The near-infrared Fundamental Plane of elliptical galaxies

Near-infrared (2.2 mu m) observations of a sample of 48 elliptical galaxies in the Coma cluster have been carried out and used to study the near-infra red Fundamental Plane (FP) of ellipticals in this cluster. An rms scatter o f 0.072 dex is found for this relation, similar to that of its optical coun terpart, using the same sample of galaxies. This corresponds to an uncertai nty of 18 per cent in distances to individual galaxies derived from this re lation. The sensitivity of the near-infrared FP to the star formation or ch anges in metallicity and stellar population among the ellipticals is explor ed and found to be small, although a likely source of scatter in this relat ion is contributions from the asymptotic giant branch (AGB) population to t he near-infrared light. Allowing for observational uncertainties, we find a n intrinsic scatter of 0.060 dex in the near-infrared FP. The cluster galax ies presented here provide the zero-point for the peculiar velocity studies , using the near-infrared FP. Changes in the slopes of the D-sigma and L-sigma relations of ellipticals b etween the optical and near-infrared wavelengths are investigated and found to be due to variations in metallicity or age (or a combination of them). However, it is not possible to disentangle the effects of age and metallici ty in these relations. We find M/L proportional to M-alpha with alpha = 0.18 +/- 10.01 in the near -infrared and alpha = 0.23 +/- 0.01 at optical wavelengths, using the same sample of galaxies. This relation is interpreted as being due to a mass-met allicity effect or changes in age or the initial mass function slope with m ass. Using evolutionary population synthesis models, we find that the effec ts of age and metallicity decouple in the (M/L)(K) versus Mg-2 and (M/L)(K) versus (V - K) diagrams. The models suggest that the observed trends in th ese relations may be due to an age sequence, while metallicity mainly contr ibutes to the scatter.