We describe a new method for simulating ionizing radiation and supernovafeedback in the analogues of low-redshift galactic disks. In this method, whichwe call star-forming molecular cloud (SFMC) particles, we use a ray-tracingtechnique to solve the radiative transfer equation for ultraviolet photonsemitted by thousands of distinct particles on the fly. Joined with highnumerical resolution of 3.8 pc, the realistic description of stellar feedbackhelps to self-regulate star formation. This new feedback scheme also enables usto study the escape of ionizing photons from star-forming clumps and from agalaxy, and to examine the evolving environment of star-forming gas clumps. Bysimulating a galactic disk in a halo of 2.3e11 Msun, we find that the averageescape fraction from all radiating sources on the spiral arms (excluding thecentral 2.5 kpc) fluctuates between 0.08% and 5.9% during a ~20 Myr period witha mean value of 1.1%. The flux of escaped photons from these sources is notstrongly beamed, but manifests a large opening angle of more than 60 degreefrom the galactic pole. Further, we investigate the escape fraction per SFMCparticle, f_esc(i), and how it evolves as the particle ages. We discover thatthe average escape fraction f_esc is dominated by a small number of SFMCparticles with high f_esc(i). On average, the escape fraction from a SFMCparticle rises from 0.27% at its birth to 2.1% at the end of a particlelifetime, 6 Myrs. This is because SFMC particles drift away from the dense gasclumps in which they were born, and because the gas around the star-formingclumps is dispersed by ionizing radiation and supernova feedback. The frameworkestablished in this study brings deeper insight into the physics of photonescape fraction from an individual star-forming clump, and from a galacticdisk.
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