The Galactic Odd Couple [Preview]

Why do giant black holes and stellar baby booms, two phenomena with little in common, so often go together?

Black holes have a bad reputation. In many ways, it is deserved. They are the most efficient engines of destruction known to humanity. Their intense gravity is a one-way ticket to oblivion for anything that strays too close; inside them is undiscovered country from whose bourn no traveler returns. We see them only because the victims do not go quietly to their doom. Material spiraling into a black hole can heat up to millions of degrees and glow brightly. Some of its kinetic energy and momentum may be transferred to a jet of particles flowing outward at close to the speed of light. Black holes of various sizes take the rap for fusillades of radiation and plasma that astronomers observe all over the cosmos.

Yet black holes are not all-powerful. Even those found at the centers of many galaxies, supermassive black holes--whose very name connotes a voracious monster that rules its galactic roost--are minuscule by cosmic standards. They typically count for less than a percent of their galaxy's mass, and their gravity is highly concentrated. Accordingly, astronomers long assumed that supermassive holes, let alone their smaller cousins, would have little effect beyond their immediate neighborhoods. Star formation farther out in the galaxy was thought to march to the beat of a different drummer.

So it has come as a surprise over the past decade that black hole activity and star formation are closely intertwined. In many galaxies where black holes devour material greedily--generating a phenomenon that astronomers call an active galactic nucleus (AGN)--stars form at a precipitous rate in episodes known as starbursts. How can these two seemingly disconnected processes be so intimately related?

Today the AGN-starburst connection is a revolutionary area of research. Beautiful Hubble Space Telescope images are allowing astronomers to pick apart the complex events at the hearts of galaxies, the Chandra X-ray Observatory is peering into places hidden to Hubble, and theorists are trying to make sense of it all. This research bears on some of the most basic questions in astronomy: How did the dark early universe come to light up with billions of stars? Did supermassive black holes need a helping hand to grow to be so big? Could they be agents of creation as well as destruction?

Galaxies on Steroids

BOTH ACTIVE GALACTIC NUCLEI and starbursts are among the most spectacular phenomena in the universe. An AGN is a luminous and compact source of light at the center of a galaxy. Quasars are the most extreme example. Pumping out as much power as a billion to a trillion suns, AGNs can outshine the rest of their host galaxies. The supermassive black holes that are thought to power them pack a million to a billion times the sun's mass inside a region smaller than 1,000 times the sun's diameter. Like a falling rock, material spiraling toward the hole picks up speed and releases energy as it collides with other material. In so doing, it gives off radiation at all wavelengths: radio, infrared, optical, ultraviolet, x-ray, gamma-ray.

Starburst galaxies rival the brilliance of AGNs. They are places where gas condenses into stars at a rate equivalent to producing up to 1,000 suns a year--1,000 times faster than stars currently form in our own galaxy. Some starbursts are confined to comparatively small regions, only hundreds of light-years across, located near the center of a galaxy; others occur on much larger scales, sometimes tens of thousands of light-years across. Starbursts often take place in galaxies that are going through, or have recently undergone, a close encounter or merger with a neighboring galaxy. The tidal forces between the two galaxies disrupt gas and cause it to fall inward, greatly accelerating the normal process by which interstellar clouds collapse and form stars. A starburst typically lasts about 10 million years before running out of gas (literally).

Like AGNs, starburst galaxies shine at a wide range of wavelengths. Much of their power output is simply the light of the stars that have been formed. Starbursts tend to be especially bright sources of infrared radiation, which is produced when interstellar dust absorbs and reradiates starlight. Starbursts also produce a lot of x-rays, which pour forth from massive stars, especially as they die. A massive star goes out with a bang: a supernova explosion, which generates x-rays directly, scatters hot x-ray-emitting debris, and leaves behind a neutron star or a smallish black hole, capable of cannibalizing a companion star and spewing x-rays. The surrounding interstellar gas, heated by all the stellar activity, gives off x-rays, too.

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