2009MNRAS.397...75G

We present a Fundamental Plane (FP) analysis of 141 early-type galaxies in the Shapley supercluster at z = 0.049 based on spectroscopy from the AAOmega spectrograph at the Anglo-Australian Telescope and photometry from the WFI on the European Southern Observatory/MPI 2.2-m telescope. The key feature of the survey is its coverage of low-mass galaxies down to σ ∼ 50km/s. We obtain a best-fitting FP relation r_e∝ σ^1.06±0.06₀<I>^{–0.82±0.02}_e in the R band. The shallow exponent of σ₀ is a result of the extension of our sample to low velocity dispersions. Considering the subsample of σ₀> 100km/s galaxies, the FP relation is r_e∝ σ^1.35 <I>^–0.81_e, consistent with previous studies in the high-luminosity regime. We investigate the origin of the intrinsic FP scatter, using estimates of age, metallicity and α/Fe. We find that the FP residuals anticorrelate (>3σ) with the mean stellar age in agreement with previous work. However, a stronger (>4σ) correlation with α/Fe is also found. These correlations indicate that galaxies with effective radii smaller than those predicted by the FP have stellar populations systematically older and with α overabundances larger than average, for their σ. Including α/Fe as a fourth parameter in the FP, the total scatter decreases from 0.088 to 0.075dex and the estimated intrinsic scatter decreases from 0.068 to 0.049dex. Thus, variations in α/Fe account for ∼30 per cent of the total variance around the FP, and ∼50 per cent of the estimated intrinsic variance. This result indicates that the distribution of galaxies around the FP are tightly related to the enrichment, and hence to the time-scale of star formation. Our results appear to be consistent with the merger hypothesis for the formation of ellipticals which predicts that a significant fraction of the scatter is due to variations in the importance of dissipation in forming merger remnants of a given mass.