High Resolution Mid-Infrared Imaging of Ultraluminous Infrared Galaxies
Observations of ultraluminous infrared galaxies (ULIRGs) with an achieved resolution approaching the diffraction limit in the mid-infrared from 8 to 25 μm using the Keck Telescopes are reported. We find extremely compact structures, with spatial scales of less than 0."3 (diameter) in six of the seven ULIRGs observed. These compact sources emit between 30% and 100% of the mid-infrared energy from these galaxies. We have utilized the compact mid-infrared structures as a diagnostic of whether an AGN or a compact (100–300 pc) starburst is the primary power source in these ULIRGs. In Markarian 231, the upper limit on the diameter of the 12.5 μm source, 0."13, shows that the size of the infrared source must increase with increasing wavelength, consistent with AGN models. In IRAS 05189-2524 and IRAS 08572+3915 there is strong evidence that the source size increases with increasing wavelength. This suggests heating by a central source rather than an extended luminosity source, consistent with the optical classification as an AGN. The compact mid-infrared sources seen in the other galaxies cannot be used to distinguish the ultimate luminosity source. If these ULIRGs are powered by compact starbursts, the star formation rates seen in the central few hundred parsecs far exceed the global rates seen in nearby starburst galaxies, and approach the surface brightness of individual clusters in nearby starburst galaxies.
Additional Information© 2000. The American Astronomical Society. Received 1999 September 26. Accepted 1999 October 26. We thank J. Aycock and R. Campbell for assistance with the observations, R. Goodrich, R. Moskitis, and the entire Keck staff for their help establishing the visitor port and to Barbara Jones, Rick Puetter, and the Keck team that brought the LWS into service, enabling these observations. The W. M. Keck Observatory is operated as a scientific partnership between the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. It was made possible by the generous financial support of the W. M. Keck Foundation. This research has made use of the NASA/IPAC Extragalactic Database which is operated by the Jet Propulsion Laboratory, Caltech under contract with NASA. B. T. S., G. N., K. M., and E. E. are supported by grants from the NSF and NASA. J. A. S. and B. T. S. are supported by the SIRTF Science Center at Caltech. SIRTF is carried out at the Jet Propulsion Laboratory. E. E. B., A. J. W., N. Z. S., and A. S. E. were supported by NASA grant NAG5-3042. This work was carried out in part (M. W. W., M. R.) at the Jet Propulsion Laboratory, operated by the California Institute of Technology, under an agreement with NASA. The development of MIRLIN was supported by NASA's Office of Space Science. The National Radio Astronomy Observatory (J. J. C.) is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
Published - Soifer_2000_AJ_119_509.pdf
Submitted - 9911045.pdf