Young red supergiants and the near-infrared light appearance of disk galaxies

Disk galaxies often show prominent nonaxisymmetric features at near-infrared (NIR) wavelengths. Such features may indicate variations in the surface density of stellar mass, contributions from young red supergiants in star-forming regions, or substantial dust obscuration. To distinguish among these possibilities, we have searched for spatial variations in the 2.3 μm photometric CO index within the disks of three nearby galaxies (NGC 278, NGC 2649, and NGC 5713). This index measures the strength of the absorption bands of molecular CO in stellar atmospheres and is strong in cool, low surface gravity stars, reaching the largest values for red supergiants. We observe significant spatial CO index variations in two galaxies (NGC 278 and NGC 5713), indicating that the dominant stellar population in the NIR is not everywhere the same. Central CO index peaks are present in two galaxies; these could be due to either metallicity gradients or to recent star formation activity. In addition, significant azimuthal CO index variations are observed in NGC 278. Because strong azimuthal metallicity gradients are physically implausible in disk galaxies, these features are most naturally explained by the presence of a young stellar population. The fraction of 2 μm light due to young stellar populations in star-forming regions can be calculated from our data. Overall, young stellar populations can contribute ∼3% of the NIR flux of a (normal) galaxy, which is consistent with other global properties (e.g., the near-IR Tully-Fisher relation). Locally, this fraction may rise to ∼33%. Thus, young stars do not dominate the total NIR flux, but can be locally dominant in star-forming regions, and can bias estimates of spiral arm amplitude or other nonaxisymmetric structures in galaxies' mass distributions.