THE SCATTER IN THE NEAR-INFRARED COLOR-MAGNITUDE RELATION IN SPIRAL GALAXIES

We have determined a dust-free colour-magnitude (CM) relation for spir al galaxies, by using I - K colours in edge-on galaxies above the plan e. We find that the scatter in this relation is small and approximatel y as large as can be explained by observational uncertainties. The slo pe of the near-IR CM relation is steeper for spirals than for elliptic al galaxies. We suggest two possible explanations. First, the differen ce could be caused by vertical colour gradients in spiral galaxies. In that case these gradients should be similar for all galaxies, on aver age similar to 0.15 dex in [Fe/H] per scaleheight, and should increase for rater galaxy types. The most likely explanation, however, is that spirals and ellipticals have intrinsically different CM relations. Th is means that the stars in spirals are younger than those in elliptica ls. The age, however, or the fraction of young stars in spiral galaxie s would be determined solely by the luminosity of the galaxy, and not by its environment.