9 JANUARY 1998
Using the new Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope (HST) combined with archival images from the HST's Wide Field Planetary Camera (WFPC), the astronomers were able to trace where the dense dust and gas reside in the central regions of several Seyfert galaxies. Dust does not absorb very much light in the near infrared, as it does in the wavelength range seen by the human eye. Thus, Regan and Mulchaey were able to locate the galaxies' dust by comparing the near infrared images taken with NICMOS to the visible images taken with WFPC.
Seyfert galaxies are a type of active galaxy characterized by tremendous amounts of energy emanating from their centers. A typical Seyfert galaxy emits the equivalent energy of 1 trillion suns over a volume the size of the solar system or smaller. The only means by which astronomers know how such a large amount of energy can be produced in such a small volume is a black hole. As material falls into a black hole, it is converted to energy. The black holes in Seyfert galaxies are believed to be very massive, perhaps the equivalent of 10 million suns.
Although Seyfert galaxies produce tremendous amounts of energy, the black holes in them are so efficient at converting mass into energy that the typical Seyfert requires only one solar mass of material to fuel the activity for an entire year. While the galaxy itself is more than capable of supplying this amount of fuel, it is not clear how the mass from the outer parts of the galaxy is transported to the center of the galaxy where the black hole resides. "Finding out how this dense gas gets into the central light year around a black hole is one of the goals of active galaxy research," says Dr. Regan, a Hubble Fellow at the Carnegie Institution's Department of Terrestrial Magnetism.
Two false-color images on the world wide web show active galaxies NGC 1667 (GIF 1 Mb) and NGC 3982 (GIF, 93 Kb), respectively. NGC 1667 is 220 million light years away. The resolution of the dust map is 50 light years. "The photo shows that the dust is spiraling into the center of the galaxy and that the dust is not symmetrical around the center of the galaxy," says Dr. Regan. "This is important because up till now only nested stellar bars were thought capable of forcing the gas into the center of a galaxy."
NGC 3982 is 55 million light years away. The resolution of the dust map is 12 light years. "In this galaxy, we see that the same spiral arms extend from the nucleus to the rest of the galaxy. The dust map reveals features that cannot be seen in the ground-based image or in the NICMOS image alone," says Dr. Regan.
A stellar bar occurs in a galaxy when the orbits of the stars align into orbits with an elongated shape. Stellar bars are very effective at driving dust and gas radially inward towards the nucleus. For this reason, bars have been proposed as the mechanism by which active galaxies are fueled. "We don't see the classic signatures of bars in the color maps of these galaxies,"' notes Dr. Regan. "Our images suggest instead that material spirals in towards the center." The scientists say that a full understanding of how the spiral mechanism works requires further theoretical modeling.
A further mystery is why more galaxies are not Seyferts. Recent work indicates that most nearby galaxies, including our own Milky Way galaxy, harbor supermassive black holes. However, only a small fraction of these galaxies contain powerful energy sources in their centers. "The simplest explanation is that in most galaxies the black hole is not currently being fed," says Dr. Mulchaey, a postdoctoral research fellow at the Carnegie Observatories. "When a black hole doesn't eat, it doesn't produce any energy."
Although not all the Seyfert galaxies that Regan and Mulchaey looked at show spiral dust lanes into the central regions, none show the dust signatures associated with stellar bars. "The next step is to study some non-active galaxies in a similar way to see how they are different," notes Mulchaey. The team has already scheduled such observations for 1998 with the Hubble Space Telescope.
This work was supported by NASA through HST observing grant GO-07330.