SIMBAD references

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 σ_e. Since I_e and σ_e are distance-independent measurements, the thickness of the FP is often expressed in terms of the accuracy with which I_e and σ_e can be used to estimate sizes R_e. We show that: (1) The thickness of the FP depends strongly on morphology. If the sample only includes E-SRs, then the observed scatter in R_e is ∼16 per cent, of which only ∼9 per cent is intrinsic. Removing galaxies with M_* < 10¹¹ M_☉ further reduces the observed scatter to ∼13 per cent (∼4 per cent intrinsic). The observed scatter increases 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 σ_e are nearly independent, whereas the R_e-I_e and R_e-σ_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.