Internal extinction, population incompleteness bias and the faint-end of the B-band Tully-Fisher relation

We analyse the distribution of three different samples of spiral galaxies of the Local Universe in the B-band luminosity-maximum rotational velocity (LB-VMax) plane, i.e. the B-magnitude Tully-Fisher (TF) relation. The first sample comprises 25+31 late-type (from Sa to Im-BCD) galaxies, selected from, respectively, the Virgo and the Ursa Major clusters. In addition, a sample of 20 group extreme late-type (ELT) spiral (from Sd to Sm) galaxies and a sample of 13 dwarf late-type galaxies are selected. First, we analyse the influence of three different corrections for internal extinction (IE) on both the slope and the zero-point of the relation. These recipes reflect either i) the morphological type dependence of the IE or ii) the assumption of a “sandwich-model” for the distribution of the dust and the stellar components within the disk or iii) the existence of the metallicity-mass relation. As a first result we find that the latter recipe of IE correction systematically makes Sa-Sc galaxies brighter than later spirals. Since the former galaxies mainly populate the region of the bright, fast rotators, the slope of the corresponding TF relation becomes much steeper than in the other two cases. Under the hypothesis that the ELT spiral galaxies in clusters and in groups share the same LF, we show that the claimed skew of the group ELT spiral galaxies with respect to the extrapolated TF relation of more luminous systems (i.e. the local TF calibrators) is due both to the population incompleteness bias affecting such a template and to the type segregation. Finally, we confirm that the distribution of dwarf galaxies with comparable rotational and random velocities in the LB-VMax plane is consistent with the relation of more luminous systems, while the rotationally supported dwarf galaxies are underluminous with respect to the ELT galaxies of the same VMax. However, the latter results may suffer from the heterogeneous methods of measurement of dwarf galaxy distances that were adopted.