Growth and anisotropy of ionization fronts near high-redshift quasars in the MassiveBlack simulation

We use radiative transfer to study the growth of ionized regions around the brightest, z = 8 quasars in a large cosmological hydrodynamic simulation that includes black hole growth and feedback (the MassiveBlack simulation). We find that in the presence of the quasars the comoving H IIbubble radii reach 10 Mpc h⁻¹ after 20 Myr while with the stellar component alone the H II bubbles are smaller by at least an order of magnitude. Our calculations show that several features are not captured within an analytic growth model of Stromgren spheres. The X-ray photons from hard quasar spectra drive a smooth transition from fully neutral to partially neutral in the ionization front. However, the transition from partially neutral to fully ionized is significantly more complex. We measure the distance to the edge of bubbles as a function of angle and use the standard deviation of these distances as a diagnostic of the anisotropy of ionized regions. We find that the overlapping of nearby ionized regions from clustered haloes not only increases the anisotropy, but also is the main mechanism which allows the outer radius to grow. We therefore predict that quasar ionized bubbles at this early stage in the reionization process should be both significantly larger and more irregularly shaped than bubbles around star-forming galaxies. Before the star formation rate increases and the Universe fully reionizes, quasar bubbles will form the most striking and recognizable features in 21 cm maps.