OSCIR Probes Heated Dust in the Active Galactic Nucleus of the Spiral Galaxy NGC 4151 Home
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. The
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. For
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.
1. Contours of mid-infrared emission overlaid on other published
observations of the nuclear region of NGC 4151. Images (a) and (b) show
the extended N and IHW18 emission, respectively, overlaid on a HST [O
III] image which traces ionized gas in the region. Images (c) and (d )
show the same N and IHW18 emission overlaid on the radio jet, as
observed at 18 cm. In all images, the peak emission in the radio and [O
III] are aligned to correspond with the peak in the mid-IR. Figure
2. X-ray image of the central region of the galaxy NGC 4151 obtained
with the satellite Chandra. This image shows that the X-ray emission
from a massive cloud of nuclear gas that is heated by the central black
hole is extended in approximately the same direction as the m
id-infrared emission detected by OSCIR.
Gemini's mission is to advance our knowledge of the Universe by providing the international Gemini Community with forefront access to the entire sky.
The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.
The Gemini Observatory provides the astronomical communities in five partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, the Brazilian Ministério da Ciência, Tecnologia e Inovação and the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership. Sitemap | Website Chilean Mirror