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7 Conclusion: Observational errors and selection effects

The mean error of the $\log V_{\rm m}$ determinations are in most of the cases better than 0.01, and the HI-fluxes, in terms of external errors, can be considered as correct within $\simeq$15% of uncertainty at the 2-$\sigma$ level. Errors on radial velocities are generally smaller than 20 km s-1, the mean error over the whole sample being about 8 km s-1.

Part of the observed HI-profiles given in Fig. 5 were recognized as confused, because one or several other galaxies are within the beam. These objects are marked with an "a'' or a "c'' in the last column of Table 2, and individual comments, mainly about possible companions or data quality, may be found in Appendix A; references related to these comments are listed in Appendix B.

Considering possible selection effects, no dependence of the detection rate on inclination, line width, or distance, were observed in the investigated redshift range ($V \leq 10~000$ km s-1). However, the 10 percents of missing objects (i.e. unsuccessfully observed) indicates a possible cut-off of the $\log V_{\rm m}$ distribution. It is indeed probable that, due to noise fluctuations or base-line effects, one tends to miss very narrow as well as very wide HI-profiles. This means that one has to be careful when determining for example the Hubble constant H0 from the inverse Tully-Fisher relation, because a systematic effect, comparable to the Malmquist bias in the direct case, is expected. This debate is presented in details by Ekholm & Teerikorpi (1997), and will be further discussed in a forthcoming paper.

It has been shown also that the HI detection rate is slightly correlated with morphological type. This (expected) result implies that it is necessary to process separately the different types when using either the inverse TF relation, or the TF-21 relation (relation between the 21-cm magnitude and $\log V_{\rm m}$).

Acknowledgements

We have made use of data from the Lyon-Meudon Extragalactic Database (LEDA) compiled by the LEDA team at the CRAL-Observatoire de Lyon (France).


  
Table 2: -excerpt. Astrophysical HI-parameters

\epsfig {file=DS1487T2.eps,angle=-90}


\begin{tabular} {l} Column 1: PGC or LEDA galaxy name; \\ Column 2: most usual g... ...'II'' when the profile was already published in Papers I or II. \\ \end{tabular}


  
\begin{figure*} \centering \epsfig {file=DS1487F5.ps,height=21cm}\end{figure*} Figure 5: -excerpt. 21-cm line profiles of galaxies listed in Table 2; profiles are classified according to their PGC name appearing in the top right corner of each panel. The usual name of the galaxy (NGC, UGC, IC, ESO, MCG...) is written above each panel. Ordinate and abscissae axis are graduated respectively in km s-1 and mJy. Note that heliocentric radial velocities are expressed in terms of optical redshift $c \frac{\Delta \lambda}{\lambda}$. The horizontal line represents the baseline of the profile, from which the maximum is estimated


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