Theoretical correlations between emission measure and integrated intensity. Left: relationship between the integrated intensity of Mg II k and the integrated intensity of Hα derived from the theoretical models computed with a micro-turbulence equal to 5 km s⁻¹ and for different values of temperature and slab thickness. Top right: integrated Mg II k intensity versus EM. Bottom right: integrated Hα intensity versus EM. The horizontal (vertical) lines limit the range of Mg II k (Hα) intensities. Integrated intensities are in erg s⁻¹ cm⁻² sr⁻¹ and EM are in cm⁻⁵. The small slab thickness models (stars) are new models. They fit quite well in the box of the observations.