At Last: the Enigmatic Centre of the Milky Way Sighted!
31 oktober 1990
Observing with the ESO New Technology Telescope, three European astronomers have discovered two previously unknown celestial objects which are seen in the direction of the mysterious centre of our galaxy, the Milky Way. One of the new objects has a comparatively blue colour and appears to be the actual Galactic Centre, at a distance of about 28 000 light-years. It has required a telescope with the penetrating power of the NTT to see through the dense, interstellar dust clouds (which effectively hide the central area from our view) in order to obtain the first optical image of the Galactic Centre.
The newly found objects, provisionally designated GZ-A and GZ-B, will now be studied with all available means in order to unravel the true nature of the Galactic Centre. In particular, it may become possible to learn whether it is a compact cluster of hot, young stars, or rather a black hole.
How to look through the interstellar clouds
Although we receive much infrared radiation from the general area surrounding it, the very Centre of the Milky Way Galaxy has so far only been detected by means of observations of its radio emission. Some years ago, radioastronomers established that the radio-source Sgr A* in the southern constellation of Sagittarius is connected to this centre. However, until now all observations in other wavebands (infrared, optical, ultraviolet, X-ray and gamma-rays) did not reveal any object at the exact position of the radio source; this is mainly because the very dense interstellar clouds of dust and gas in front of the Galactic Centre absorb most of the radiation at the shorter wavelengths.
Another observational problem is the great number of foreground objects like stars and nebulae, which are located in the space between us and the Galactic Centre. In particular, there is one comparatively bright star which is seen very near the line of sight to the Centre and partly obstructs the view. For some time this star was even thought to be the Centre itself, but this has since been disproved.
To obtain the present image of the Galactic Centre, the astronomers resorted to a clever observational approach. Assuming that the Galactic Centre (as other energy-rich regions) has a blue colour, it would be natural to attempt to register the blue light from this direction. However, blue light is particularly strongly absorbed in the interstellar clouds and less than one million millionth of what is emitted at the Centre actually gets through. For this reason there is not enough blue light to be registered, even with the NTT. In the infrared spectral region, the interstellar absorption is much less and relatively more infrared light can therefore pass through the clouds. But if the Centre is blue, it may not emit much infrared radiation and accordingly, it would not be observable at these wavelengths either.
Thus the astronomers reasoned that the best chance to see the image of the Galactic Centre is by observing at wavelengths, intermediate between blue and infrared. The present observations were therefore made in the spectral interval 850 - 1100 nm, just outside the red spectral region. At these wavelengths, one millionth of the light gets through. As will be seen, this strategy bore fruit!
What was hidden behind the bright star
A total of five 40-min exposures of the sky area in the direction of the Galactic Centre was obtained with the NTT and further image processing was done with the MIDAS software, developed by ESO. To begin, the five exposures were added together to produce the "deepest" optical image  ever produced of this region, showing a wealth of very faint objects. Among those identified in the picture are virtually all of the infrared-emitting sources, which have been detected in earlier infrared surveys, but which are not directly connected to the Galactic Centre.
In the next step, the NTT picture was "computer-sharpened" (by means of a powerful algorithm developed by ESO astronomer Leon Lucy) so that the diameters of the stellar images in the field were reduced to 0.4 arcseconds. Because of the very low “noise'' in this picture (due to the very large number of photons registered with the large NTT during a long exposure time), the images are extremely “clean'' and well-defined. This permitted the astronomers to detect that the image of one of the two relatively bright stars in the picture is slightly elongated, a clear indication of the presence of other fainter images, nearly coincident with that of the bright star.
When the perfectly round image of the other bright star in the field was "subtracted" from that of the elongated star (whereby the image of the latter is removed from the picture), two previously unknown star-like objects emerged on the computer screen. Comparing their positions with that of the radiosource Sgr A*, the objects (designated GZ-A and GZ-B) were found to lie within 0.3 and 0.5 arcseconds of the Galactic Centre, respectively; this is well within the uncertainty of the radio position. The distance between the two objects is only 0.7 arcseconds. The very close positional coincidence of GZ-A with SgrA*, and the fact that no infrared radiation has been detected from GZ-A, which therefore presumably has a blue colour, strongly indicates that this object is indeed identical with the optical image of the Galactic Centre.
The presence of these two objects, until now hidden in the glare of the bright star, has in the meantime been confirmed by means of another, shorter NTT exposure, obtained under very good observing conditions by ESO astronomer Jorge Melnick.
Is there a black hole at the Galactic Centre?
The astronomers have measured the brightness of GZ-A as seen on the computer-processed picture and they have also estimated how much its light is weakened during the passage to us. This makes it possible to calculate the amount of light emitted by GZ-A. They find that GZ-A shines with an intensity that is a few million times larger than that of the Sun and that it could be a very compact cluster of several hot stars (of spectral class O7). Such clusters have been found at the centres of some star-forming regions in the Milky Way and also in the Tarantula Nebula (30 Doradus) in the Large Magellanic Cloud.
However, it is also possible that one or both of the two new objects are related to a black hole at the Galactic Centre. It would be surrounded by hot gas which emits strong radiation from ionized atoms and from electrons moving at very high (relativistic) velocities in a magnetic field.
It should be feasible to ascertain which of the two possibilities is the correct one by means of spectroscopic observations. They will be attempted with the NTT as soon as possible, but will require extraordinarily favourable observing conditions in order to succeed.
This important discovery is creating much excitement in the astronomical community and has significantly increased our chances of finally being able to unravel the true nature of the enigmatic Galactic Centre.
A preliminary, scientific account of this investigation has been given on IAU Circulars nos. 5125-6, issued by the Telegram Bureau of the International Astronomical Union on October 30, 1990.
Hans Zinnecker (Institut für Astronomie und Astrophysik, Universität Würzburg, Fed. Rep. Germany), Michael R. Rosa (ST-ECF, ESO-Garching) and Andrea Moneti (ESO La Silla Observatory).
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