Kinematics of the local universe VI. B-band Tully-Fisher relation and mean surface brightness

Continuing our study of the type dependence of the TF relation, we present a new attempt to reduce the scatter of that relation by using the observed mean surface brightness of galaxies. It is shown that the zero-point of the relation is a continuous function of , well described by a third or fourth degree polynome. Taking this effect into account, the TF scatter decreases by 20 percents, and the uncertainty in the derived TF distancesisreducedaswell,whilethecorrelationcoefcientin- creasesfrom0.67to0.84(diameterrelation).Thisstudyisbased onthestatisticalanalysisoftheKLUN(KinematicsoftheLocal Universe) sample of 5271 spiral galaxies, both for diameter and magnitude, direct and inverse TF relations. The absolute cal- ibration is performed using two sets of good cepheid distance measurements: the rst one, mostly from the HST programmes, relies on distance moduli generally agreed in the literature, the secondoneisasubsetofobjectswhosecepheiddistancemoduli are derived from geometrical distances measured by HIPPAR- COS. The mean surface brightness dependence is rst revealed by the inverse relations, and further studied in the direct way through the normalized distance method whose principle was rst set by Bottinelli et al. (1986), and a rened form recently applied by Theureau et al. (1997b). As a consequence of the smaller scatter of the resulting TF relation, the extracted un- biased subsample is deeper and contains more objects than in previous studies. The Hubble constant H0 may thus be esti- mated from the largest unbiased sample of eld galaxies ever used (577 objects reaching velocities as large as 6000 km s 1 , using the diameter relation). We obtain H0 =5 6 3k m s 1 Mpc 1 and H0 =5 1 4k m s 1 Mpc 1 from HST and HIP-