Radio galaxies and quasars look different and have been traditionally classified as different objects. But for quite some time now, many astronomers have suspected that those differences are not real but are only apparent, a 'visual illusion' which arises because of our special observation point from the Earth.

The unification scheme for radio galaxies and quasars. Photo: ESA

ISO home pageIn their description, radio galaxies and quasars would both be powered by a supermassive black hole at their centres, surrounded by a wall of dust. A quasar is then simply the object viewed from an angle such that the observer can peer over the dust wall directly into the central powerhouse, while a radio galaxy would be oriented in such a way that the central emission is blocked by the dust. New results obtained by a German team of astronomers using ESA's Infrared Space Observatory, ISO, which is able to see the heated dust, confirm this idea.

The Universe contains billions of massive galaxies like our own Milky Way. A few of them are very peculiar: they look like normal galaxies when seen in visible light with an optical telescope. However, imaged with a radio telescope, they display two giant symmetric lobes coming from the galaxy's central core and reaching out to millions of light-years - a much larger structure than that seen in the visible. These objects have been traditionally classified as 'radio galaxies'. But there are other objects which are also powerful radio-emitters, although they look much different from radio galaxies in visible light. For instance, 'quasi-stellar radio sources', or quasars as they are more commonly known, which show an extremely bright core when viewed in visible light.

Having studied powerful radio sources for more than half a century, astronomers are now certain that a strong radio source is related to the existence of a super-massive black hole in the centre of a galaxy. It was proposed in the eighties that most of the differences among the variety of objects hosting one of these 'monsters' in their centres could be simply a consequence of an 'optical illusion': an effect due to the different orientation of the object with respect to the observer on Earth.

The hypothesis, called the 'unification theory', proposes that the central black hole is surrounded by a ring-like 'wall' of dust. This 'doughnut' of dusty material would block the light emitted in its direction from the vicinity of the black hole. Therefore, only the objects oriented in such a way that astronomers can look at the centre of the dust ring - from above or below the plane of the ring - will reveal their central powerful emission, as quasars do. In all other objects, the black hole and its characteristic emission remain hidden behind the dust.

How could the 'unification theory' be tested? By detecting radiation from the object other than radio or optical; radiation that is not blocked by the dust in the ring nor affected by the relative spatial orientation of the observer and the dust ring. Far-infrared radiation precisely fulfils those requirements. ESA's Infrared Space Observatory, ISO, a pioneer facility in infrared space astronomy, is the first telescope capable of observing the sky at such wavelengths.

New results from ISO
A German team of astronomers led by Klaus Meisenheimer, at the Max-Planck-Institut for Astronomie in Heidelberg, used the ISO spectrophotometer, ISOPHOT, which was developed in their institute, to compare the far-infrared brightness of radio-galaxies and quasars. If these objects are the same, as the unification theory proposes, they should look very similar in the far-infrared.

They observed a sample of 10 pairs of radio-galaxies and quasars, each of them comprised of objects of similar radio-luminosity (a measure of their brightness at radio wavelengths) and distance. And indeed they found that quasars and radio galaxies can not be distinguished by their observed mid- and far-infrared properties.

"We prove for the first time that very hot and luminous quasar cores are embedded even in feeble radio galaxies at large distances," says Meisenheimer. "This is a first hint that one of the major predictions of the radio-galaxy/quasar unification scheme is fulfilled."

Other objects also support unification scheme
This new result adds to previous findings with ISO, which also favour the unification scheme. Last year a group headed by ESA astronomers Jean Clavel and Bernhard Schulz, at the ISO Data Centre in Villafranca (Madrid, Spain), found evidence that the so-called Seyfert 1 and Seyfert 2 galaxies are also the same type of object but viewed at a different angle.

Both these categories of galaxy also have a much brighter nucleus than normal galaxies and are also thought to be powered by a black hole at their cores. Their bright, compact, nuclei made them seem somehow related, yet studies of their properties revealed puzzling and detailed differences, making these objects stand as an enigma over several decades. The unification theory once again came to the rescue. Apparently Seyfert 1 and Seyfert 2 galaxies represent a common class of object. But Seyfert 1 galaxies are observed face-on, while Seyfert 2 galaxies are seen at a certain inclination; the internal edge of the doughnut, where the dust is hot and emits strongly in the mid-infrared, is revealed in the Seyfert 1 galaxies, but that same part of the ring is hidden in Seyfert 2 galaxies. That would explain the differences in their emission. Clavel and Schulz observed a sample of 57 objects with ISO to verify this prediction. They were even able to measure the amount of dust in the ring.

So, are Seyfert galaxies, in turn, the same as radio galaxies and quasars? Essentially yes, astronomers say, but their central black holes are smaller.

The next task of the groups working in the field will be to observe more sources with more sensitive instruments. Astronomers are looking forward to working with ESA?s next infrared space observatory, Herschel, to be launched in 2007.