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The Astronomical Journal Volume 119 Number 2 Create an alert RSS this journal

B. T. Soifer et al. 2000 The Astronomical Journal 119 509 doi:10.1086/301233

High Resolution Mid-Infrared Imaging of Ultraluminous Infrared Galaxies*

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B. T. Soifer1,7, G. Neugebauer1, K. Matthews1, E. Egami1, E. E. Becklin2, A. J. Weinberger2, M. Ressler3, M. W. Werner3, A. S. Evans4,8, N. Z. Scoville4, J. A. Surace5 and J. J. Condon6

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Abstract References Cited By

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 0farcs3 (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, 0farcs13, 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.


Footnote
*  Based in part on observations obtained at the W. M. Keck Observatory which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration.
Keywords

galaxies: individual (Arp 220, Markarian 231, Markarian 273, UGC 5101, IRAS 05189-2524, IRAS 08572+3915, IRAS 17208-0014)


Dates

Issue 2 (2000 February)

Received 1999 September 26, accepted for publication 1999 October 26



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