Caltech Astronomers Obtain the Most Detailed Infared Image of the Environment of an Active Black Hole

TORONTO — Sophisticated imaging techniques applied on the Keck Telescope have uncovered a new structure in a nearby active galaxy.

The image and associated research are being presented today at the semiannual meeting of the American Astronomical Society. Alycia Weinberger, a doctoral student in physics at the California Institute of Technology, and her collaborators have used the computer-intensive technique of speckle imaging and the 10-meter W. M. Keck Telescope atop Mauna Kea, Hawaii, to image the nucleus of NGC 1068.

This galaxy, found in the constellation Cetus at a distance of about 50 million light years, reveals a a bright active nucleus at infrared wavelengths. This nucleus has long been thought to harbor a black hole as its central engine and, because it is bright and nearby, has been intensely studied by astrophysicists.

The accompanying false color image shows an elongated structure, which is over 100 light-years across, centered on a bright point-like infrared nucleus. In contrast, the bright disk of the galaxy NGC 1068 is over 30,000 light-years across at visual wavelengths.

Made at a wavelength of 2.2 microns, Weinberger's near-infrared image has the capability to reveal structures which are only 12 light years across. This is an extremely small distance by galactic standards, as small as about three times the distance between the Sun and its nearest stellar neighbors. Although taken from a ground based observatory, this image has resolution as fine as what the Hubble Space Telescope achieves in the visual part of the spectrum. The space telescope does not currently have an infrared camera, but is scheduled to receive one in 1997. The elongated feature discovered by the Caltech group has not been seen in Hubble's optical images.

There are two very interesting aspects of this image. First, the image is elongated, and second the axis of the emission points in a different direction than previously observed visual emission. The near-infrared light used to make this picture typically traces the distribution of hot dust and cool stars.

However, in NGC 1068, it is very unlikely that there could be dust 100 light-years from the central black hole which would be hot enough to produce the observed emission. Rather, Weinberger says, it is likely that the observed extended near-infrared light is from stars. Furthermore, since it points in a different direction, this newly resolved infrared emission is likely to come from an entirely different source than previously observed visual emission.

It has long been proposed that stellar bars are a way of funneling material to an active nucleus. As gas moves in a non-circular distribution of stars, such as what may be seen in Weinberger's image, it is forced into orbits likely to take it near the central black hole. This provides a continuous mechanism for "feeding" the central engine.

"The significance of this research is that it finds a brand-new feature in this galaxy. And even more, this new feature may provide observational evidence for a theoretically predicted means of channelling material to the black hole on very small scales," Weinberger says. The image is by no means detailed enough to show the in-fall of the matter itself, Weinberger stresses. For this, one would need a resolution of less than a light-year, and there is currently no way to make such finely detailed pictures.

Nonetheless, the quality of this image is unparalleled because it relies on the unique resolving power of Caltech's 10-m Keck Telescope and the technique of speckle interferometry to remove the distorting effects of Earth's atmosphere. With this technique, a series of very rapid exposures are made of the object, freezing the atmospheric distortions that cause stars to "twinkle." Then the distortions are removed in computer post-processing. As the largest infrared telescope in the world, the Keck Telescope provides the best obtainable resolution.

Weinberger is currently completing work on her doctorate. She will continue doing observations to support this research, a part of her thesis. "It will be exciting to look at NGC 1068 with similar resolution in other infrared wavelengths," she says. "The more information we have across the spectrum the more we'll understand about the nature of this extended emission."

Also collaborating in this research are her thesis supervisor, Gerry Neugebauer and Keith Matthews both of the Caltech physics department.