Posted: July 30, 2008
In a landmark study of more than 2,000 spiral galaxies, Hubble astronomers have found that barred spiral galaxies were far less abundant 7 billion years ago than they are today in the local Universe, an important ramification for galaxy evolution.
Galaxy bars – the ribbon of stars and gas that slice through the nucleus of a spiral galaxy – are thought to form when stellar orbits within the galaxy become unstable and deviate from a circular path. These tiny elongations in the orbit grow until they get locked into place, making a bar, which becomes even stronger as it attracts more and more stars from the galaxy’s inner region. Since huge amounts of gas are forced towards the galaxy’s centre, fueling new star formation, creating central bulges and even feeding massive black holes, bars are perhaps one of the most important catalysts for changing the morphology and behaviour of a galaxy.
"Bars pull stars and gas out of their normal circular orbits into the central regions, perhaps even funneling gas to the central supermassive black hole,” says Nicholas Scoville, COSMOS principal investigator. “Without this fueling, the black holes would be starved and the central regions of galaxies devoid of young stars."
Four barred spiral galaxies in the recent Hubble census. The galaxies are at different locations from Earth (see individual captions). Image: NASA, ESA, K. Sheth (Spitzer Science Center, California Institute of Technology, Pasadena, Calif.), and P. Capak and N. Scoville (California Institute of Technology).
COSMOS, or the Cosmic Evolution Survey, allowed Kartick Sheth of the Spitzer Science Centre at the California Institute of Technology, and colleagues, to analyse over ten times more spiral galaxies than ever before studied in one single research programme, revealing that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with nearly 70 percent of their modern counterparts.
"The recently forming bars are not uniformly distributed across galaxy masses, however, and this is a key finding from our investigation," says 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."
Astronomers already know that evolution is generally faster for more massive galaxies that form their stars early and fast, compared with low-mass galaxies that form more slowly, but now they have evidence for these low-mass galaxies also forming their bars slowly, too.
"The formation of a bar may be the final important act in the evolution of a spiral galaxy," says Sheth. "Galaxies are thought to build themselves up through mergers with other galaxies. After settling down, the only other dramatic way for galaxies to evolve is through the action of bars."
Our own Milky Way Galaxy is another fine example of a massive barred spiral which likely formed earlier rather than later, but by studying bars in the more distant galaxies as well, astronomers will learn more about how bar formation and evolution occurred here in our own cosmic backyard.