Gemini North probes the gas, dust and stars at the center of the active galaxy NGC 1068
December 12, 2005
Two upcoming papers based on optical and mid-infrared data from the Gemini North telescope focus on the dynamic core of the spiral galaxy NGC 1068. Described here are summaries of these papers.
Mid-infrared View of the Environment Surrounding NGC 1068’s Central Black Hole
Rachel Mason (Cerro Tololo Inter-American Observatory, NOAO) and her United States/Gemini staff team used MICHELLE, the mid-infrared imager and spectrometer at the 8-meter Gemini North telescope, to obtain deep 10 micron spectra of the nucleus and central ionization cones of NGC 1068. The superb spatial resolution of the MICHELLE spectra, ~0.4 arcseconds or 30 parsecs at NGC 1068, let the team investigate the close environment of the central back hole on much finer scales than previous work. This allowed Mason et al. to discover that the mid-infrared emission from NGC 1068 originates in two distinct components:
- Emission from the central region (within a diameter of 0.4 arcseconds) is dominated by the infrared-bright obscuring torus. This region is unresolved in the Gemini data, which is consistent with the compact (~3 parsec) size determined by mid-infrared interferometry.
- At larger scales, the mid-infrared flux is dominated by diffuse emission from the AGN-heated dust within the ionization cones (Figure 1). Since the areas within the ionization cones are relatively devoid of material, they have a direct line of sight to the black hole itself. This means that the radiation emitted by the black hole can directly heat the dust present along the inner walls of the cones. The dust absorbs the high-energy photons from the central engine and then re-radiates them in the mid-infrared.
The most striking aspect of the mid-infrared data is the variation in the shape of the spectra (slope of the continuum and strength of the silicate feature and fine structure line fluxes) over small distances (Figure 2). There is significant structure in the dust in the circumnuclear regions even on subarcsecond scales. The 9.7-micron silicate feature appears to be present in the cone.
Figure 1 (left). Central portion of the 11.6 micron acquisition image of NGC 1068, with the position of the MICHELLE spectroscopic slits.
Figure 2 (right). MICHELLE mid-infrared spectra extracted in 0.4 arcsecond steps over the central 0.4 x 6.0 arcsecond region of NGC 1068 along a position angle of 20 degrees.
Optical probing of the jet of gas and of stellar velocities
Joris Gerssen (University of Durham) and his United Kingdom/Gemini staff team used the GMOS-North integral field unit (IFU) to map the velocities of the ionized gas and stars in the central region of NGC 1068 at optical wavelengths. The resulting datacube is rich in complex features, particularly in the morphology of the [O III] doublet and the H-beta line. To interpret the observed structure in the lines the authors constructed an atlas of velocity components by Gaussian fitting of each line.
The atlas contains many features that cannot be readily associated with physical structures in the nucleus; however, some components are likely associated with the expected biconic outflow. Both the H-beta 468.1 nanometer line of hydrogen and the [OIII] 500.7 nanometer line of ionized oxygen are very broad, indicative of velocities between 1000 to 2000 kilometers per second. Centroids of the radial velocity of the gas range from –1000 to 0 kilometers per second over the spatial extent of the jet (Figure 3). Both the gas and stellar data present a very rich and complex morphology. Indeed, a third set of distinct features found in the datacube are suggestive of distinct high-velocity gas flows and a large disk-like structure.
Additionally the stellar kinematics of the nuclear region were mapped through the detection of the Mg b absorption line feature in the GMOS spectra. The resulting position angle and kinematic axis in the GMOS velocity map (Figure 4) is consistent with previously published data on the motions of carbon monoxide in the same region.
Figure 3. The top panels (each covering an area of 10.3 x 7.3 arcseconds) show a series of one Angstrom wide wavelength slices through the data across the [OIII] 500.7 nanometer line of doubly ionized oxygen. Across this line the central morphology of NGC 1068 develops various subcomponents corresponding to different velocities. Lower panels show a similar “movie” across the H-beta 486.1 nanometer line of hydrogen.
Figure 4. . Maps of the stellar kinematics in the center of NGC 1068 (central panel: stellar velocities; right panel, stellar velocity dispersion). Left panel shows an image of the continuum stellar light of the center.
For more details, see “Spatially-resolved mid-infrared spectroscopy of NGC 1068: the nature and distribution of the nuclear material”, by R. E. Mason, T. R. Geballe, C. Packham, N. A. Levenson, M. Elitzur, R. S. Fisher and E. Perlman, The Astrophysical Journal, in press 2006 (or astro-ph/0512202); and “GMOS IFU observations of the stellar and gaseous kinematics in the centre of NGC 1068”, by J. Gerssen, J. Allington-Smith, B. W. Miller, J. E. H. Turner and A. Walker, The Monthly Notices of the Astronomical Society, in press 2006 (or astro-ph/0510475).