The difficulty of getting high escape fractions of ionizing photons from high-redshift galaxies: a view from the FIRE cosmological simulations
Year: 2015
Type: article
Abstract: We present a series of high-resolution (20–2000 M⊙, 0.1–4 pc) cosmological zoom-in simulations at z ≳ 6 from the Feedback In Realistic Environment (FIRE) project. These simulations cover halo masses 109–1011 M⊙ and rest-frame ultraviolet magnitude MUV = −9 to −19. These simulations include explicit models of the multi-phase ISM, star formation, and stellar feedback, which produce reasonable galaxy properties at z = 0–6. We post-process the snapshots with a radiative transfer code to evaluate the escape fraction (fesc) of hydrogen ionizing photons. We find that the instantaneous fesc has large time variability (0.01–20 per cent), while the time-averaged fesc over long time-scales generally remains ≲ 5 per cent, considerably lower than the estimate in many reionization models. We find no strong dependence of fesc on galaxy mass or redshift. In our simulations, the intrinsic ionizing photon budgets are dominated by stellar populations younger than 3 Myr, which tend to be buried in dense birth clouds. The escaping photons mostly come from populations between 3 and 10 Myr, whose birth clouds have been largely cleared by stellar feedback. However, these populations only contribute a small fraction of intrinsic ionizing photon budgets according to standard stellar population models. We show that fesc can be boosted to high values, if stellar populations older than 3 Myr produce more ionizing photons than standard stellar population models (as motivated by, e.g. models including binaries). By contrast, runaway stars with velocities suggested by observations can enhance fesc by only a small fraction. We show that ‘sub-grid’ star formation models, which do not explicitly resolve star formation in dense clouds with n ≫ 1 cm−3, will dramatically overpredict fesc.
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: bronze
Funders: National Science Foundation, David and Lucile Packard Foundation, U.S. Department of Energy, National Aeronautics and Space Administration, Alfred P. Sloan Foundation, Simons Foundation, Gordon and Betty Moore Foundation, California Institute of Technology, Northwestern University, University of California, San Diego, Nuclear Physics
Awards: G1286236842, G1400302619, G1422262692, G1816284028, G2046453982, G2238354798, THE INSTALLATION OF THE COSMIC ORIGINS SPECTROGRAPH (COS) ON THE HUBBLE SPACE TELESCOPE (HST) AS PART OF ITS LAST SERVICING MISSION HAS REVOLUTIONIZE, Collaborative Research: CDS&E: FIRE: Physically-Predictive Cosmological Simulations of Galaxy Formation with Resolved Feedback, G2821925151, G3083819904, G3108277084, G3201778039, G3591616885, G365575497, G42916606, G5714521999, G5996804137, G6025746495, Collaborative Research: CDS&E: FIRE: Physically-Predictive Cosmological Simulations of Galaxy Formation with Resolved Feedback, G7075399390, Collaborative Research: CDS&E: FIRE: Physically-Predictive Cosmological Simulations of Galaxy Formation with Resolved Feedback, Explaining Peculiar Thermonuclear Supernovae, G8330914465, G8561699031, MRI-R2: Acquisition of a Compute Cluster for High-Fidelity Simulations of Gravitational Wave Sources -- Facilitating LIGO and Enabling Multi-Messenger Astronomy, G8970859722, G969889393
OpenAlex ID: W2165698997