OSCIR Probes Heated Dust in the Active Galactic Nucleus of the Spiral Galaxy NGC 4151
May 12, 2003
In 1943 the American astronomer Carl K. Seyfert reported that a small percentage of galaxies have very bright nuclei that are the source of broad optical emission lines seen in their spectra. Nearly stellar in appearance, these nuclei are responsible for the emission of strong lines produced by atoms in a wide range of ionization stages. Today these objects are known as Seyfert Galaxies and the spiral galaxy NGC 4151 is a good example of this type of object. Seyfert Galaxies belong to a broader class of objects called Active Galactic Nuclei (AGN) that are characterized by energetic emission across the entire electromagnetic spectrum, from the radio to X-rays. The power source for an AGN is thought to be a central supermassive black hole where energy is generated by matter accreting onto the black hole itself and onto its surrounding accretion disk.
Radomski of the University of Florida and his collaborators used the
mid-infrared camera OSCIR mounted on Gemini North to observe the
nuclear region of NGC 4151 at the wavelengths of 10.8 and 18.2 microns.
At a distance of 13.2 Mpc (42 million light years) NGC 4151 harbors one
of the nearest and best-studied active galactic nuclei. Indeed,
monitoring of NGC 4151 has shown that its AGN is highly variable in
both continuum and line emission. The mid-infrared emission of NGC 4151
has been suggested to arise from either thermal emission from dust
grains or synchrotron emission.
Gemini observations conducted by Radomski et al. show that the
mid-infrared emission in NGC 4151 is compact but resolved. It extends
~3.5 arcseconds, or 200 pc, across the nucleus at a position angle that
matches the narrow line region as observed in the optical at [OIII]5007
by the Hubble Space Telescope (Figure 1). The most
likely explanation for the extended nature of the mid-infrared emission
is that the central engine is heating dust in the narrow line region.
The authors find no extended emission associated with the proposed
central torus and are able to place an upper limit of < 35 pc on its
mid-infrared size. The fact that the elongated shape of the
mid-infrared emission is coincident with the emission seen in the
optical, radio, and X-rays (Figure 2) seems to
confirm that the high energy radiation emitted by the black hole is
collimated into a narrow beam that illuminates gas and dust lying in
certain quadrants of the central region of the galaxy.
more detailed information, refer to the paper by Radomski, J., Pina, R.
K., Packham, C., Telesco, C. M., De Buizer, J., Fisher, R. S. and
Robinson, A. "Resolved Mid-Infrared Emission in the Narrow-Line Region of NGC 4151", The Astrophysical Journal, vol. 587, pp. 117-122, April 10, 2003.
The University of Florida developed the mid-infrared camera OSCIR with support from NASA and the National Science Foundation.