relations but have a wide spread. In our sample there are just 10 galaxies and their representative points have been not plotted in Figure \[CO\_100\].

X-ray component.
================

We are interested to understand what relations exist between L$_X$, the X-ray luminosity, and the other global galaxy properties. From the literature, it is known the existence of a proportionality between L$_X$ produced by discrete sources and L$_B$, the blue luminosity of the whole galaxy. This relation has been studied by @ciotti and compared by @beuing with soft X-ray fluxes measured by ROSAT satellite. It appears that late-type galaxies have a global X-ray luminosity directly proportional to L$_B$, while early-type systems are dominated by emission produced by hot diffuse gas and their $L_X$ is proportional to the square power of the blue luminosity, as discussed by @beuing. For this reason, the early and late-type galaxies are discussed separately.

Late-type galaxies
------------------

With our data, the X-ray luminosity of galaxies with morphological type later than Sb can be fitted by a linear relation as a function of L$_B$(dotted line in Fig. \[X1\], left panel). The direct proportionality is expressed by the equation: $$Log L_X = Log L_B - 3.85
\label{XBlate}$$ with a r.m.s. 