Washington, July 30 : In a landmark study of more than 2,000 spiral galaxies from the largest galaxy census conducted by NASA's Hubble Space Telescope, astronomers found that so-called barred spiral galaxies were far less plentiful 7 billion years ago than they are today, in the local universe.
A frequent sign of the maturity of a spiral galaxy is the formation of a ribbon of stars and gas that slices across the nucleus.
The study's results confirm the idea that bars are a sign of galaxies reaching full maturity as the "formative years" end.
The observations are part of the Cosmic Evolution Survey (COSMOS).
This new detailed look at the history of bar formation, made with Hubble's Advanced Camera for Surveys, provides clues to understanding when and how spiral galaxies formed and evolved over time.
A team led by Kartik Sheth of the Spitzer Science Center at the California Institute of Technology in Pasadena discovered that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with nearly 70 percent of their modern counterparts.
Bars have been forming steadily over the last 7 billion years, more than tripling in number.
"The recently forming bars are not uniformly distributed across galaxy masses, however, and this is a key finding from our investigation," explained Sheth. "They are forming mostly in the small, low-mass galaxies, whereas among the most massive galaxies, the fraction of bars was the same in the past as it is today," he added.
The findings, according to Sheth, have important ramifications for galaxy evolution.
"We know that evolution is generally faster for more massive galaxies: They form their stars early and fast and then fade into red disks. Low-mass galaxies are known to form stars at a slower pace, but now we see that they also made their bars slowly over time," he said.
Bars form when stellar orbits in a spiral galaxy become unstable and deviate from a circular path.
"The tiny elongations in the stars' orbits grow and they get locked into place, making a bar," explained team member Bruce Elmegreen of IBM's research Division in Yorktown Heights, N.Y. "The bar becomes even stronger as it locks more and more of these elongated orbits into place. Eventually a high fraction of the stars in the galaxy's inner region join the bar," he added.
According to team member Lia Athanassoula of the Laboratoire d'Astrophysique de Marseille in France, "The new observations suggest that the instability is faster in more massive galaxies, perhaps because their inner disks are denser and their gravity is stronger."