A team of US scientists led by a research astronomer at MIT's Haystack Observatory has reported a discovery that may shed new light on the origin of quasars and their possible relationship to colliding galaxies.
The team led by Colin J. Lonsdale of Haystack included his sister, Carol J. Lonsdale of the California Institute of Technology's Infrared Processing and Analysis Center; her husband, Harding E. Smith of the University of California, San Diego; and Philip Diamond of the National Radio Astronomy Observatory.
Using a technique known as very long baseline interferometry (VLBI), the team concluded that the microwave laser radiation coming from an infrared galaxy is concentrated into a compact region at the galaxy nucleus. A source of energy so compact is very likely a quasar, the team believes.
Quasars are the most distant and powerful objects in the known universe. Astronomers currently believe that the radiation seen from these objects, from radio waves and through infrared, optical, ultraviolet and X-rays, ultimately arises from material falling into an enormous black hole in the middle of a galaxy. However, the origin of this most violent of natural phenomena is very much a matter of debate.
How did the black hole get there? Where does the material inferred to be falling into it come from, and why does it fall in instead of just orbiting the hole? Are black holes present in all galaxies, and if so, why do only a tiny minority show this quasar activity?
One way of attacking these questions is to study another class of galaxies, which emit prodigious amounts of infrared radiation but otherwise appear relatively normal. The power emitted in the infrared by these galaxies is so great that they actually rival quasars, and many astronomers suspect that the two types of object may be related.
The problem is that infrared galaxies are very dusty. This dust hides the nuclear source of energy so that optical telescopes, even the Hubble Space Telescope, cannot see very far into the center. It is known that many of the most luminous infrared galaxies arise from a collision between two galaxies, which causes much of their dust and gas to settle into the center of the merged system. There, something enormously powerful lurks unseen, heating the dust and making it glow with the power of hundreds of billions of suns. The origin of this energy has been controversial ever since these luminous infrared galaxies were discovered by NASA's IRAS (Infrared Astronomical Satellite) mission a decade ago. Is the hidden powerhouse a newly born quasar?
A dramatic indication that the answer is yes comes from new radio observations reported in an article in the July 14 issue of the journal Nature.
Using VLBI, the team studied maser (microwave laser) radiation at a wavelength of 18 centimeters coming from the nucleus of a luminous infrared galaxy named Arp 220. Arp 220 emits more energy than a trillion stars like the sun, and a thousand times more than the entire Milky Way Galaxy.
The existence of such maser emission itself is not news: Arp 220 had long been known to emit powerful maser radiation from OH (hydroxyl) molecules. The surprising result, however, shows that the gas responsible for it is concentrated into an extraordinarily compact region in the nucleus of the galaxy. The gas cannot emit maser radio emission without huge amounts of infrared radiation to energize it, so the infrared radiation must also be coming from a very small region. It is hard to imagine a source of energy this compact without thinking of a quasar, so the obvious interpretation is that the monster which powers these galaxies is in fact a quasar buried by dust and gas. The argument is not yet conclusive, but the buried quasar idea gets a big boost from the team's observations.
If the team's inferences about a buried quasar in Arp 220 are correct, it is reasonable to suspect that quasars are "born" in galaxy collisions, which send huge amounts of dust and gas spiraling into the center to be devoured by a massive black hole. When the dust clears, we might catch a glimpse of the naked powerhouse itself, and recognize it as one of the familiar quasars. Until then, the monster reveals its presence only through a muted but enormously powerful glow from the dusty shroud enveloping it.
The new results offer a powerful new way to explore the phenomena, because radio waves can penetrate the dust and gas. Dr. Lonsdale's team is planning comprehensive new observations of this and other luminous infrared galaxies with OH maser emission, which have the potential to make images of the OH gas as it spirals into the central black hole. These and similar investigations may gradually unlock some of the deeper mysteries of quasars. Colin J. Lonsdale, Haystack Observatory.
A version of this article appeared in the August 17, 1994 issue of MIT Tech Talk (Volume 39, Number 2).