The luminosity function of Seyfert 1 galaxy nuclei and BL Lac objects, and the X-ray background

The luminosity function of Seyfert 1 and BL Lac object nuclei is constructed. The main results of this study are that: The luminosity function of Seyfert 1 nuclei is steep, which implies a rather large volume density of such nuclei fainter than M/sub v/=-18 (using H/sub o /=50 km s/sup -1//Mpc). The volume density of BL Lac nuclei in the range -23<M/sub v/<-21 is at least as large as that of the Seyfert 1 nuclei. A consequence of this fact is probably that BL Lac nuclei suffer a smaller cosmological evolution than do the Seyfert 1 nuclei (or quasars), as suspected by Setti and Woltjer (1977), as the surface density of faint BL Lac objects is much smaller than that of quasars. If the steep luminosity function of the Seyfert 1 nuclei extends to M /sub v/=-16.5, these objects may easily account for most of the X-ray background in the range 2 to 10 keV. (81 refs).