Unveiling a galaxy's power source.

Unveiling a Galaxy's Power Source

In visible light, the galaxy Arp 220 looks unimpressive, but at infrared wavelengths it shines brighter than 100 Milky Ways. Astronomers have now identified two closely spaced sources of intense radiation buried deep within the galaxy's core. The finding suggests that Arp 220 represents a late stage in the merger of two spiral galaxies, with each galaxy having an active nucleus powered by the movement of matter toward a black hole.

"We've caught it at a unique time in its evolution, when the nuclei are very close together," says James R. Graham of the California Institute of Technology in Pasadena.

Once the obscuring dust and gas dissipate, he says, the object will likely emerge as a quasar, emitting prodigious amounts of energy at visible and ultraviolet wavelengths.

Graham and his colleagues report the unveiling of Arp 220 in the May 1 ASTRO-PHYSICAL JOURNAL LETTERS.

At a distance of 250 million light-years from Earth, Arp 220 represents the nearest example of an ultraluminous infrared galaxy. "It is of crucial importance to understand the origin, nature, and ultimate fate of the activity in the ultraluminous infrared galaxies because they are an important constituent of the local universe," the researchers write.

In visible light, Arp 220 appears as a blob split by a strip of dust. But images obtained at a wavelength of 2.2 microns reveal a pair of intense infrared sources hidden behind the dust.

The infrared measurements, combined with earlier radio-wave observations and the galaxy's disturbed appearance, provide convincing evidence that Arp 220 is the product of a galactic merger, the team asserts. "When you compare the radio map with the infrared map and find they are virtually identical, that clinches it," Graham says. Overall, the merged system has some of the characteristics of an elliptic galaxy.

Astronomers have now discovered close pairs of nuclei in four of the 10 known ultraluminous galaxies and have found evidence suggesting mergers in all 10. Arp 220 stands out because its two nuclei are only about 1,000 light-years apart -- the smallest nuclear separation measured to date.

"What's remarkable is that the actual time the two galactic nuclei will stand at that separation is very short," Graham says. "So either we're very privileged to have caught Arp 220 at this particular stage in its evolution, or there's something we don't quite understand that maybe halts the evolution of the binary at that separation."

If black holes are indeed responsible for the observed activity, the merger will probably lead to a system consisting of two black holes orbiting each other. "If this is correct, then a black hole binary ... may be an essential ingredient for many quasars," Graham and his co-workers contend.

According to this scenario, a galactic merger triggers the intense activity responsible for the brightness of ultraluminous galaxies by funneling gas and dust toward the system's center. With an increased fuel supply, the black holes already present in the individual galaxies grow, generating more and more power. When radiation pressure and interstellar winds finally sweep away the dust, the system emerges as a quasar.