Your Friendly Neighborhood ULIRG The two main strategies used to study the background leave something to be desired. The intensive surveys push technology to and beyond its limits, and population synthesis is rather abstract. Astronomers have therefore developed a third strategy: scour the nearby universe for counterparts to the distant type 2 galaxies.
They have found their answer in the galaxy NGC 6240. It is one of the black sheep of the Milky Way’s neighborhood—a member of an exotic class known as ultraluminous infrared galaxies (ULIRGs). Such galaxies emit most of their total energy output in the far-infrared, a telltale sign that they are saturated with dust. Because dust consists of heavy chemical elements that are synthesized in stars and scattered through space when those stars die, prodigious amounts of dust imply prodigious star formation.
Whereas the Milky Way is making a few new stars a year, NGC 6240 must be churning out hundreds. Not only is NGC 6240 wracked by star formation, it is cursed with one of the most voracious black holes in the nearby universe.
The overall spectrum of NGC 6240 has the same shape as that of the cosmic background. It contains all the ingredients we need to explain the background, although we still need to mix them in the right proportions.
Seeing what NGC 6240 looks like, astronomers have realized that the unexpected prevalence of type 2 AGNs in the early universe has a natural explanation: the AGNs were accompanied by bursts of star formation. Stars spewed dust, which hides the holes from our view. Indeed, an accumulating body of evidence indicates that star formation and black-hole feeding were much more common in the past than today. The two processes seem to have hit their peak at roughly the same era in cosmic history.
Why do AGNs and starbursts occur hand in hand? No one yet knows. It seems quite likely that the two processes have the same underlying cause: galaxy collisions, which cause gas to spiral toward the center of the galaxy, whereupon it either forms stars or falls into the hole. Nearly all ULIRGs, including NGC 6240, show signs of having undergone a collision with another galaxy. On the other hand, not all AGNs seem to be associated with major collisions.
Many researchers think the connection between AGNs and starbursts may be much tighter than merely having a common source of fuel. Black holes could directly stoke the fires of star formation, or stars could help funnel material into the hole. Stars and supermassive holes might even be symbiotic, unable to exist without one another. Such connections might account for the correlation between the properties of galaxies and their central holes [see “The Hole Shebang,” by George Musser; Scientific American, October 2000].
Fortified by studies of NGC 6240 and its ilk, astronomers have used population synthesis to see whether AGNs and starbursts could explain not just the x-ray background but also the optical and infrared backgrounds. The answer appears to be no. Joint observations by Chandra and the SCUBA instrument, which observes at submillimeter wavelengths between the far-infrared and radio, have failed to note much overlap. Omar Almaini of the Royal Observatory in Edinburgh, Scotland, and his collaborators estimate that up to 30 percent of the cosmic infrared background is ultimately generated by AGNs. Hasinger and his colleagues have combinedXMM and Infrared Space Observatory measurements of the Lockman Hole, putting a lower limit—15 percent—on the AGN contribution to the infrared background.
Elese N. Archibald of the Joint Astronomy Center in Hilo, Hawaii, and her co-workers have explained these findings as a natural sequence of galaxy formation. In their scenario, each galaxy forms around a seed black hole of relatively low mass (10 to 1,000 solar masses). At first, starlight dominates the total output of the galaxy, because the little hole still has to grow. The hole does so exponentially by swallowing material as fast as it can. After about 500 million years, the hole is so fat—a billion solar masses—that infalling material outshines the stars. A quasar is born. After a while, this quasar has eaten all the available fuel and falls asleep until new gas falls into the center, waking it up. The hole may also merge with another of like size.