The near-infrared Fundamental Plane of elliptical galaxies

Near-infrared (2.2 μm) observations of a sample of 48 elliptical galaxies in the Coma cluster have been carried out and used to study the near-infrared Fundamental Plane (FP) of ellipticals in this cluster. An rms scatter of 0.072 dex is found for this relation, similar to that of its optical counterpart, using the same sample of galaxies. This corresponds to an uncertainty of 18 per cent in distances to individual galaxies derived from this relation. The sensitivity of the near-infrared FP to the star formation or changes in metallicity and stellar population among the ellipticals is explored and found to be small, although a likely source of scatter in this relation is contributions from the asymptotic giant branch (AGB) population to the near-infrared light. Allowing for observational uncertainties, we find an intrinsic scatter of 0.060 dex in the near-infrared FP. The cluster galaxies presented here provide the zero-point for the peculiar velocity studies, using the near-infrared FP. Changes in the slopes of the D--σ and L--σ relations of ellipticals between 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 metallicity in these relations. We find M/L∝Mα with α=0.18±0.01 in the near-infrared and α=0.23±0.01 at optical wavelengths, using the same sample of galaxies. This relation is interpreted as being due to a mass--metallicity effect or changes in age or the initial mass function slope with mass. Using evolutionary population synthesis models, we find that the effects of age and metallicity decouple in the (M/L)K versus Mg2 and (M/L)K versus (V−K) diagrams. The models suggest that the observed trends in these relations may be due to an age sequence, while metallicity mainly contributes to the scatter.