Sparks of luminous energy fly when two galaxies meet. But scientists studying those galactic unions can't say yet if the bulk of the emission comes from the galaxies' offspring of bright young stars, or from the black holes they think lie at the heart of the new union.
Recently, a group of astronomers, including a brother, sister and spouse, found evidence for supernovae and black holes in Arp 220, a recent merger of two spiral galaxies. They think that much of Arp 220's luminosity is created by massive young stars that give rise to the supernovae. But when the dust settles in several million years, they suspect that a luminous quasar will have formed from the black holes at its heart.
"The sky is littered with objects in various stages of evolution," said Dr. Colin Lonsdale of MIT's Haystack Laboratory. "If we can figure this puzzle out -- what happens when galaxies collide -- then we can go and look at systems that are in different stages of merging and check out our ideas about how these systems evolve."
Dr. Lonsdale worked on the Arp 220 research with his sister, Dr. Carol Lonsdale of the California Institute of Technology, and her husband, Professor Harding Smith of the University of California at San Diego, and Dr. Philip Diamond of the National Radio Astronomy Observatory in Socorro, NM.
Dr. Lonsdale commented on the unusual constellation of siblings and spouses working together on the research team. "It's not that unusual for astronomers to marry and work together," he said, referring to his sister and her husband, "but I think our team is bordering on the unique."
Their work adds to our understanding of galaxy formation and how black holes -- which are believed to lie at the center of all galaxies, even our own Milky Way -- may become visible for a while as extremely bright quasars.
The Arp 220 galaxy displays a brilliant luminosity, about one hundred times brighter than normal, but at invisible infrared wavelengths. It's the closest good example of what scientists call an LIG, or Luminous Infrared Galaxy.
There are two leading schools of thought on the source of LIGs' luminosity, and the Lonsdale team's research suggests that both schools may be right. The first theory -- that new stars forming as a result of the collision release the luminous energy -- can be confirmed by the new research. But their evidence indicates that the other hypothesis -- that matter being sucked into the black holes at Arp 220's heart is the energy source -- may also be correct.
It's now a question of which energy source is predominant, and Dr. Lonsdale believes that could vary from one galactic collision to the next.
"We think we've found something that really sheds a lot of light on this problem," he said. "We're starting to get concrete numbers, but we need to try to reduce the uncertainties. We still don't know which of the two provides the dominant energy source, though at the moment I'd probably put my money on star formation in Arp 220."
The researchers published their work in two papers in the January 20 issue of the Astrophysical Journal of Letters.
Dr. Lonsdale said the research also indicates that the accepted model for masers (naturally occurring microwave equivalents of lasers) in external galaxies requires major revision, a discovery that may provide a new way for astronomers to measure the characteristics of black holes in colliding galaxy systems. By studying the masers in fine detail, astronomers can trace the gas motions caused by the gravity of hidden black holes, and estimate the black holes' contribution to Arp 220's luminosity.
WHEN WORLDS COLLIDE
When two galaxies collide and merge, they send massive clouds of gas and dust spiraling inward toward the center of the merging system, effectively blocking the center of activity from view.
Thus, the heart of the new merged galaxy, where all the action is taking place, can't be studied by direct optical observation, even with the most powerful telescopes. Instead, scientists must look at radio and infrared wavelengths to calculate mass and luminosity and to make inferences about other aspects of the galaxy, like age and stage of life.
The astronomers gathered information on Arp 220, the 220th object in Halton Arp's Atlas of Peculiar Galaxies, using 17 radio telescopes set up in Europe and the United States. They found about a dozen tiny, unresolved points of radio emission scattered randomly over a region measuring a few hundred light-years in size. (A light-year is about six trillion miles.) Each of those points of energy is caused by a supernova explosion that has occurred in the last few years of observation, said Dr. Lonsdale.
"When you get a burst of star formation [which happens when galaxies collide], you get stars forming in a wide variety of masses. The higher-mass stars that will eventually turn into supernovae burn their hydrogen fuel quickly. So one of the signatures of active, ongoing star formation is a high frequency of supernovae," he said. Each supernova fades from the view of radio telescopes after only a decade or so.
Right now, Arp 220 boasts the brightest group of supernovae known to exist -- each one as bright in radio waves as the brightest supernova ever discovered -- and it is a prodigious producer. By contrast, the Milky Way currently generates only a single supernova every century or so; Arp 220 is thought to produce as many as two per year. This implies the formation of huge numbers of bright, young stars in a starburst, which can account for the vast energy output of Arp 220.
But the astronomers also found indications that at least some of Arp 220's luminosity is generated by the black holes at its heart.
"The evidence for supermassive black holes [at Arp 220's center] remains circumstantial at this point, but if the masers are indeed tracing such objects in Arp 220, the holes can be expected to contribute significantly to the overall energy budget of the galaxy," Dr. Lonsdale said.
How much of Arp 220's total energy budget is produced by the black holes and how much by the starburst is still unclear, but the researchers hope to figure that out with additional observations of the newly merged galaxy.
Dr. Lonsdale's research at the Haystack Observatory was funded by the National Science Foundation.
A version of this article appeared in MIT Tech Talk on February 25, 1998.