Dark 'x' Across Nucleus of M51
8 June 1992The NASA/ESA Hubble Space Telescope has provided astronomers with what may be their first direct view of an immense ring of dust which fuels a massive black hole at the heart of the spiral galaxy M51, located 20 million light-years away. Surprisingly, they found that the ring is standing almost perpendicular to the relatively flat spiral galaxy, like a top spinning on its side with respect to the floor. Even more surprising is the discovery of a secondary ring or dust lane which is contrary to all expectations.
These observations are being reported by Dr. Holland Ford (Johns Hopkins University and Space Telescope Science Institute), and his Faint Object Spectrograph Team co-investigators at the 180th meeting of the American Astronomical Society in Columbus, Ohio.
"Pictures of M5l taken with the [Hubble] Planetary Camera show a striking, dark "X" silhouetted across the nucleus. The "X" is due to absorption by dust and marks the exact position of the nuclear black hole," says Ford. If these ideas are correct, M51 provides the first direct view of a torus [ring] which both fuels a massive black hole and hides the hole from direct view from anyone outside the ionization cone [narrow cone of light emitted from the near-vicinity of the black hole]
Commonly called the Whirlpool Galaxy, M51 is one of the nearest and brightest galaxies, having an angular diameter 1/3rd the width of the Full Moon. The galaxy is spectacular because it is tilted nearly face-on to Earth, allowing for an unobstructed view of its the bright core. M51 is especially noteworthy because its well-defined spiral arms are unusually bright, and the end of one of the spiral arms projects across a small, dusty and distorted satellite galaxy.
Previous observations made with both radio and optical telescopes have revealed energetic activity in the core of the galaxy. Hot ionized gas in the center of M51 is moving at speeds of up to two million miles per hour. Ground-based observations also show a pair of radio and corresponding optical "bubbles" which form a double-lobed structure across the nucleus.
The new HST images now offer the best glimpse yet of the near-vicinity of the "powerhouse" driving these fireworks. The pictures reveal an hour-glass structure formed by two bright beacons of light that are so energetic they cause interstellar gas caught in their beams to glow through ionization. This double cone "searchlight" is bisected by the widest bar of the dark "X". "This suggests that the dark band in the 'X', which is perpendicular to the ionization cone, may be a rotating torus of cold gas and dust seen edge-on," says Ford.
The second bar of the "X" is both interesting and puzzling. The dust in this linear feature could be a second disk seen edge on, or possibly rotating gas and dust in M5l interacting with the jets and ionization cones. "The safest interpretation is that the slash is a caution sign, showing that we do not yet fully understand what is happening in the center of M51," says Ford.
The edge-on torus, estimated to be 100 light-years across, hides the black hole and its disk of infalling hot gas. This accretion disk buried deep inside the torus is presumably the source of the ionizing radiation. The dusty ring confines the radiation from the accretion disk so that it can only escape through the "donut hole" of the torus as a pair of oppositely-directed cones of light.
The ring also determines the axis of a jet of material being accelerated away from the black hole. The dust ring is tilted so that it is perpendicular to the plane of M51. The high-speed jet thus lies in the galaxy's plane and plows into the gas and dust in M51 disk. The jet inflates a bubble of hot gas on either side of the black hole. This is analogous to a fire hose directed against a large pile of sand. The fire hose inflates a cavity of water and sand which expands and advances into the sandpile.
The resulting optical and radio emission from this "blowtorch" in M5l is several times brighter than the radio emission in the center of our Milky Way Galaxy. This means that the M51 black hole is more energetic than the million-solar mass black hole suspected to lie at the center of our Galaxy.
The power produced by the M51 black hole is still a hundred times fainter than the nuclei of bright Seyfert galaxies (a class of active galaxy having a brilliant point-like nucleus) which are thought to harbor 100 million solar mass black holes. Ford suggests the M51 core may be classified as a "miniature Seyfert nucleus."
A black hole's level of activity is determined not only by its mass, but the rate at which surrounding material falls into it. The gravitational interaction between M51 and its companion is likely causing gas to fall into the nucleus.
Astronomers will attempt to confirm the existence of the dusty torus and the black hole by using spectroscopy to measure the rotation velocity of the ring. Just as the orbital speed of the planets in our solar system can be used to estimate the mass of our Sun, the black hole's mass can be estimated by the rotation velocity of the torus.