V. I. Harvey 1, S. Satyapal 2, M. L. Luhman 3, J. Fischer 3, P. E. Clegg 4, P. Cox 5, M. A. Greenhouse 2, S. D. Lord 6, M. A. Malkan 7, H. A. Smith 8, L. Spinoglio 9, G. J. Stacey 10, & S. J. Unger 4
1 University of Nevada, Las Vegas/NASA Goddard 2 NASA Goddard Space Flight Center 3 Naval Research Laboratory 4 Queen Mary and Westfield College 5 Institut d'Astrophysicque Spatiale 6 California Institute of Technology 7 University of California, Los Angeles 8 Smithsonian Astrophysical Observatory 9 CNR-Istituto di Fisica dello Spazio Interplanetario 10 Cornell University
We present far-infrared spectroscopy of the ultraluminous infrared Seyfert 1 galaxy Mrk 231 obtained with the ISO Long Wavelength Spectrometer. Based upon its optical spectra, its compact unresolved nucleus, and its variable radio source, Mrk 231 has been classified as a classic Seyfert 1. On the other hand, it has some of the characteristics of a starburst galaxy, such as strong FeII emission, and the detection of a powerful OH megamaser. Our observations reveal a far-infrared spectrum distinctly different from classical starburst galaxies. Like the prototypical ULIG Arp 220, the far-infrared spectrum shows weak or absent far-infrared emission lines and many strong absorption lines. This could arise from a stellar population deficient in the most massive stars, or extremely high extinction, extending into the far-infrared. In this paper, we investigate these possibilities. In addition, we have obtained near-infrared 3.3 micron dust feature observations of this source, which provides a measure of the starburst activity in galaxies. Combining the near-infrared imaging with the ISO spectroscopy, we discuss the origin of the dominant luminosity source in this galaxy. We also discuss the properties of MRK 231 with respect to other ULIGs studied by the ISO extragalactic LWS consortium.