Host-galaxy stars can dominate the ionizing radiation field of the circumgalactic medium in galaxies at Cosmic Noon

Elucidating the processes that shape the circumgalactic medium (CGM) is crucial for understanding galaxy evolution. Absorption and emission diagnostics can be interpreted using photoionization calculations to obtain information about the phase and ionization structure of the CGM. For simplicity, typically only the metagalactic background is considered in photoionization calculations, and local sources are ignored. To test this simplification, we perform Monte Carlo radiation transfer on 12 cosmological zoom-in simulations from the Feedback in Realistic Environments (FIRE) project with halo masses $10^{10.5}-10^{13} \mathrm{M}_{\odot}$ in the redshift range $z = 0-3.5$ to determine the spatial extent over which local sources appreciably contribute to the ionizing radiation field in the CGM. We find that on average, the contribution of stars within the galaxy is small beyond one-tenth of the virial radius, $R_{\mathrm{vir}}$, for $z < 1$. For $1<z<2$ and $M_{\mathrm{vir}} \sim 10^{11.5}$, the radius at which the contribution to the ionizing radiation field from stars within the galaxy and that from the UV background are equal is roughly 0.2 $R_{\mathrm{vir}}$. For $M_{\mathrm{vir}} > 10^{12} \mathrm{M}_{\odot}$ at $z \sim 1.5-2.5$ and for all $M_{\mathrm{vir}}$ considered at $z>3$ , this transition radius can sometimes exceed 0.5 $R_{\mathrm{vir}}$. We also compute the escape fraction at $R_{\mathrm{vir}}$, finding typical values of less than $0.1$, except in higher-mass halos ($M_{\mathrm{halo}} \gtrsim 10^{12} \mathrm{M}_{\odot}$), which have consistently high values of $\sim 0.5-0.6$. Our results indicate that at low redshift, it is reasonable to ignore the ionizing radiation from host-galaxy stars outside of 0.2 $R_{\mathrm{vir}}$, while at Cosmic Noon, local stellar ionizing radiation likely extends further into the CGM and thus should be included in photoionization calculations.