Ordinary-looking spiral galaxies can hide a blazing black hole core
Photograph a Seyfert galaxy in visible light and it resembles any other spiral. Examine other wavelengths and its tiny core turns out to shine about as brightly as all the stars of a galaxy like the Milky Way combined, fed by gas spiralling into a supermassive black hole.
The first clue came in 1908, when Edward Fath and Vesto Slipher at Lick Observatory took spectra of so-called spiral nebulae. Most showed the dark absorption lines typical of starlight, but NGC 1068 displayed six bright emission lines. In 1926 Edwin Hubble classed it and two similar objects as lying beyond our galaxy, and in 1943 Carl Seyfert described a group of galaxies with brilliant, star-like centres producing broad emission lines. The class now bears his name.
Seyfert galaxies and quasars are the two biggest families of active galaxies and share the same engine: an accretion disk around a central black hole, thought to generate the ultraviolet glare. The difference is degree. A Seyfert nucleus roughly matches the combined starlight of its galaxy, so the host remains visible, while a quasar's nucleus overwhelms its surroundings. Seyferts are also nearer and dimmer, which makes them easier to study. Once thought to make up about a tenth of all galaxies, they appear in roughly 16 percent when faint and dust-obscured examples are counted.
Astronomers split them by their spectra. Type I nuclei show broad emission lines, type II only narrow ones, with in-between cases labelled 1.2 through 1.9. Line width tracks speed: gas swirling fast close to the black hole smears its light into broad lines, while slower gas farther out gives narrow ones. Broad lines can change within short periods, but narrow ones barely vary, confirming they come from a much larger zone; in NGC 1068 the line-emitting region spans more than a thousand light-years.
That variability is a tool. Reverberation mapping times how the broad lines respond to changes in the central glow, revealing the structure of the emitting region and, for 35 active nuclei, the mass of the black hole. Radiation pressure also sets a floor: the hole must be heavy enough that its gravity keeps the glowing disk from blowing apart.
Source: Seyfert galaxy
