A Near-Infrared Spectroscopic Study of 60 Micron Peakers

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Abstract
We present near-infrared (NIR) spectroscopy of a sample of IRAS-selected galaxies with warm far-infrared colors and spectral energy distributions peaking near 60 μm. Of the 21 60 μm peakers classified by optical spectroscopy as Seyfert 2 galaxies, we confirm the existence of dust-obscured broad lines in four (Mkn 3, Mkn 1210, Mkn 463, IRAS 20460). We also propose that two additional 60 μm peakers may have weak broad lines in the NIR (UGC 02024, IRAS 06488), but we require higher quality data for confirmation. All four of the 60 μm peakers with confirmed broad NIR lines are known to have broad lines in polarized light, while the two possible candidates have not been observed with spectropolarimetry so far as we are aware. These would be good candidates for future spectropolarimetric observations on large telescopes. Within our sample of 60 μm peakers, the average ratio of He I/Brγ is larger for the starburst galaxies relative to the Seyferts. This is consistent with an evolutionary scenario for 60 μm peakers, with the starburst class representing an early phase of evolution following an interaction event and the Seyfert class representing a later stage in which the nuclear starburst has faded and has either collapsed to form a central black hole or at least to fuel an existing one. The He I/Brγ ratio for the starburst class of 60 μm peakers is typical of a young starburst with a burst age less than 5 Myr, which is in agreement with their optical spectra. Various near-infrared spectroscopic features ([Fe II]/Paβ, K-band continuum shape, CO absorption, and line widths) are generally consistent with their optical spectroscopic classifications. The exceptions are: IRAS 00160, IRAS 08007A, and IRAS 10567, which are classified as starburst via optical spectroscopy, yet some of their NIR spectral features point toward the existence of a dust-obscured AGN. The stellar absorption CO feature, an indicator of star formation, is detected more strongly in the starburst 60 μm peakers compared to the Seyfert class, presumably because of dilution by hot dust emission from the active nucleus in the Seyfert 60 μm peakers.

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