Galactic black hole: X marks the spot?

It looks more like a pirate's treasure map than a picture taken by the Hubble Space Telescope. Researchers this week released a Hubble photo depicting a dark X that may mark the exact location of a black hole believed to be hiding at the heart of a spiral galaxy called M51.

When Hubble radioed the image to Earth late last year, technicians at NASA's Goddard Space Flight Center in Greenbelt, Md., were so captivated with the bizarre graphic that they immediately brought it to the attention of Admiral Richard Truly, then administrator of NASA. But the image -- along with other recent Hubble findings -- is proving far more than a pictorial curiosity, says Holland C. Ford, an astronomer with Johns Hopkins University and the Space Telescope Science Institute in Baltimore. The observations suggest that M51 and some of its relatives, all of which sport moderately luminous centers, share a common lineage with a group of galaxies known as Seyferts, which possess cores 100 times as bright.

"This tells us there's a real continuity in physical phenomena from the most luminous to the least luminous; we don't have a half-dozen different galaxy types that we're dealing with," says Ford. He presented the findings June 8 at a meeting of the American Astronomical Society in Columbus, Ohio.

For years, astronomers have suspected that relatively small black holes fuel the energetic activity found at the core of M51 and several other galaxies, called LINERs (for low-ionization narrow emission-line region), just as larger black holes may power the more luminous cores of Seyfert galaxies. But the theory faced a major obstacle: As observed from Earth, not all LINERs and Seyferts radiate in the same pattern.

To address that problem, researchers speculated that the differences might stem from the orientation of doughnut-shaped clouds of gas and dust thought to surround the proposed black holes in these galaxies. Viewed edge-on, such a doughnut would hide the black hole, and the galaxy's center would seem to emit only narrow bands of light. However, an observer looking at the ring face-on -- straight through the hole of the doughnut -- would probably detect a wide band of frequencies. Thus, a diverse group of galaxies with active nuclei might possess similar powerhouses.

But another problem remained: No one had ever found direct evidence of a dusty doughnut.

Ford and his colleagues weren't looking for the elusive structure when they used Hubble's wide-field/planetary camera to study M51's nucleus last December. They merely wanted to follow up on groundbased radio and optical observations of M51, a flat, spiral galaxy seen nearly face-on from Earth. Those studies had provided tantalizing evidence that M51 has an active nucleus: Its core contains hot, ionized gas moving at speeds of up to 2 million miles per hour, as well as material packed into two gas-inflated bubbles.

Two newly released Hubble images provide the sharpest views yet of M51's core. One shows a pair of cone-shaped searchlights streaming out from the center in opposite directions, each leaving a glowing trail of ionized gas. The other image, taken at a different wavelength, shows the dark X, with the fatter arm bisecting the apex of the twin searchlights.

Ford and his co-workers propose that this arm represents an edge-on view of the doughnut that researchers have long sought -- a rotating ring of cold gas and dust that somehow got tipped out of the plane of the flattened galaxy. The doughnut may obscure the "central engine" -- the presumed black hole at the core of M51 -- as well as infalling material from an inner disk of hot gas needed to feed the black hole. The researchers speculate that the doughnut, about 100 light-years in diameter, also directs the ionizing radiation emitted by the infalling matter. Light passing through the hole would emerge as twin cones, similar to the image Hubble obtained.

Since a rotating doughnut has a characteristic light spectrum, researchers intend to test these ideas by analyzing emissions near the fat arm of the X. This week, astronomers will analyze spectra taken with a telescope in Hawaii. Later this year, Ford's team plans to use Hubble's faint-object spectrograph for a similar study.

The thinner arm of the X remains a puzzle. It could represent a second doughnut seen edge-on or gas and dust interacting with the galaxy's energetic core, says Ford. In either case, it indicates that astronomers still don't fully understand what drives the fireworks at the center of M51.