey will be complete by virtue of looking at 'all' the galaxies in the region," said Julianne Dalcanton, head of the ANGST survey at the University of Washington in Seattle. "This gives us a multi-color picture of when and where all the stars in the local universe formed."

A galactic fossil record

The survey covered galaxies ranging 6.5 million light-years to 13 million light-years from Earth. A light-year is the distance light will travel in a year, or about 6 trillion miles (10 trillion kilometers).

Many nearby galaxies contain stars that are positive relics compared with the younger stars in more distant galaxies. Astronomers can guess how those younger stars may evolve based on the older nearby stars.

"Using the galaxies in the nearby universe as a 'fossil record,' we can compare them with young galaxies far away," Dalcanton said. "This comparison gives us a history of star formation and provides a better understanding of the masses, structures and environments of the galaxies."

The formation of galaxies remains poorly understood, although evidence is mounting that massive galaxies grow by clumping together and merging with smaller brethren. This step-by-step evolution of gradually larger galaxies competes with the theory that galaxies simply start from scratch.

However, some observations suggest that galaxies do have the power to expand rapidly. Astronomers found a star factory 12.3 billion light-years away that churns out 4,000 stars per year, compared with our Milky Way's 10 stars annually. That burst of star-making activity suggests such a galaxy would only need 50 million years to grow into one of the largest ever observed.

Stellar evolution

Many such large spiral galaxies created most of their stars early on, according to a separate new study. Also relying on data from the Hubble galactic survey, the astronomers examined the outer disk of M81 and found that most stars formed more than 7 billion years ago, when the universe was half its current age. The supernova deaths of massive stars within such mammoth galaxies rapidly enriched them with heavy elements such as carbon.

"We were surprised by how quickly the elements formed and how the subsequent star-formation rate for the bulk of the stars in M81 changed after that," said Benjamin Williams, an astronomer at the University of Washington in Seattle.

Astronomers had believed that star populations become increasingly younger further out within galaxies, but Hubble data indicated that older stars can also lurk in a galaxy's outer arms. Recent simulation work has presented another possible explanation — stars may wander all over spiral galaxies. The survey should provide further data to flesh out current theories.

"With this information, we will be able to trace the complete cycle of star formation in detail," Dalcanton said.

The galactic survey results were submitted for publication in The Astrophysical Journal Supplement Series. The research on M81's star-formation has been submitted to The Astronomical Journal.

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