left panel. The above formula agrees with the expected relation for X-ray emission coming from hot diffuse gas, as discussed by @beuing.

The relation still hold if $D^2_{kpc}$ (kpc$^2$) is used. It becomes: $$Log L_X = 2 Log D^2_{kpc} + 1.51$$ ($\sigma$=0.85) for 226 early-type galaxies from E to Sb.

Many galaxies with high blue luminosity, indication of high masses and of a recent star formation, lie quite far from the mean line, with a behaviour different than that of late-type galaxies.

If the X-ray fluxes are compared with FIR luminosity, the disagreement with the behaviour found in late-type galaxies is more evident. The plot L$_X$ vs. L$_{FIR}$ for early-type galaxies shows the representative points of the galaxies above the relation (\[XFIR\]) for late-type galaxies (Fig.\[X2\], right panel). To understand this apparent disagreement, we should use a theoretical analysis of the far infrared emission, as explained in the next Section 4.

Active galaxies
---------------

Active galaxies (Seyfert 1, Seyfert 2 and Liners) have X-ray, B and FIR fluxes that are not linked together. This happens because, to the emission mechanisms stimulating the light emission at the different wavebands described for non active galaxies, adds an X-ray emission coming from nucleus. 