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J.T. Radomski, R.K. Pina (University of Florida Infrared Astrophysics Group)
We present 0.25" resolution mid-infrared images at 10.8 and 18.2 microns of three luminous infrared galaxies: Arp 220, Cygnus A, and NGC 6240. These images were obtained with the University of Florida mid-IR camera/spectrometer OSCIR at the Keck II 10-m telescope. All three of these galaxies show complex subarcsecond nuclear morphology. At 10.8 microns, Arp 220 is clearly resolved into its eastern and western nuclei (separated by 1"). The eastern nucleus of Arp 220 is resolved into two components separated by 0.36". Recent HST/NICMOS images (Scoville et al. 1998 ApJ 492) show the western nucleus to have a crescent shape in the near-IR. These authors suggest this apparent morphology is due to extinction of the nuclear region by a nearly edge-on circumnuclear dust ring. With less susceptibility to extinction in the mid-IR, we find the western nucleus to be azimuthally symmetric. In addition, the western nucleus displays a marginally resolved core of ~ 0.2" (obtained by subtracting the PSF in quadrature). This size is consistent with the spatial extent of radio supernovae in the VLBI observations of Smith et al. (1998 ApJ 493).
For Cygnus A, the central 1 square arcsecond region is found to consist of 3 (and possibly 4) components at both 10.8 and 18.2 microns. The central component is unresolved at both wavelengths and contains 57% of the flux within the inner 1 sq. arcsec. The blackbody limiting size for this unresolved component is 8.6 milliarcseconds, which corresponds to a linear size of 9.6pc at a distance of 230Mpc. The two other components lie above and below the axis of the radio jet. The observed 10.8 and 18.2 micron flux densities in the central 1 sq. arcsecond region account for 82% and 44% of the color corrected IRAS 12 and 25 micron flux densities respectively.
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