21cm Line Widths and Distances of Spiral Galaxies

Abstract

The Tully-Fisher relation is analysed for the sample of field spirals with known IR magnitudes m_H. They exhibit a pronounced distance effect in the M(IR) vs.log ∆v₂₁ relation and in the log ∆v₂₁ vs. M(IR) relation. This means that in addition to the Malmquist effect another bias must be present. A discussion of possible bias sources reveals that, in addition to type differences, the line width ∆v₂₁ itself has probably influenced the sample. Indeed, a nearly complete sample of Sbc, Sc galaxies does not show any distance bias in the (Malmquist-free) log v₂₁ vs. M(B) relation. This demonstrates that even the latter relation can be applied only to well defined samples of type-restricted galaxies, if unbiased distances are aimed at.