Massive Stars as Important Contributors to Two Micron Light

Near infrared light at 2m is relatively insensitive to the presence of hot young stars and dust in galaxies, and there has been recent interest in using it as a mass tracer in spiral galaxies. I present evidence that young, cool supergiant stars, whose presence is indicated by strong CO absorption in a 2:36m bandpass, dominate the 2m light from active star forming regions in the galaxy NGC 1309. The galaxy's quiescent regions, in contrast, do not show evidence of young supergiants. It follows that the 2m light comes from diierent stellar populations in diierent places, and large changes in the 2m surface brightness need not imply correspondingly large features in the galaxy's mass distribution.