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Published July 1990 | Published
Journal Article Open

The IRAS bright galaxy sample. V. Multibeam photometry of galaxies with L(IR) ⩾ 10^(11) L_☉


Forty-seven galaxies from the IRAS Bright Galaxy Sample with infrared luminosities L_(IR)⩾ 10^(11) L_☉ have been measured at 1.3, 1.65, and 2.2 µm with beam diameters of 17", 33", and 55". These measurements, combined with 5" and 10" observations presented in an earlier paper, provide an opportunity to study the spatial distribution of the near-infrared emission in luminous IRAS galaxies. It is found that the unusually red near-infrared colors known previously for many of these galaxies are confined to the nuclear regions, whereas the outer disk regions have near-infrared colors essentially appropriate for a normal stellar population. Since dust reddening and emission are required to explain the unusual nuclear colors, it follows that the observed effects of dust in these galaxies are also confined primarily to the nuclei. Thus, it is probable that the far-infrared emission, the bulk of the entire luminosity in infrared luminous galaxies, is highly concentrated about the nuclei, and that the physical processes responsible for the unusual properties of infrared luminous galaxies tend to occur within the central regions, with diameters ≾1-3 kpc. The nuclei are found to have considerably higher 2.2 µm luminosities than are found in classical "starburst" nuclei, implying that infrared luminous galaxies are characterized by extremely high radiation densities in their central regions, presumably due to intense star formation activity and/or the presence of a dust-enshrouded quasar. However, the nuclei of the galaxies studied are typically not as luminous at 2.2 µm as classical Seyfert nuclei, which may be partly attributable to extinction from dust at near-infrared wavelengths, particularly for those sources in the sample that have been identified in the literature as having Seyfert nuclei. Finally, the large diameter beam measurements are used to obtain estimates of the total near-infrared emission. It is found that, since most of the infrared luminosity is coming from the nuclei, the global near-infrared properties of infrared luminous galaxies are not good tracers of infrared activity. Also, the contribution from the observed stellar emission to the total observed luminosity is found to be ≾25% for most of the galaxies in the sample, considerably smaller than the value for typical low-luminosity spiral galaxies.

Additional Information

© 1990 American Astronomical Society. Received 9 January 1990; revised 26 March 1990 Special thanks are given to S. E. Persson for the use of his infrared photometer, and to Dave Tennent, John Henning, and Mike Doyle for their much needed help in getting it all working. Thanks are also given to the night assistants at the Palomar 60 in. telescope, Russ Day, Will McKinley, Jeff Phinney, and Skip Staples, and the entire staff at Palomar Observatory. A number of useful comments and references were provided by the anonymous referee, and we greatly appreciate his/her time and effort in reviewing this work. The Palomar 60 in. telescope is owned jointly by the California Institute of Technology and the Carnegie Institution of Washington. Ground-based astronomy at Caltech is supported by a grant from the National Science Foundation.

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