May 14, 1998
Although the cause-and-effect relationships are not yet clear, the views provided by complementary images from two instruments aboard NASA's Hubble Space Telescope are giving astronomers new insights into the powerful forces being exerted in this complex maelstrom. Researchers believe these forces may even have shifted the axis of the massive black hole from its expected orientation.
The Hubble wide-field camera visible image of the merged Centaurus A galaxy, also called NGC 5128, shows in sharp clarity a dramatic dark lane of dust girdling the galaxy. Blue clusters of newborn stars are clearly resolved, and silhouettes of dust filaments are interspersed with blazing orange-glowing gas. Located only 10 million light-years away, this peculiar-looking galaxy contains the closest active galactic nucleus to Earth and has long been considered an example of an elliptical galaxy disrupted by a recent collision with a smaller companion spiral galaxy.
Using the infrared vision of Hubble, astronomers have penetrated this wall of dust for the first time to see a twisted disk of hot gas swept up in the black hole's gravitational whirlpool. The suspected black hole is so dense it contains the mass of perhaps a billion stars, compacted into a small region of space not much larger than our Solar System.
Resolving features as small as seven light-years across, Hubble has shown astronomers that the hot gas disk is tilted in a different direction from the black hole's axis -- like a wobbly wheel around an axle. The black hole's axis is identified by the orientation of a high-speed jet of material, glowing in X-rays and radio frequencies, blasted from the black hole at 1/100th the speed of light.
This gas disk presumably fueling the black hole may have formed so recently it is not yet aligned to the black hole's spin axis, or it may simply be influenced more by the galaxy's gravitational tug than by the black hole's.
"This black hole is doing its own thing. Aside from receiving fresh fuel from a devoured galaxy, it may be oblivious to the rest of the galaxy and the collision," said Ethan Schreier of the Space Telescope Science Institute, Baltimore, MD. Schreier and an international team of co-investigators used Hubble's Near Infrared Camera and Multi-Object Spectrometer to probe deeper into the galaxy's mysterious heart than anyone has before.
The hot gas disk viewed by Hubble investigators is perpendicular to the galaxy's outer dust belt, while the black hole's own internal accretion disk of superhot gas falling into it is tilted approximately diagonally to these axes.
"We have found a complicated situation of a disk within a disk within a disk, all pointing in different directions," Schreier said. It is not clear if the black hole was always present in the host galaxy or belonged to the spiral galaxy that fell into the core, or if it is the product of the merger of a pair of smaller black holes that lived in the two once-separate galaxies.
Having an active galaxy just 10 million light-years away from Earth rather than hundreds of millions or billions of light-years distant offers astronomers a unique laboratory for understanding the elusive details of the behavior of supermassive black holes as fueled by galaxy collisions.
"Though Hubble has seen hot gas disks around black holes in other galaxies, the infrared camera has for the first time allowed us to peer at this relatively nearby, very active, but obscured black hole region," Schreier added.
The team of astronomers is awaiting further Hubble data to continue its study of the disk, as well as ground-based spectroscopic observations to measure the velocity of entrapped material around the black hole. This will allow the astronomers to better calculate the black hole's mass. The current results are scheduled to appear in the June 1, 1998 issue of Astrophysical Journal Letters.
Images and further information related to these results are available on the Internet.
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).
A ground-based telescopic view (upper left insert) shows that the dust lane girdles the entire elliptical galaxy. This lane has long been considered the dust remnant of a smaller spiral galaxy that merged with the large elliptical galaxy. The spiral galaxy deposited its gas and dust into the elliptical galaxy, and the shock of the collision compressed interstellar gas, precipitating a flurry of star formation. Resembling looming storm clouds, dark filaments of dust mixed with cold hydrogen gas are silhouetted against the incandescent yellow-orange glow from hot gas and stars behind it.
Brilliant clusters of young blue stars lie along the edge of the dark dust rift. Outside the rift the sky is filled with the soft hazy glow of the galaxy's much older resident population of red giant and red dwarf stars.
The dusty disk is tilted nearly edge-on, its inclination estimated to be only 10 or 20 degrees from our line-of-sight. The dust lane has not yet had enough time since the recent merger to settle down into a flat disk. At this oblique angle, bends and warps in the dust lane cause us to see a rippled "washboard" structure.
The picture is a mosaic of two Hubble Space Telescope images taken with the Wide Field Planetary Camera 2, on Aug. 1, 1997 and Jan. 10, 1998. The approximately natural color is assembled from images taken in blue, green and red light. Details as small as seven light-years across can be resolved. The blue color is due to the light from extremely hot, newborn stars. The reddish-yellow color is due in part to hot gas, in part to older stars in the elliptical galaxy and in part to scattering of blue light by dust -- the same effect that produces brilliant orange sunsets on Earth.
Centaurus A (NGC 5128) Fast Facts:
Right Ascension: 13 : 25.5 (hours : minutes) Declination: -43 : 01 (degrees : minutes) Apparent Magnitude: 7.0 Apparent Diameter: 18.2 (arc minutes) Distance: 10 million light-years Constellation: Centaurus (southern sky)
Hubble image credit: E.J. Schreier, (STScI) and NASA
Team members are: E.J. Schreier, A. Marconi, D. Axon, N. Caon, and D. Macchetto (STScI)
Groundbased image credit: NOAO
PHOTO NO.: STScI-PRC98-14b
[UPPER LEFT] - A close-up high resolution Wide Field Planetary Camera 2 image of the dramatic dust disk which is thought to be the remnant of a smaller spiral galaxy that merged with the large elliptical galaxy. The shock of the collision compressed interstellar gas, precipitating a flurry of star formation and giving the material a fleecy pattern. Dark filaments of dust mixed with cold hydrogen gas are silhouetted against the incandescent yellow-orange glow from stars behind it.
[LOWER RIGHT] - Hubble's Near Infrared Camera and Multi-Object Spectrometer was used to peer past the dust to discover a tilted disk of hot gas at the galaxy's center (white bar running diagonally across image center). This 130 light-year diameter disk encircles a suspected black hole which may be one billion times the mass of our Sun. The disk feeds material to presumably an inner, unresolved accretion disk that is made up of gas entrapped by the black hole. The red blobs near the disk are glowing gas clouds which have been heated up and ionized by the powerful radiation from the active nucleus.
The false-color NICMOS image was taken on Aug. 11, 1997 at a wavelength of 1.87 microns ("Paschen alpha"), characteristic of ionized Hydrogen.
Credit: E.J. Schreier, (STScI) and NASA
Team members are: Ethan J. Schreier, Alessandro Marconi, David J. Axon, Nicola Caon, Duccio Macchetto ( STScI), Alessandro Capetti - (Osservatorio Astronomico di Torino, Italy), James H. Hough, Stuart Young ( University of Hertfordshire, UK), and Chris Packham (Isaac Newton Group, Islas Canarias, SPAIN)
1848-1949: Despite Herschel's observation, astronomers paid little attention to Centaurus A for about 100 years because they considered it another one of those nebulous, fuzzy objects originally thought to be in our own galaxy. Even Edwin Hubble called it a nebulous object. Later astronomers with more powerful telescopes identified many of these objects as galaxies. Centaurus A also was largely ignored because of the lack of large optical telescopes in the southern hemisphere, where this galaxy can be seen.
1949: Astronomers had just developed another technique to study celestial objects besides optical light: special instruments to collect radio waves. Using an 80-foot radio antenna at Dover Heights, Australia and radio waves reflected off the nearby sea, astronomers John Bolton, G. Stanley, and Bruce Slee were the first to identify Centaurus A as a powerful radio galaxy. Radio waves from Centaurus A were among the first to be linked to an extragalactic object.
1954: Studying Centaurus A with telescopes at Palomar Observatory in California, Walter Baade and Rudolph Minkowski confirmed that it is a galaxy. The pair also proposed that the peculiar structure of Centaurus A - with a bar of dark dust bisecting the galaxy - is the result of a merger between two galaxies, a giant elliptical and a small spiral.
1969-1971: Stuart Bowyer, using a sounding rocket, detected X-rays emanating from Centaurus A. In late 1970, the UHURU satellite confirmed that the X-rays were indeed associated with the galaxy. In 1971, Bill Kunkel and Hale Bradt used a telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile to observe infrared light originating from a compact source in the galaxy's core. X-ray observations in the early 1970s with the Astronomical Netherlands Satellite (ANS) and the Orbiting Solar Observatory (OSO-7) showed changes in the intensity of X-ray emissions. Significant changes over a very short time indicated that the emission source was confined to a small region of space. The results suggested that a black hole could be the culprit.
1975: Looking at the galaxy in visible light with a CTIO telescope, Victor Blanco discovered a faint jet out in the galaxy. He also noted blue stellar objects in this region.
1975-76: Using information from the SIGMA satellite and high-flying research balloons, Josh Grindlay and R.D. Hall detected gamma ray emissions which were probably coming from the nucleus.
Late 1970s: John Graham, using a CTIO telescope, found a series of faint shells of gas in the outer regions of the galaxy. These shells could have been produced by collisions of gas from a galaxy merger.
1979: Using the Einstein Observatory, Ethan Schreier discovered an X-ray jet emanating from the nucleus of Centaurus. A. Working with Eric Feigelson and Jack Burns, Schreier used the Very Large Array in New Mexico to find the jet's radio counterpart.
1981: With the Einstein Observatory, Schreier and Feigelson also observed extended X-ray emissions in the galaxy, including ridges of emissions along the north and south edges of the dust lane.
1996: Schreier used the Hubble Space Telescope's Wide Field and Planetary Camera 2 (WFPC2) to study the dust lane's polarization properties and the distribution of young stars along its northern edge. The young, blue stars provided further proof that Centaurus A, an elliptical galaxy, merged with a spiral galaxy. Elliptical galaxies such as Centaurus A would not have had enough dust and gas to form clusters of new stars.
1997: Kenneth Kellermann, Anton Zensus, and Marshall Cohen, using the Very Long Baseline Array (VLBA) in New Mexico, observed that the core of Centaurus A is only 10 light-days across, making it the smallest known extragalactic radio source. They also noted that the energy produced by this source varies in intensity every day. This core has a mass of about 100 million suns.
1997-98: Schreier used WFPC2 to further examine the nucleus and the bright, young stars. Since WFPC2 cannot peer through the dusty core, Schreier and Alessandro Marconi turned to the Hubble telescope's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to explore the core. They found a very compact nucleus and also discovered a central, warped disk. The disk could have been warped by a collision with another galaxy.