The stellar mass Fundamental Plane: the virial relation and a very thin plane for slow rotators

Early-type galaxies - slow and fast rotating ellipticals (E-SRs and E-FRs) and S0s/lenticulars - define a Fundamental Plane (FP) in the space of half-light radius R-e, enclosed surface brightness I-e, and velocity dispersion sigma(e). Since I-e and sigma(e) are distance-independent measurements, the thickness of the FP is often expressed in terms of the accuracy with which I-e and sigma(e) can be used to estimate sizes R-e. We showthat: (1) The thickness of the FP depends strongly onmorphology. If the sample only includes E-SRs, then the observed scatter in R-e is similar to 16 per cent, of which only similar to 9 per cent is intrinsic. Removing galaxies with M-* < 10(11) M-circle dot further reduces the observed scatter to similar to 13 per cent (similar to 4 per cent intrinsic). The observed scatter increases to similar to 25 per cent usually quoted in the literature if E-FRs and S0s are added. If the FP is defined using the eigenvectors of the covariance matrix of the observables, then the E-SRs again define an exceptionally thin FP, with intrinsic scatter of only 5 per cent orthogonal to the plane. (2) The structure within the FP is most easily understood as arising from the fact that I-e and sigma(e) are nearly independent, whereas the R-e-I-e and R-e - sigma(e) correlations are nearly equal and opposite. (3) If the coefficients of the FP differ from those associated with the virial theorem the plane is said to be 'tilted'. If we multiply I-e by the global stellar mass-to-light ratio M-*/L and we account for non-homology across the population by using Sersic photometry, then the resulting stellar mass FP is less tilted. Accounting self-consistently for M-*/L gradients will change the tilt. The tilt we currently see suggests that the efficiency of turning baryons into stars increases and/or the dark matter fraction decreases as stellar surface brightness increases.