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No "Missing Mass" in Opaque Spiral Galaxies?
27. syyskuuta 1990
A long-term astronomical study of spiral galaxies, initiated almost a decade ago at the European Southern Observatory, has recently produced intriguing results about the presence of cold matter in the Universe. They have a direct bearing on the so-called "missing mass" problem, one of the major unsolved riddles in astronomy.
The ESO Atlas
In 1972, ESO embarked on the production of the first modern, photographic atlas of the southern sky. More than 1200 large, blue-sensitive photographic plates were obtained with the 1-m ESO Schmidt telescope on La Silla; the 606 best of these formed the basis for the "ESO Quick Blue Atlas of the Southern Sky" which was ready in 1980. This Atlas showed celestial objects up to 100 times fainter than recorded in earlier southern atlasses and, not unexpectedly, many new and interesting discoveries were made on it.
A comprehensive catalogue of more than 16,000 bright galaxies, stellar clusters and galactic nebulae was compiled by Swedish astronomer Andris Lauberts as a result of a careful, visual inspection of the Atlas photographs. It was published by ESO in 1982 and included the first systematic classification by type (elliptical, spiral, irregular) of southern galaxies.
Accurate measurements of 15,000 southern galaxies
In 1982, Andris Lauberts and the Dutch astronomer Edwin Valentijn embarked upon an even more ambitious project. With a fast, high-precision microphotometer at the ESO Headquarters in Garching, they scanned the images of 15,467 galaxies in this Catalogue, first on the blue-sensitive Atlas plates, and then on red-sensitive plates, also obtained with the ESO Schmidt telescope. In this way, the photographic images were registered as arrays of numbers which could be stored in a computer. In the end the immense database comprised more than 4 Gigabytes. It has been stored on optical disks, making it possible to display the blue and red images of all of these galaxies instantaneously, a very efficient tool for many astronomical investigations.
By means of sophisticated computer programmes, each galaxy was automatically analyzed and classified according to type, brightness, colour, size, the angle from which it is viewed, etc. In all, each galaxy was characterized by about 200 different parameters.
Opaque spiral galaxies
New and exciting results have now been obtained by Edwin Valentijn, following an extremely detailed computer analysis of 9,381 southern spiral galaxies, identified as such in the above mentioned database.
His investigation began with a comprehensive study of the surface brightness of these galaxies (that is, the way the brightness varies over the galaxies' surface) in relation to the angle under which they are seen (spiral galaxies seen face-on ressemble pinwheels, while they look like compass needles when they are seen from the side). The dependance of the surface brightness on the viewing angle, statistically spoken, makes it possible to estimate the opaqueness of these galaxies, that is how strongly light passing through them is absorbed.
To his great surprise, and contrary to the conventional view that spiral galaxies are rather transparent, Valentijn found that these galaxies are quite opaque and therefore contain many more clouds of interstellar matter than thought before, also in their outer regions. Like the dimly visible headlights of cars on a foggy morning, the light from many of the stars within these galaxies barely penetrates these clouds.
Most important, a comparison with infrared measurements from the IRAS satellite indicates that this matter must be very cold; the temperature is less than 20 degrees above the absolute zero (T < 20 K). It is most likely to consist of molecular clouds, such as those known since some time in our own galaxy, the Milky Way.
The clouds are too cold and dark to be seen directly, but from our own galaxy it is known that much of the mass of molecular clouds is made up of molecular hydrogen gas, H2, which is extremely difficult to observe.
The missing mass?
The mass of a spiral galaxy can be determined by accurate measurements of the motions of its stars and atomic hydrogen gas (H I); this is done by means of optical and radio doppler spectroscopy. The more rapid the motions are, the heavier is the spiral galaxy. Previously, in virtually all cases the mass of a galaxy determined this way, has been found to be significantly larger than the combined mass of all the stars and interstellar matter actually visible within the confines of that galaxy. Thus, a large fraction of the mass must be "invisible" - this is known as the problem of the "missing mass".
There have been many attempts to explain this. Some scientists believe that galaxies may have large haloes of hot gas, not visible with present astronomical instruments. Others have invoked the presence of large numbers of exotic elementary particles, including neutrinoes, or massive "cosmic strings".
However, if the spiral galaxies contain many more interstellar, molecular clouds than detected before, then perhaps the mass of these clouds makes up for the "missing" amount?
To look into this, Valentijn and the Spanish astronomer Ignacio González-Serrano have studied half a dozen of the most "mass-missing" spiral galaxies, for which extensive doppler spectroscopy has been made. In all cases, the astronomers find that the mass of the molecular clouds inferred from the opaqueness corresponds exactly to the amount that was "missing". Thus, for these "classical missing-mass" galaxies at least, there is no longer any need to invoke the presence of any exotic "missing mass".
This, of course, does not mean that the "missing mass" problem has now been definitively solved. More observations are needed to provide more knowledge about the molecular clouds in these spiral galaxies, in particular difficult measurements of the radio emission from some of the other molecules expected to be present in the clouds, for instance carbon monoxide (CO). It is important, however, that Valentijn's findings offer a natural explanation of the "missing mass", which is in agreement with all available observations and which would eliminate the need for additional, exotic ingredients.
There is also "missing mass" in elliptical galaxies and in clusters of galaxies, but such objects were not included in the present study.
The Milky Way
Our own galaxy, the Milky Way, is a typical spiral galaxy, so how opaque is it, and what about the "missing mass" thought to exist here in our immediate neighbourhood?
Hoping to cast new light also on this problem, Edwin Valentijn again turned to the computerized database. He noted that about 60,000 additional, faint galaxies, situated much further out in space, are seen in almost the same direction as the nearly 16,000 galaxies for which computerized images are available. Counts of the nearby as well as of the more distant galaxies revealed that there are 60% more of these objects "above" the Milky Way plane (in the Northern Galactic Hemisphere) than in the opposite direction.
There are two possible explanations for this phenomenon. In the first case the distribution in space of these galaxies is not uniform, but this would require that the Universe is non-uniform on a scale of 1000 - 2000 million lightyears which is not very probable according to current cosmological research.
The more likely explanation is that it reflects the position of our Sun in the Milky Way, which has been shown by accurate measurements to be about 40 lightyears above ("North" of) the galactic plane. Indeed, if our Milky Way were as opaque as the spiral galaxies of the same class in the computer database were found to be, then the interstellar absorption in the Milky Way should be significantly larger towards the South (looking through the central plane where most of the absorbing material is) than towards the North (looking through less absorbing regions, high above the plane). Hence more galaxies would be seen in the North than in the South.
In summary, the discovery that spiral galaxies are more opaque than thought before may therefore also apply to our own Galaxy. Again more observations, particularly in the southern hemisphere (for instance with the SEST telescope on La Silla) will be needed to clarify the situation.
Note that, as in the spiral galaxies in the study, the presence of cold and opaque molecular clouds near the galactic plane would not show up in conventional studies of interstellar absorption which rely on the reddening of starlight. This is because these opaque clouds simply block all light from objects located behind them.
A preliminary account by Valentijn of the opaqueness of spiral galaxies has just appeared in the science journal Nature (346, p.153). More detailed accounts, also including a discussion about the”missing mass'' will appear in papers being published in the Proceedings of IAU Symposium 144 (Kluwer, Dordrecht) and (with Ignacio González-Serrano) in the European journal Astronomy and Astrophysics.
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