HI line flux (flux corresponding to the area under the 21-cm line
profile) and IRAS fluxes are treated separately from the classical
magnitudes (*UBV*) because they are obtained and corrected in a
completely different way.

The HI line flux is generally expressed in
converted in magnitude *m*21 according to the formula adopted in RC3:

where *f* is the area of the 21-cm line profile expressed in
. This formula is equivalent to the one used
in RC3: , where *fwm* is the flux in
. The standard error *sm*21 on *m*21 is given as a function
of the radiotelescope according to Bottinelli et al. (1990).

The HI line magnitude *m*21 has been corrected for self-absorption
effect following Heidmann et al. (1972):

The adopted free parameter is . If inclination is higher than the maximum correction is limited to -0.82 mag.

Both magnitudes *m*21*c* and *btc* are used to calculate a HI color index *hi*
initially defined by de Vaucouleurs et al. (1976):

This parameter is interesting as it is directly connected to the
hydrogen contents per unit of *B*-flux (see RC3, p51 Rel. 78).
The relation between *hi* and morphological type code *t* is presented
in Fig. 12 (click here). It shows a clear correlation which validates the
use of morphological type code as an observable parameter.

**Figure 12:** Mean HI color index *hi* vs. morphological type code *t*

IRAS fluxes at 60 m and 100 m are converted in the so-called far-infrared flux according to the relation:

where *f*60 and *f*100 are IRAS fluxes at 60 m and 100 m
expressed in Jansky. This relation is equivalent to the relation
given in RC3. The term 14.75 comes from the arbitrary zero-point of 20
in RC3 (p.43, Rel. 49). The factor 2.58 comes from IRAS *Point
Source Catalog* (1988).

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