Centaurus A, also known as NGC 5128, is a peculiar galaxy in the constellation of Centaurus. It is gravitationally bound into a group of galaxies which also contains Messier 83. Its common name derives from the early days of radio astronomy (1940s and 1950s), when astronomers named radio sources by letter (A,B,C), with "A" designating the strongest source in a given constellation.
Visible: DSS and Visible: AAO,
This pair of visible-light images shows some of the characteristics that have led astronomers to classify Centaurus A as a peculiar galaxy. First of all, note that the DSS image (above left) is over-exposed in the central regions. However, this brings out fainter emission features elsewhere in the field of view, and allows us a better view of dust (seen in silhouette) in the outer regions. You should see that the bright stellar content of the galaxy resembles an elliptical galaxy , a spherical configuration of hundreds of millions of stars. An obscuring band of dust, however, is more often associated with spiral galaxies! This dust band encircling Centaurus A is heavily warped, suggesting that something strange has taken place here. Furthermore, if you look closely at the northern edge of the dust lane, about 5 arcmin to the northwest (upper right) of the image center, you will discover a region where young and hot blue stars have recently formed. This phenomena is usually seen along the dusty arms of spiral galaxies.
So what explains these apparent dichotomies? Before answering that question, let us peek at a close-up visible-light photograph of Centaurus A, courtesy of the the Hubble Space Telescope (HST). Another close up view of the inner regions was one of the first images obtained by the Very Large Telescope Very Large Telescope, a ground-based telescope in Chile.
Astronomers now believe that Centaurus A is the remnant of an intergalactic collision, in which a dusty spiral galaxy and a bright elliptical galaxy merged hundreds of millions of years ago. A black hole is presumed to lie at the center of the resulting peculiar galaxy. The near-infrared camera aboard HST has provided evidence of this black hole. Galaxies with a central black hole and/or massive episodes of star formation are known as "active galaxies." Additional evidence attesting to the peculiar nature of NGC 5128 appears later in this gallery.
For now, let us turn our attention to the ground-based near-infrared image of Centaurus A (above). In this short-exposure mosaic of images taken at three near-IR wavelengths, the most noticeable feature is the bright central condensation of stellar emission. The intervening dust lane is much less conspicuous. This is due to the fact that near-infrared light can pierce through all but extremely dense concentration of dust. In a real sense, the 2MASS image lifts (most of) the obscuring veil of dust. The surrounding speckles of light are foreground stars within our own Milky Way Galaxy.
Mid-Infrared: Spitzer and Far-Infrared: IRAS
This set of images shows Centaurus A at longer infrared wavelengths. In this regime, most of the IR emission is produced by dust grains. The mid-infrared picture (above left) was obtained with a camera aboard the Spitzer Space Telescope at a wavelength of 3.6 to 8 microns. This image clearly reveals the flattened inner disk of the spiral galaxy that once rammed into the elliptical galaxy. The low-resolution far-infrared photograph (above right) is able to distinguish only a central peak of long-wavelength emission, stretched along the same orientation as the dust disk seen in the Spitzer image.
The radio image (above) was obtained at a wavelength of 6 cm at the Very Large Array, a series of radio telescopes in New Mexico, and it shows a remarkable feature unlike any seen in the previous photographs. A pair of narrow "jets" appears to be shooting out of Centaurus A, with the radio emission becoming more diffuse at greater distances from the galaxy center. The radio jets consist of plasma, a high-temperature stream of matter in which atoms have been ionized and molecules have been split apart.
What makes these jets so unusual is their orientation. They are perpendicular to the dust disk, as seen when the radio emission contours are overlaid on a visible-light photograph. What could explain such a feature? Before answering that question, let us examine the x-ray image of NGC 5128.
The x-ray view of Centaurus A was obtained by the Chandra X-ray Observatory (CXO), one of NASA's "Great Observatories." The most dominant feature is an x-ray jet extending 25,000 light years towards the northeast (upper left). A less prominent jet extends towards the southwest. The apparent brightness difference between the jets is probably due to the viewing geometry. Astronomers believe the first jet is moving towards us, while the second is moving away. These observations provide additional evidence that an invisible and supermassive black hole is nestled in the center of Centaurus A. Why?
By definition, no light can escape from a black hole (BH), and therefore astronomers cannot "see" a black hole directly. However, they can infer its existence through the gravitational effect that a BH exhibits on its surrounding environment. As matter falls towards the BH, it creates an accretion disk, a thin disk of super-heated matter that spins at very high speed around the BH. As an analogy, think of how bath water swirls down into the drain upon emptying the tub. With temperatures of millions of degrees, it is this matter that accounts for the x-ray emission peak seen in the central core of Centaurus A.
The origin of the radio and x-ray jets is still a subject of intense debate among research astronomers. Most agree that the ultimate power source for the jets is some sort of explosive event(s) near the BH event horizon Matter is preferentially "squirted" out from the central core in a direction that is perpendicular to the accretion disk because of the intense magnetic fields that are thought to permeate the matter in the disk. The magnetic field lines act as barriers to any charged particle (i.e., ion) attempting to cross, thereby forcing the plasma stream to shoot out along the path of least resistance.
Isolated x-ray sources that are clustered around the galaxy center can also be seen in the CXO image. These emission sources are likely to be discrete neutron stars and/or stellar-sized black holes. To appreciate the substantial improvements in resolution and sensitivity of CXO, one merely needs to look at the image of Centaurus A obtained by ROSAT, a previous x-ray observatory. The graininess of the image attests to the relatively low "signal-to-noise" ratio of the image.
Before closing our visit to this interesting gallery, let us quickly look at yet
one more image of Centaurus A, obtained by the (now extinct)
Compton Gamma-Ray Observatory.
Gamma rays represent the most powerful
type of electromagnetic radiation (light). In this wide-angle image, Centaurus
A is clearly visible. However, the low-resolution of the gamma ray detectors
prevents us from studying any of the finer details (such as jets) from this
intriguing celestial object.
Centaurus A has a prominent dust lane that absorbs the ultraviolet light from the stars in the galaxy. This galaxy has a super massive black hole at its center that emits jets of high energy particles, traced by the X-ray emission observed by Chandra. At the intersection of the jets and clouds of Hydrogen gas approximately 50,000 light years away from the galaxy, several regions of Ultraviolet (UV) emission can be seen in the North-East (upper left) just beyond the X-ray emission. This UV light may be from young stars formed in a burst of recent star formation triggered by the compression of the gas clouds by the X-ray jet.