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We have measured the depth of the 2.3 $\mu$m CO absorption bands in 19 high luminosity galaxies selected from the IRAS catalog. We used the CGAS spectrometer on the IRTF with a beamsize of 2.7 arcsec and a spectral resolution of 120.
Our main result is that we find a relationship between the depth of the CO absorption and the $H - K$ color of a galaxy. Specifically:
\begin{itemize} \item Galaxies with $H - K$ colors in the range 0.25 to 0.6 have a deeper CO absorption than normal galaxies, which have $H-K$ colors of less than 0.25. Their near infrared emission is probably dominated by late-type supergiants which have been reddened by interstellar dust. \item Galaxies with $H - K$ colors redder than 0.7 have much weaker than normal CO absorption. Their near-infrared emission must therefore be dominated by something other than starlight. Of the five objects with the reddest $H - K$ colors, three are ultraluminous galaxies and two are off-nuclear components of the interacting system Arp 299. \end{itemize}
