FOR RELEASE: 9:20 a.m., January 8, 2002

WASHINGTON --- Astronomers today presented evidence that the vast bar-shaped structures common in spiral galaxies sow the seeds of their own destruction by driving interstellar gas towards the galactic center, where the mass buildup ultimately disrupts the orbits of the bar stars. The report is being presented to the American Astronomical Society meeting in Washington, D.C., by Drs. Mousumi Das, Stuart Vogel, Peter Teuben and Andrew Harris of the University of Maryland (College Park), Michael Regan of the Space Telescope Science Institute, Kartik Sheth of California Institute of Technology (CA), Tamara Helfer of the University of California (Berkeley), and Michele Thornley of Bucknell University (PA).

The results are based on the Survey of Nearby Galaxies (SONG) using the Berkeley-Illinois-Maryland Association (BIMA) radio telescope at Hat Creek, California. Emission from the Carbon Monoxide molecule (CO) was observed at a wavelength of 2.6 millimeters, corresponding to a frequency of 115 Gigahertz (GHz). The telescope is operated by the Universities of California (Berkeley), Illinois, and Maryland. More information about BIMA can be found at http://bima.astro.umd.edu.

Many galaxies, including the Milky Way Galaxy in which we live, are spiral galaxies: great rotating collections of billions of stars. In addition to the familiar spiral arms, more than half of all spiral galaxies contain bar-shaped structures that cross their centers. These bars can be as long as 30,000 light years long (the Milky Way has a modest bar buried within its central regions). In extreme cases, the bar is dominant, with the spiral arms that are the hallmark of spiral galaxies trailing from the ends of the bar.

These bars play an extremely important role in a galaxy's evolution. Recent observations and theoretical work have shown that the gravity from a bar is the mechanism that drives interstellar gas from the outer parts of a galaxy inward toward the central regions, and into the galactic nucleus itself. Here this gas fuels tremendous bursts of star formation: a majority of massive stars are born in such starbursts in the nuclei of galaxies. Bars may also channel the material that falls into black holes within active galactic nuclei, releasing enormous power in radiation and particles from tiny regions at the centers of some galaxies. Recent theoretical work and observations show that bars can feed these large accumulations of mass in galactic nuclei, gas with mass exceeding 100,000,000 times the mass of the sun.

The new observations indicate that bars are less pronounced (shorter and rounder) in galaxies with greater central mass concentrations. This is the best evidence so far that supports theoretical computer simulations of bar disruption. Such models predicted that gravity from such large mass accumulations would disrupt the orbits of stars in the bar, causing the bar to dissolve and terminating this portion of a galaxy's evolution. The stars lost from the bar are thought to contribute to the growth of the more spherical galactic bulge that surrounds the nucleus on large scales.

This evidence for bar destruction implies that bars do not persist for the entire lifetime of a galaxy, and shows how galaxies transform between the barred and unbarred state. Since most spiral galaxies have bars, bars may reform during the approximately 10 billion year lifetime of a galaxy. Bar formation might also be triggered by tidal interactions and collisions between galaxies.

Dr. Mousumi Das, the study's lead author, compares the effect to indigestion on a truly cosmic scale: ``After swallowing hundreds of millions of solar masses of gas, the center of the galaxy rebels and dismantles the structures that caused it to grow so large. The cycle may begin again in a few billion years, but that's at least a few billion years of relief."

Dr. Stuart Vogel, one of the co-authors and Director of the Laboratory for Millimeter-wave Astronomy at the University of Maryland, commented ``There's a lot happening in galactic centers, so we were quite surprised to find such a clear indication that galaxy bars self-destruct as their centers grow more massive.'' He noted that ``it will be important to confirm the trend using the next generation of millimeter-wave radio telescopes, in particular CARMA and ALMA, will be able to observe yet more distant barred galaxies at earlier stages of evolution.'' These future observations could also determine the lifetimes and importance of galactic bars at different stages of the Universe's evolution.

SONG also includes optical and infrared images obtained at the Kitt Peak National Observatory in Arizona (operated by AURA) and Palomar Observatory in California (operated by the California Institute of Technology). The 44 spiral galaxies observed by SONG are relatively nearby, with an average distance of about 40,000,000 light years.

This material is based upon work supported by the National Science Foundation (University Radio Facilities unit of the Division of Astronomical Sciences), and by the state of Maryland. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation (NSF).