In work that provides a glimpse of the early universe and improves our understanding of quasars-the most powerful energy sources in the universe-an international team of scientists led by an MIT astronomer has detected for the first time large amounts of molecular gas in a very distant quasar.
The discovery, reported in the October 13 issue of Nature, shows that molecular gas was present in the galaxies associated with quasars early in the history of the universe, possibly providing the raw material for some of the first generations of stars.
The new findings also help forge a link between quasars and another class of powerful and exotic objects, the so-called ultraluminous infrared galaxies. Indeed, the scientists believe that the quasar they studied, known as the Cloverleaf, could be a transition between an infrared galaxy and a full-blown quasar.
The scientific team, led by Dr. Richard Barvainis of MIT's Haystack Observatory, included Linda Tacconi of the Max Planck Institut fur extraterrestrische Physik in Germany; Robert Antonucci of the University of California at Santa Barbara; Danielle Alloin of CNRS/Observatoire de Paris in France; and Paul Coleman of the Kapteyn Astronomical Institute in The Netherlands (Dr. Coleman is now at the New Mexico Institute of Mining and Technology).
THE CLOVERLEAF
Quasars are thought to be ignited when gas is heated while falling into a supermassive black hole at the center of a galaxy. The quasar in question lies at a distance of 10-15 billion light years from Earth, which means the scientists viewed the object as it was some 10-15 billion years ago. The light they observed left the quasar when the universe was only one-seventh its current age, and has been traveling through space ever since.
On its way toward Earth, the light passed by or through a galaxy lying between us and the quasar, and as a result of the galaxy's gravitational field the quasar light was refocused-the intervening galaxy acts as a `gravitational lens.' In this rare event, the lens split up the light of the quasar into four separate spots, resembling a four-leaf clover, hence the name.
MOLECULAR GAS
The discovery of molecular gas-specifically, carbon monoxide-in the Cloverleaf was made using the high-precision radio telescopes of the Institute de Radioastronomie Millimetrique, or IRAM, a French-German-Spanish consortium. The telescopes are located on the Plateau de Bure in southeastern France, where the discovery was first made, and at the summit of Pico Veleta in the Spanish Sierra Nevada mountains, where follow-up studies were done.
Carbon monoxide, CO, is prevalent throughout our own Milky Way galaxy and is commonly seen in other nearby galaxies as well. On Earth carbon monoxide gas is generally regarded as a noxious pollutant and poison, but in space astronomers find its existence to be most useful. This is because CO is known to be a tracer of the much more abundant but less easily observed hydrogen molecule, which dominates the mass of the giant molecular clouds that are known to be the birth sites of new stars in galaxies.
A significant aspect of the new result is that it shows that molecular gas existed in great abundance even when the universe was quite young, only a few billion years after the Big Bang, possibly providing the raw material for some of the first generations of stars in the Universe.
Young stars are very difficult to see directly in visible light at the distance of the Cloverleaf, but sensitive radio telescopes such as those of IRAM are now beginning to detect their constituent molecular gas. This gas probably lies in a galaxy which serves as `host' to the supermassive black hole that produces the powerful visible light of the quasar. The galaxy supplies the fuel for the black hole in the form of dust and gas that are driven into its nucleus during a collision or merger with another galaxy. Such collisions, in addition to stirring up the existing gas, may also substantially augment the gas content of the quasar host galaxy.
The detection of molecular gas in the Cloverleaf indicates a large reservoir of fuel available to power the central engine. Indeed, the mass in molecules found in the Cloverleaf is greater than that of all the material, including the stars, in the Milky Way. The Cloverleaf's host galaxy may thus be dominated by molecular gas rather than stars, suggesting that it is an extremely young, or even primordial, galaxy.
LINK TO INFRARED GALAXIES
The new findings also help forge a link between quasars and another class of powerful and exotic objects, the so-called ultraluminous infrared galaxies.
These galaxies, unlike quasars, are relatively weak in visible light, but glow with tremendous luminosity in the infrared region of the spectrum. Their molecular content is quite similar to that of quasars, supporting a recent theory that quasars and infrared galaxies are related objects in a different phase of evolution.
In this scenario, the infrared galaxies are very young quasars, seen as they appear shortly after a merger with another galaxy. The strong visible light of the quasar is blocked by a veil of dust and gas that fuels but also hides the black hole there. The dust is warmed upon absorbing the quasar light, and then reradiates at infrared wavelengths. In this way the primary quasar energy is reprocessed and an infrared galaxy is born.
Later, when much of the gas and dust has been either consumed by the black hole, blown away or turned into stars, the quasar light becomes directly visible. In the case of the Cloverleaf, there is still a large quantity of gas present in the host galaxy, and the infrared radiation is still quite strong, but the visible radiation of a quasar is also clearly seen. This suggests that the Cloverleaf may be a transition object, one which has just emerged from the infrared galaxy stage to become a full-blown quasar.
Another, more radical, possibility is that the only difference between a quasar and an ultraluminous infrared galaxy is in its orientation relative to the line of sight to Earth. Dust and gas may block the view to the center along some directions but not others, allowing the quasar to be seen only by certain observers. Others would see what appears to be an infrared galaxy. A prediction of this idea, which is consistent with the CO observations, is that both types of objects should be equally rich in molecular gas.
Whatever the interpretation, the new discovery of molecules in the Cloverleaf improves our understanding of quasars and helps establish a unified picture of quasars and their powerful infrared counterparts among the galaxies.
Dr. Barsainis' work was funded by the National Science Foundation through a grant to the Haystack Observatory.
A version of this article appeared in the October 19, 1994 issue of MIT Tech Talk (Volume 39, Number 8).
