On the origin of the scatter around the Fundamental Plane: correlations with stellar population parameters

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 Sigma similar to 50 km s(-1). We obtain a best-fitting FP relation r(e) proportional to Sigma(1.06 +/- 0.06)(0)>(-0.82 +/- 0.02)(e) in the R band. The shallow exponent of Sigma(0) is a result of the extension of our sample to low velocity dispersions. Considering the subsample of Sigma(0) > 100 km s(-1) galaxies, the FP relation is r(e) proportional to Sigma(1.35) >(-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 alpha/Fe. We find that the FP residuals anticorrelate (> 3 Sigma) with the mean stellar age in agreement with previous work. However, a stronger (> 4 Sigma) correlation with alpha/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 alpha overabundances larger than average, for their Sigma. Including alpha/Fe as a fourth parameter in the FP, the total scatter decreases from 0.088 to 0.075 dex and the estimated intrinsic scatter decreases from 0.068 to 0.049 dex. Thus, variations in alpha/Fe account for similar to 30 per cent of the total variance around the FP, and similar to 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.