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10. HI-line and IR fluxes

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 tex2html_wrap_inline2307 converted in magnitude m21 according to the formula adopted in RC3:


equation519
where f is the area of the 21-cm line profile expressed in tex2html_wrap_inline2313. This formula is equivalent to the one used in RC3: tex2html_wrap_inline2315, where fwm is the flux in tex2html_wrap_inline2319. 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):


equation528

The adopted free parameter is tex2html_wrap_inline2327. If inclination is higher than tex2html_wrap_inline2329 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):


equation534

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.

  figure537
Figure 12: Mean HI color index hi vs. morphological type code t

IRAS fluxes at 60 tex2html_wrap_inline2349m and 100 tex2html_wrap_inline2351m are converted in the so-called far-infrared flux according to the relation:


equation542
where f60 and f100 are IRAS fluxes at 60 tex2html_wrap_inline2357m and 100 tex2html_wrap_inline2359m 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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