Hazy, hot, and hidden: dust-laden clouds at the centers of some galaxies may enshroud titanic starburst or baby quasars

Natural History, July-August, 2003 by Charles Liu

Galaxies illuminate the universe, and not just with visible light. Think, for a moment, about an incandescent light bulb--it's not only bright, it's hot, too; and the heat we feel comes mostly from infrared radiation emitted by the bulb's filament. Similarly, stars in galaxies pour out visible light, as well as plenty of radiation at wavelengths beyond the visible, such as infrared and ultraviolet. A typical galaxy (or more precisely, its constituent stars) emits that invisible radiation much the way the light bulb does: roughly proportional to the amount of visible light it emits. Our Milky Way, with its several hundred billion stars, is a fine example of a galaxy that emits light broadly over the range of the electromagnetic spectrum.

There are, of course, some spectacular exceptions. In the 1980s surveys by NASA's Infrared Astronomical Satellite revealed an entire class of galaxies shining far more intensely in infrared light than they did in visible light. We astronomers, straightforward as always, named the group "luminous infrared galaxies"--LIRGs for short--and the brightest among them, "ultra-luminous infrared galaxies" (ULIRGs). Some of the LIRGs emit more than 90 percent of their energy as infrared radiation; the brightest ULIRGs generate a hundred times more infrared energy than the total energy output of the Milky Way--at all wavelengths combined.

Those findings were more than puzzling. No normal population of stars can produce so much infrared energy without generating a corresponding amount of visible light. Something else in those galaxies must be converting most of the visible light into infrared light. For that matter, the total energy output, whatever the wavelength, was (and remains) a mystery. Whatever generates the observed energies streaming out of a LIRG must be far more powerful than any collection of stars known.

In the past twenty years astronomers have learned that the "infrared converter" is a cloud of intervening dust. Now, in a recently published study, Aaron S. Evans, an astronomer at Stony Brook University in New York State, and his collaborators have focused on the riddle of the total output energy. They haven't identified the energy source churning inside LIRGs--mainly because of an obscuring cloud of dusty gas--but they have offered new insight on how LIRGs might be put together.

According to recent measurements, about half of the background light generated in the universe today (not including the cosmic background radiation left over from the big bang) originates from LIRGs. LIRGs typically radiate more energy in just a few seconds than our Sun does in millennia. It is also known that the energy from LIRGs originates from compact regions near their galactic centers.

So what is the likely source of such energy? Think about how stars arise [see "Universe: Dust to Dust," by Neil deGrasse Tyson, May 2003]. Deep within interstellar clouds of gas and dust, dense clumps form and collapse under their own weight. Eventually the collapsed matter becomes so dense and hot that nuclear fusion begins, and the clumps become fledgling stars. Millions of years later, the stars' radiation ionizes the clouds still surrounding them and pushes the clouds away, unveiling the bright new stars to the rest of the universe. But while the stars are being formed, all that's visible from the outside is the dust and gas surrounding them.

The clouds that enshroud the center of a LIRG are thousands of times more massive than typical star-bearing clouds, but they may be analogous. Current models suggest that the object they hide--the energy source for the LIRG's radiation--could take one of two forms. It might be an aggregate of billions of stars formed in a massive burst in the recent past. Or it might be a single supermassive black hole--a newborn quasar, still swaddled in a cocoon of dusty gas--much larger than the black hole now thought to lie at the center of the Milky Way [see "Peering at the Edge of Time," by Fulvio Melia, June 2003]. In that case, the energy output would come from huge amounts of potential energy released by matter falling into the black hole.

In either case, the dust surrounding the energy source would absorb visible and ultraviolet light and re-emit it later as infrared light. That's why almost all the radiation coming from LIRGs is infrared. All that remains, then, is to confirm either one of the two scenarios; if one is confirmed, astronomers will know which objects, stars or black holes, emit half of the background light in the universe.

But as much as we would like to glimpse the center of a LIRG, the dusty, surrounding clouds present a daunting observational obstacle. Dust not only quenches visible light, but it also degrades our view. Imagine watching from a distance as a powerful searchlight shines into a pea-soup fog. From a distance you can see a bright glow coming from a spot in the fog, but you can't make out its source. That's exactly what we have to deal with when we look at a LIRG. Dust in the universe, like earthly fog, scatters light; it turns straight beams of light into a crisscrossing mishmash of unfocused glare. And because the cloud itself is aglow in infrared light--also an effect of dust--seeing inside it is all the harder.