The THESAN project: connecting ionized bubble sizes to their local environments during the Epoch of Reionization

An important characteristic of cosmic reionization is the growth of ionized gas bubbles surrounding early luminous objects. Understanding the connections between the formation and coalescence of these bubbles and their originating astrophysical sources is equally critical. We present results from a study of bubble sizes using the state-of-the-art THESAN radiation-hydrodynamics simulation suite, which self-consistently models radiation transport and realistic galaxy formation. We employ the mean-free path method, and track the evolution of the effective ionized bubble size at each point ($R_{\rm eff}$) throughout the Epoch of Reionization. We show there is a slow growth period for regions ionized early, but a rapid flash ionization process for regions ionized later as they immediately enter a large, pre-existing bubble. We also find that bright sources are preferentially in larger bubbles, and find consistency with recent observational constraints at $z \gtrsim 9$, but tension with idealized Lyman-alpha damping-wing models at $z \approx 7$ when the size distribution is complex. We find that high overdensity regions have larger characteristic bubble sizes, but the correlation decreases as reionization progresses, likely due to the runaway formation of large percolated bubbles. Finally, we compare the redshift at which a region transitions from neutral to ionized ($z_{\rm reion}$) with the time it takes to reach a given bubble size and conclude that $z_{\rm reion}$ is a reasonable local probe of small-scale bubble size statistics ($R_\text{eff} \lesssim 1$ cMpc). However, for larger bubbles, the correspondence between $z_{\rm reion}$ and size statistics weakens due to the time delay between the onset of reionization and the expansion of a large bubble, particularly at high redshifts.