Title: The effect of local photoionization on the galaxy properties and the circumgalactic medium in simulations of Milky Way-sized galaxies

Abstract: In this study, we investigate the impact of local stellar radiation in cosmological zoom simulations of the formation of Milky Way-sized galaxies. We include the radiation field as an additional feedback component that is computed alongside gravity with a tree code in an optically thin approximation. We resimulate the initial conditions of five Milk Way-like systems taken from the Auriga project with and without stellar radiation, and study the effects of local stellar radiation on several properties of the galaxies and the circumgalactic medium (CGM). Similar to previous findings, we observe that local stellar radiation can modify gas cooling in the circumgalactic medium and thus suppress star formation and the surface densities of young stars and HI gas, while having little impact on the total gas content. Even though the magnitude of the effect we find is smaller than reported in earlier work, the radiation field is thus clearly an important component in simulations of late time galaxies. In particular, it also suppresses the peak of the rotation curve and reduces the mass of the stellar bulge. In the CGM region, the young stellar radiation exceeds the external UVB and dominates the radiation field within the virial halo at all redshifts. Nevertheless, we find that the local stellar radiation has overall little impact on the radial density and temperature profile of the CGM gas. The metallicity profile is slightly reduced, however, as well as the HI and MgII column densities within $\sim 0.3\,R_{\rm vir}$. In contrast, we find that the OVI column density is hardly impacted by the radiation field. Additional effects can be expected from the radiation of the central AGN during phases of quasar activity, which has not yet been included in the simulations of the present study.