The σ-L Correlation in Nearby Early-Type Galaxies

Early-type galaxy velocity dispersions and luminosities are correlated. The correlation estimated in local samples (≤100 Mpc) differs from that measured more recently in the Sloan Digital Sky Survey (SDSS). This is true even when systematics in the SDSS photometric and spectroscopic parameters have been accounted for. We show that this is also true for the Nearby Early-Type Galaxies (ENEAR) sample if galaxy luminosities are estimated using distances that have been corrected for peculiar motions. We then show that, because the estimate of the "true" distance is derived from a correlation with velocity dispersion, in this case the D_n-σ relation, using it in the σ-L relation leads to an artificially tight relation with a biased slope. Making no correction for peculiar velocities results in a σ-L relation that is very similar to that of the SDSS, although with larger scatter. We also measure the σ-L correlation in a mock ENEAR catalog, in which the underlying galaxy sample has the same σ-L correlation as seen in the SDSS. The mock catalog produces the same D_n-σ relation as the data, the same biased slope when D_n-σ distances are used to estimate luminosities, and good agreement with the input σ-L relation when redshift is used as the distance indicator. This provides further evidence that the true σ-L relation of ENEAR galaxies is indeed very similar to that of SDSS early types. Our results suggest that the local σ-L relations based on fundamental plane distances should also be reevaluated. Our findings also have important implications for black hole demographics; the best direct estimates of the masses of supermassive black holes come from local galaxies, so estimates of the black hole mass function are more safely made by working with the M_•-σ correlation than with the M_•-L correlation.