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We analyze the available sample of double quasars, and investigate theirphysical properties. Our sample comprises 85 pairs, selected from the SloanDigital Sky Survey (SDSS). We derive physical parameters for the engine and thehost, and model the dynamical evolution of the pair. First, we comparedifferent scaling relationships between massive black holes and their hosts(bulge mass, velocity dispersion, and their possible redshift dependences), anddiscuss their consistency. We then compute dynamical friction timescales forthe double quasar systems to investigate their frequency and their agreementwith scenarios for quasar triggering. Comparing typical merging timescales toexpected quasar lifetimes, the fraction of double quasars should be roughly afactor of 10 larger than observed. Additionally, we find that, depending on thecorrelations between black holes and their hosts, the occurrence of doublequasars could be redshift-dependent. Comparison of our models to the SDSSquasar catalog suggests that double quasars should be more common at highredshift. We compare the typical separations at which double quasars areobserved to the predictions of merger simulations. We find that thedistribution of physical separations peaks at ~30 kpc, with a tail at largerseparations (~100-200 kpc). These large separation pairs are instead consistentwith unequal mass mergers where gas is dynamically perturbed during the firstpericentric passage, but the gas reaches the black hole only at the nextapocenter, where the pair is observed.

Double quasars: probes of black hole scaling relationships and merger scenarios