Undetected Black Holes: Far and Near
In this talk I will discuss two undetected populations of black holes. In the faraway Universe, the first lensed z > 6 quasar (J0439+1634) was recently discovered. We predicted that the observed population of quasars should contain many mildly magnified sources, with unresolved image separations, while current selection criteria could miss a substantial population of lensed high-z quasars. We estimated the fraction of these "phantom quasars" depending on the slope of their luminosity function. A new analysis of 22 Chandra sources at z > 6 is also presented. Additionally, we showed how future detections of z > 8 quasars will constrain their growth parameters, especially using observational priors on luminosities. Uncertainties in the determination of growth parameters decrease by a factor ~2-3 when a quasar's detection redshift goes from z=6 to z=9.
In the nearby Universe, we studied the properties of a putative population of intermediate-mass black holes wandering in the Milky Way. First, using the Illustris TNG50 simulation, we investigated the dynamics of these objects, finding that ~84% are sinking, their orbits circularize with time, and their highest number density is found in the central 1 kpc. Their typical velocity with respect to the galactic center is ~180 km/s, while with respect to the local gas it is ~88 km/s. We finally showed that these sources are observable in a wide range of frequencies, with observability fractions ranging from ~10% (X-ray) to ~20% (sub-mm) of the population. Remarkably, they are always visible when passing through molecular clouds.