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 m21 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 sm21 on m21 is given as a function of the radiotelescope according to Bottinelli et al. (1990).
The HI line magnitude m21 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 m21c 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 f60 and f100 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).