One of Chandra's recent images (below) shows new evidence for a super-massive black hole at the heart of the Andromeda galaxy. It's not the black hole itself (as the name suggests, a black hole's gravity is so strong that not even light can escape) but the glow of very hot matter falling into it. And that's where the puzzle is.

The Andromeda spiral is thought to be a near twin of our own Milky Way galaxy. It's also one of our nearest celestial neighbors: At a distance of 2 million light-years, it's practically in our backyard.

Astronomers had already suspected the presence of a super-massive black hole at Andromeda's center, based on observations from the Hubble Space Telescope and ground-based observatories. The evidence was gleaned by observing the motions of stars near the galaxy's center, explains Eliot Quataret, an astronomer at the Institute for Advanced Studies in Princeton, New Jersey.

"You look at the motion of the stars," Quataret says, "and you infer that there's a large amount of mass that you don't see." Calculations have shown that if it exists, the Andromeda black hole contains 30 million times as much mass as the sun.

Chandra's new X-ray image seems to confirm the black hole's existence. It reveals several bright spots within the Andromeda spiral, including one at the galaxy's center -- just at the place where astronomers would expect to see the so-called accretion disc, which contains material that is plunging into the black hole.

There's just one troubling detail: the temperature of the gases in the accretion disc, as measured by Chandra's spectrometer, is "only" 1.8 million degrees Fahrenheit (1 million degrees Kelvin). That's barely hot enough to produce X-rays, and much "colder" than the temperatures expected for a black hole of this size.

How to explain this surprisingly low temperature? "I don't have any good ideas right now," Quataret says.

And there's another wrinkle: Observations of the Andromeda spiral made some two decades ago by an X-ray satellite called Einstein registered X-rays 10 times brighter than Chandra detected. Such variability, Quataret says, would take place over a scale of centuries or millennia, not decades.

The bottom line? If Chandra's Andromeda image really does show the accretion disc of a super-massive black hole, Quataret says, "It really is a puzzle."