TRANSVERSE DISSECTIONS OF THE FUNDAMENTAL PLANES OF ELLIPTIC GALAXIESAND CLUSTERS OF GALAXIES

Seen edge-on, the fundamental plane (FP) of elliptical galaxies is ver y thin and tilted, which corresponds to a very small dispersion (less than or similar 12%) of the M/L ratio at each location on the FP, and to a trend by a factor approximately 3 of M/L along the FP, respective ly. In this Letter we investigate two possible origins of the tilt: a systematic change of the stellar initial mass function (IMF), and a tr end in the relative distributions of the bright and dark matter. In bo th cases we find that major changes in either the IMF or the bright/da rk matter relative distributions are required along the FP to account for its tilt, while its constant thickness requires an extremely small dispersion in the IMF or in the bright/dark matter distribution at ev ery location on the FP. In both cases fine-tuning seems required to ac count for the tilt and yet preserve the small, constant thickness of t he FP. We suggest observations that may distinguish between the two op tions. Also clusters of galaxies appear to define an equally thin but untilted FP (i.e., M/L = constant). We discuss some implication of the existence of such cluster FPs for galaxy formation and cluster evolut ion.