The scatter in the near-infrared colour-magnitude relation in spiral galaxies

We have determined a dust-free colour-magnitude (CM) relation for spiral galaxies, by using I - K colours in edge-on galaxies above the plane. We find that the scatter in this relation is small and approximately as large as can be explained by observational uncertainties. The slope of the near-IR CM relation is steeper for spirals than for elliptical galaxies. We suggest two possible explanations. First, the difference could be caused by vertical colour gradients in spiral galaxies. In that case these gradients should be similar for all galaxies, on average 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. This means that the stars in spirals are younger than those in ellipticals. The age, however, or the fraction of young stars in spiral galaxies would be determined solely by the luminosity of the galaxy, and not by its environment.