Galactic views through thick and thin.

Galactic views through thick and thin

More luminous than ordinary galaxies but less intensely radiant than quasars, Seyfert galaxies fall somewhere between the two extremes. Now an astronomer reports evidence that a doughnut-shaped ring of opaque material obscures the very center of at least one Seyfert galaxy. Moreover, this arrangement allows some light to escape from the galaxy's center -- like a search-light emerging from the doughnut's hole. Reflections of that light from dust clouds near the doughnut allow astronomers to peek into the otherwise hidden galactic center.

"For the first time, we know what the center of a [Seyfert] galaxy looks like from two different directions," says Joseph S. Miller of the University of California at Santa Cruz. In the direct line of sight, we get an obscured view of the galaxy's nucleus, he says. From the dust-cloud reflections, we see clearly into the galactic center.

To identify this effect, Miller used a special instrument to study polarized light coming from the center of the Seyfert galaxy known as NGC 1068. The galaxy's hidden nucleus, less than a light-year across, is such a tiny fraction of the galaxy's full width that no telescope could pick it out. Despite its size, however, that nucleus sends out an immense quantity of radiation in the form of visible light and radio waves.

Miller's discovery may force revisions in the way Seyfert galaxies are classified. Traditionally, astronomers have divided Seyfert galaxies into two types, based on the characteristics of their emitted light. In general, typical spectra from Seyfert galaxies contain prominent lines signifying the emission of particular wavelengths of light by various elements present in their central regions. The spectra of Type 1 Seyfert galaxies feature a broader hydrogen line than those of Type 2.

Miller's results show that NGC 1068 looks like a Type 2 galaxy when observed through the doughnut of obscuring matter and like a Type 1 galaxy when viewed by reflected light. "It could well be the case that there's just one kind of Seyfert galaxy, and it depends on which way we're looking as to what we call it," Miller says. Alternatively, Type 1 galaxies may have thinner rings hiding less of their central regions than Type 2 galaxies. More recent observations show that NGC 1068 is not unique.

Although astronomers don't yet understand the nature of a Seyfert galaxy's obscuring ring or disk, light from these galaxies shows evidence that their centers often contain large quantities of intensely heated gas (SN: 4/27/85, p.262). "As we go from different Seyfert galaxies to quasars, we've determined that the structure of this [obscuring] disk changes," says Miller. "Quasars appear to have a very thin disk. It is only in Seyfert galaxies that we see a big, thick disk." Those characteristics may be related to how much energy these central sources put out. "We now have the possibility of constructing a complete picture of what's going on in the nuclei of quasars and active galaxies."