In autumn and winter night sky, there is a spectacular galaxy which can be seen even with the naked eye: the "Great Nebula" in Andromeda or M31 (the 31st object in the Messier catalogue). Although Andromeda Galaxy is faint for the naked eye, it is one of the farthest objects which can be seen with the unaided eye. Under a dark area, the Andromeda Galaxy looks like a fuzzy "cloud" with the width of the full moon. At a distance of 2.3 million light years, Andromeda Galaxy is our nearest neighbouring "twin" galaxy. Hence, it provides us with an opportunity to study the global properties of a galaxy which is similar to our Milky Way. As M31 and the Milky Way share similar morphology and size, comparisons between the properties of both can be enlightening.
Like the Milky Way, M31 is a giant spiral-shaped disk of stars, with a central dense region of older stars. The central region is estimated to
have a few million stars which is very difficult to be resolved with large ground-based telescopes; the starlight blends to resemble a single
bright almost point-like object. In 1993, a team of astronomers using Hubble Space Telescope discovered a double nucleus in the centre of M31. Most galaxies like our Milky Way are
believed to harbour a supermassive black hole in the centre of the galaxy. Black hole is a strong gravitational pull object in which even light cannot escape from it. Although black hole is invisible (by definition), astronomers can "detect" black hole in the galactic centre by studying the orbital motions of stars closely around the black hole. Astronomers estimate that the mass of the central black hole in M31 is about 30 million suns.
If the centre of the galaxy is powered by a black hole, then we will expect to detect X-rays emitting from its surroundings. When nearby materials fall into the black hole due to its strong gravitational force, they will be heated up to millions of degrees, resulting X-ray emission. With the help of X-ray telescopes in the space, astronomers have discovered many celestial X-ray sources. Unlike looking at visible light, the spatial resolution of X-ray optics is not that high in the last two decades. Hence, astronomers often do not know the correct identification of those X-ray sources in a crowded region when comparing to optical images.
NASA's Chandra X-ray Observatory has changed this picture; it provides
the sub-arcsecond resolution equivalent to the spatial resolution of ground-based optical telescopes. Such high resolution enables it to
overcome the problems of source confusion. Soon after the launch in 1999, Chandra provides us fruitful results; one of the major targets is M31. The spatial
resolution of Chandra has been proved to be crucial in studying M31, when it resolves a huge numbers of individual X-ray sources in the central region of M31. In particular, the nucleus of M31 has finally been resolved in X-rays. This result came in only 6 months after the launch of Chandra and an intriguing feature of this observation is the complexity of this very central region. Near the optical counterpart of the double nucleus, Chandra found two sources which were just resolved. Since the positional uncertainty is comparable to the separation between the two sources, astronomers are not sure the true X-ray position of the nucleus, the central supermassive black hole.
Our knowledge has been improved two years after the first observations. A team of
astronomers led by Michael Garcia and myself obtained a deep X-ray image of M31 to
pinpoint the X-ray positions recently. This 11-hour exposure image is constructed by
accumulating 1.5-year Chandra observations, which shows a large numbers of faint X-ray sources. In particular, two X-ray emitting globular clusters near the nucleus are also visible in the Hubble image. This allows us to cross-register both X-ray and optical images to determine the relative positions ten times better than before. This remarkable measurement is very encouraging, as the optical position of the nucleus is almost on top of one of the X-ray sources. The detection of X-rays coming from the centre of the galaxy is a signature for black holes. However, the X-ray flux of the nucleus is weak, which is a long-standing puzzle for astronomers. More Chandra observations will be scheduled in coming years and they may shed light on the nature of the supermassive black hole in M31.
Chandra X-ray Observatory
Photo credits ˇGNASA/CXC/SAO
Chandra and Hubble Space Telescope images of the centre of M31. The insert shows the three Chandra sources closest to the supermassive black hole, overlaid with the intensity contours from the HST image (red). The location of the supermassive black hole is thought to be in the middle of the peanut-shaped intensity contours.
Photo credits ˇG
X-ray: NASA/SAO/CXC/M.Garcia et.al.
Optical: NASA/GSFC/T.Brown et.al