Tuning In On "The Antennae": The Ultraluminous Starburst Potential

The large amount of molecular gas and the spectacular images of its very extended and complex distribution are of special importance because they may lead to an entirely different view of the origin and evolution of the galactic system.

The new results were reported by Yu Gao and Robert Gruendl of the University of Illinois, and Kwok-Yung (Fred) Lo, Chorng-Yuan Hwang, Siow-Wang Lee, Wei-Hao Wang and Ting-Hui Lee of the Academia Sinica Institute of Astronomy and Astrophysics in Taipei, Taiwan.

"The high value of the total molecular gas mass--greater than 15 billion suns--was very unexpected because the previously accepted value was only three billion suns," said Gao, who announced the team's findings. "The large gas reservoir we discovered contains sufficient fuel to make the Antennae enter an ultraluminous phase as the colliding pair continues to merge. Eventually, the Antennae will join the ranks of the most luminous objects in the universe."

The galaxy pair NGC 4038/4039 (also known as Arp 244), was nicknamed "the Antennae" because of the resemblance of a pair of very long bright tails that were formed in the collision. Although more than 65 million light-years from Earth, the Antennae is the nearest infrared-luminous, prototypical galaxy-galaxy merger caught in the middle of the action. Accurate observations of the molecular gas are crucial to understanding its star-formation history and evolution.

Detailed imaging of the distribution of molecular hydrogen--which is generally not visible by itself except when heated by shock waves--was obtained using the Berkeley-Illinois-Maryland Association (BIMA) millimeter array in northern California. The astronomers observed the carbon monoxide emission at a wavelength of 3 millimeters to deduce the molecular hydrogen.

The discovery of the large molecular gas mass was based on a careful mapping of the carbon monoxide emission using the 12-meter radio telescope at the National Science Foundation's National Radio Astronomy Observatory in Arizona. The previously accepted value for the total molecular mass had been obtained from a single pointing observation made with the same telescope more than a decade ago. Because this galaxy system is much larger than the telescope's beam, the original observation detected only a small part of the total emission. A detailed mapping of the system's extended carbon monoxide emission was essential for more accurately estimating the total molecular gas.

In addition, the interferometric data from the 10-element BIMA array were combined with the 12-meter, single-dish data to produce a fully sampled synthesis image. "This not only recovered all the extended emission in the system but also gave the best linear resolution scale of about 1,000 light-years, comparable to the size of large associations of giant molecular clouds found in our own galaxy," Gruendl said. "These observations provide clues to the origin and evolution of the Antennae."

The Antennae has a total infrared luminosity of nearly 100 billion suns, barely enough to be classified as a luminous infrared galaxy. Such galaxies are believed to be primarily the result of collisions and mergers of gas-rich spiral galaxies, which lead to a strong enhancement of new star formation.

"Events like these may have been much more numerous in the early universe when galaxy collisions were more frequent," Gao said. "Normal gas-rich spiral galaxies are the building blocks of the luminous infrared galaxies. Late-stage mergers tend to be ultraluminous with an infrared power of more than 1,000 billion suns, comparable to the bolometric luminosity of quasars, the most luminous objects in the universe."

With such a large quantity of raw material available for star formation, the Antennae has the potential to enter an ultraluminous starburst phase in the future, Gao said.

There is a wealth of recent observations of the Antennae, including the Hubble Space Telescope optical images, the Infrared Space Observatory mid-infrared images and numerous observations made in essentially all astronomical windows from radio to X-ray. "But much of our understanding, interpretation and modeling have been derived from the much lower molecular gas mass determined more than a decade ago," Gao said. "It is important to reconsider all of these observations accordingly."

Currently, there is no existing facility capable of mapping the far-infrared emission of the Antennae at a high-enough resolution, Gao said. "However, radio continuum emission at centimeter wavelengths mapped by the Very Large Array in New Mexico can show roughly the same structure because the two are tightly correlated. We have obtained high-resolution Very Large Array data and it shows very good correspondence to our BIMA carbon monoxide map."

The carbon monoxide images of the Antennae, as well as images of another dozen more distant colliding/merging luminous infrared galaxies, were obtained by Gao and Gruendl and their team over the last few years as one of BIMA?s key projects. The goal is to statistically study an entire merger sequence to trace the merging progress and the starburst development by mapping the detailed distribution and kinematics of the star-forming molecular gas.