Astronomers catch star-forming turning into 'red and dead' elliptical galaxies

Credit: Wolfgang Steffen, Cosmovision; Marscher et al., NRAO/AUI/NSF

Astronomers catch star-forming turning into 'red and dead' elliptical galaxies.

ScienceDaily  — Astronomers using the partially completed ALMA observatory have found compelling evidence for how star-forming galaxies evolve into 'red and dead' elliptical galaxies, catching a large group of galaxies right in the middle of this change.

For years, astronomers have been developing a picture of galaxy evolution in which mergers between spiral galaxies could explain why nearby large elliptical galaxies have so few young stars. The theoretical picture is chaotic and violent: The merging galaxies knock gas and dust into clumps of rapid star formation, called starbursts, and down into the maws of the supermassive black hole growing in the merger's core. As more and more matter heaves onto the black hole, powerful jets erupt, and the region around the black hole glows brilliantly as a quasar. The jets blowing out of the merger eventually plow out the galaxy's potential star-forming gas, ending the starbursts.

Until now, astronomers had never spotted enough mergers at this critical, jet-plowing stage to definitively link jet-driven outflows to the cessation of starburst activity. During its Early Science observations in late 2011, however, ALMA became the first telescope to confirm nearly two dozen galaxies in this brief stage of galaxy evolution.

What did ALMA actually see? "Despite ALMA's great sensitiviy to detecting starbursts, we saw nothing, or next to nothing -- which is exactly what we hoped it would see," said lead investigator Dr. Carol Lonsdale of the North American ALMA Science Center at the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia. Lonsdale presented the findings at the American Astronomical Society's meeting in Austin, Texas on behalf of an international team of astronomers.