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

TL;DR

This study uses radiative transfer in a large cosmological simulation to analyze the growth, shape, and anisotropy of ionized regions around high-redshift quasars, revealing complex dynamics not captured by simple models.

Contribution

It provides new insights into the size, shape, and anisotropy of quasar ionized bubbles during reionization, highlighting the importance of overlapping regions and complex ionization transitions.

Findings

Quasar ionized bubbles reach 10 Mpc/h after 20 Myr.

Bubbles are larger and more irregular than those around star-forming galaxies.

Overlapping ionized regions increase anisotropy and growth of bubbles.

Abstract

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 quasar s the comoving HII bubble radii reach 10 Mpc/h after 20 My while with the stellar component alone the HII bubbles are smaller by at least an order of magnitude. Our calculations show that several features are not captured within an analytical 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 isotropy of ionized regions. We find that the overlapping of nearby ionized regions from clustered halos 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 21cm maps.