ACCRETION DISKS IN ACTIVE GALACTIC NUCLEI - VERTICALLY EXPLICIT MODELS

We explore a region of the parameter space accessible to accretion disks in active galactic nuclei using vertically integrated models to describe the thin-disk structure. We examine models with 0.01 less-than-or-equal-to alpha less-than-or-equal-to 3 and radii near 10(-3) pc (assuming a central black hole of mass 10(8) M.). Particular attention is paid to the role of convection. By comparing strongly convective solutions with varying mixing lengths to solutions obtained using a recent theory of convection developed for accretion disks, we find that it may be appropriate to take the mixing length to be roughly one-third of a pressure scale height when applying standard mixing-length theory to disks. We also present scalings for various critical points in the locus of steady state solutions and provide crude estimates of time scales for behavior associated with the disk instability. Our numerical results largely confirm previous work by Lin & Shields and by Mineshige & Shields. As pointed out by these authors, the disk instability solves the problem of fueling quasars by requiring that they be "on" only a fraction of the time. In addition, the fact that this duty cycle is roughly equal to the ratio of quasar-like objects to all galaxies at the height of the quasar era implies that most or all galaxies once had active cores, and therefore most present-day galaxies must still have massive black holes at their centers.