Newly-Discovered "Missing Link" Changes Ideas About Elliptical Galaxies

The elliptical galaxy NGC 3656, thought to be a remnant of a giant collision between two galaxies, is at a distance of 150 million light-years. The visible light from the stars in the galaxy is shown in red, and the radio emission from CO molecules is shown in blue.  Photo Credit: Space Telescope Science Institute (Digitized Sky Survey), L. Young, and BIMA.
FOR RELEASE: 10:00 a.m., MDT, Tuesday, June 4, 2002

Elliptical galaxies, once thought to be simple, "dead" structures, are more complex and lively than expected, according to an astronomer who has discovered a "missing link" in their life cycle. "The structure of elliptical galaxies is turning out to be much more complex than we previously thought," said Dr. Lisa Young, of the New Mexico Institute of Mining and Technology (NM Tech) in Socorro, NM. "Furthermore, the life history of these galaxies now seems more complex and more interesting than we imagined," Young added.

Young used the Berkeley-Illinois-Maryland Association (BIMA) radio telescope at Hat Creek, California, to make detailed images of molecular gas in a number of elliptical galaxies. Unexpectedly, she found that such gas -- the raw material for new stars -- forms a rotating disk inside the egg-shaped elliptical galaxies. She presented her results to the American Astronomical Society's meeting in Albuquerque, NM.

For many years, elliptical galaxies were thought to be "dead" galaxies: no new stars were being born because there was no molecular gas from which to form them. Spiral galaxies, in contrast, are rich in molecular gas and are actively forming new stars. However, astronomers recently came to realize that many elliptical galaxies (such as the ones studied by Young) do have enough raw material to sustain some star formation.

Elliptical galaxies also were thought to have very simple structure. Stellar systems such as star clusters, elliptical galaxies, and spiral galaxies can avoid collapsing under their own weight only if the stars are moving fast enough. The stars in elliptical galaxies are moving in mostly random directions, which gives these galaxies their puffy appearance. The stars of spiral galaxies all are rotating in the same direction around the center of the galaxy, which produces a very thin circular disk.

Most spiral galaxies also contain small, puffy "bulges" of randomly moving stars in their central regions. Careful studies of the motions of stars within elliptical galaxies have shown that some of these galaxies have small, thin stellar disks embedded within their puffy forms. These rotating disks inside elliptical galaxies are normally difficult to see because they typically include only about 1 in 10 of the galaxy's stars.

"The existence of these stellar disks in elliptical galaxies has been known for a couple of decades," said Young, "but before now we had little more than educated guesses about their origins."

Young's new images showing disks of molecular gas are the first observational evidence that the rotating stellar disks in elliptical galaxies must have begun as rotating gas disks. The molecular gas gradually transformed itself into stars, and the gas disk became a stellar disk. The molecular gas disks Young discovered have approximately the right masses and sizes to be the precursors of the stellar disks.

Supporting evidence for this interpretation comes from emissions in the infrared and radio portions of the spectrum (wavelengths of 100 microns and 20 cm), measured by the InfraRed Astronomy Satellite (IRAS) and by the National Radio Astronomy Observatory's Very Large Array. "These emissions imply that star formation is indeed taking place in the gas disks," said Danielle Rundle, a graduate student at New Mexico Tech. If star formation proceeds at the reasonable rates inferred from the infrared emission, the transformation from gas disks to stellar disks would probably occur over a few billion years.

Young used the BIMA array to make images of molecular gas in a sample of elliptical galaxies. The molecular gas reveals itself through radio emissions from molecules of carbon monoxide (CO) at a frequency of 115 GHz. Carbon monoxide is not the most abundant molecule in outer space, by a large margin, but it is generally the easiest molecule to detect. The signals from the 10 individual antennas of the BIMA array were combined to produce an image with much finer detail than could be obtained from any one of the antennas alone. The final image shows where the molecular gas is within the galaxy and also in what direction and how fast the gas is moving.

Young cautions that additional work remains to be done to confirm or disprove her hypothesis. For example, if star formation is occuring in the ellipticals with molecular gas, then there should be small, rotating stellar disks already present inside these ellipticals. In addition, it seems clear that at least some of the disky ellipticals can be explained by the mechanism that Young proposes, but it is not yet known whether all of the disky ellipticals could have arisen this way. Finally, mapping the molecular gas in a larger number of elliptical galaxies would help astronomers to understand the origin of the molecular gas disks and how they evolve.

Future telescopes such as the Combined Array for Research in Millimeter-wave Astronomy (CARMA) and the National Radio Astronomy Observatory's  ALMA array will make it possible to extend this type of study to many more elliptical galaxies.

The BIMA array is operated by the Universities of California (Berkeley), Illinois, and Maryland, with support from the National Science Foundation. The BIMA array is described in more detail at  http://bima.astro.umd.edu .  Young's results are based upon work supported by the National Science Foundation.

For more information: Dr. Lisa Young
  (505) 835-5104 voice
  (505) 835-5707 FAX
  lyoung@physics.nmt.edu