Massive stars as important contributors to two micron light

Abstract

Near infrared light at 2 micrometers 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 2.36 micrometer bandpass, dominate the 2 micrometer 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 2 micrometer light comes from different stellar populations in different places, and large changes in the 2 micrometer surface brightness need not imply correspondingly large features in the galaxy's mass distribution.