The difficulty of getting high escape fractions of ionizing photons from high-redshift galaxies: A view from the FIRE cosmological simulations

By Xiangcheng Ma, D. Kasen, P. Hopkins, C. Faucher-Giguére, E. Quataert, D. Keres̆, Norman Murray Caltech, Uc Berkeley, Lbnl, Northwestern and 2 more. 2015

133 citations recorded by the source corpus.

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 10^9–10^(11) M⊙ and rest-frame ultraviolet magnitude M_(UV) = −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 (f_(esc)) of hydrogen ionizing photons. We find that the instantaneous f_(esc) has large time variability (0.01–20 per cent), while the time-averaged f_(esc) over long time-scales generally remains ≲ 5 per cent, considerably lower than the estimate in many reionization models. We find no strong dependence of f_(esc) 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 f_(esc) 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 f_(esc) 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 f_(esc).

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/898926027213799c182f43a2c241a8b5d4667846

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Citations in the corpus

The origin and evolution of the galaxy mass–metallicity relation (2015, background): “Nevertheless, we confirmed that these simulations predict similar global galaxy properties to those of much poorer resolutions (see Ma et al. 2015).”
Modeling the Radio Background from the First Black Holes at Cosmic Dawn: Implications for the 21 cm Absorption Amplitude (2018, background): “Ma et al. (2015) found escape fractions between 0.01 and 0.05 for ∼109Me halos in simulations, while observations of larger galaxies at lower redshifts found escape fractions fesc∼10 −2 –0.5 (Bridge et al. 2010; Izotov et al. 2016; Vanzella et al. 2016; Vasei et al. 2016; Grazian et al. 2017).”
The Universe Is Reionizing at z ∼ 7: Bayesian Inference of the IGM Neutral Fraction Using Lyα Emission from Galaxies (2017, background): “C O ] 1 5 Fe b duction efficiency and escape fraction of hard ionizing photons (Bouwens et al. 2015a; Ma et al. 2015; Grazian et al. 2017; Vanzella et al. 2017).”
FIRE-2 simulations: physics versus numerics in galaxy formation (2017, background): “…et al. 2017a); star-formation properties of galactic nuclei (Torrey et al. 2017); escape fractions of ionizing photons needed for reionization (Ma et al. 2015, 2016b); and at least some of the diversity of star-forming and quiescent massive galaxies at high redshifts (Narayanan et al.…”
Stellar and Quasar Feedback in Concert: Effects on AGN Accretion, Obscuration, and Outflows (2015, unknown): “In a series of papers (Hopkins et al. 2014; Muratov et al. 2015; Oñorbe et al. 2015; van de Voort et al. 2015; Ma et al. 2015b,a; Faucher-Giguere et al. 2015), we showed that, on galactic scales, these models produce a quasi-steady ISM in which molecular clouds form and disperse rapidly, with phase…”
Conditions for Reionizing the Universe with a Low Galaxy Ionizing Photon Escape Fraction (2019, methodology): “Ma et al. (2015) explored ionizing photon escape using the FIRE simulations, which include advanced treatment for feedback and utilize zoom-ins to achieve high (<1 pc) resolution on halos of interest.”
The Evolution of the Intergalactic Medium (2015, methodology): “…problem of calculating fesc (which requires the daunting task of resolving the multiphase ISM), but with answers essentially ranging from zero to one (Gnedin, Kravtsov & Chen 2008; Wise & Cen 2009) with some recent simulations finding average values of fesc ∼ 10% (Kimm & Cen 2014; Ma et al. 2015).”
A case study of hurdle and generalized additive models in astronomy: the escape of ionizing radiation (2018, background): “A strong dependence on stellar feedback was confirmed also by Ma et al. (2015): although the intrinsic budget of ionizing photons is dominated by young stellar populations as expected in standard population models, the majority of escaping photons derives from intermediate age stars located in…”
Rapid Reionization by the Oligarchs: The Case for Massive, UV-bright, Star-forming Galaxies with High Escape Fractions (2019, background): “…fesc 4 Hubble Fellow. through simulations, since it depends sensitively on resolving the multi-phase ISM and the treatment of small-scale processes associated with galaxy formation, which, at present, are modeled in an approximate way (e.g., Wise et al. 2014; Ma et al. 2015; Trebitsch et al. 2017).”
GALAXY PROPERTIES AND UV ESCAPE FRACTIONS DURING THE EPOCH OF REIONIZATION: RESULTS FROM THE RENAISSANCE SIMULATIONS (2016, background): “…the photon budget in reionization models and has attracted much attention from numerical studies (e.g., Gnedin et al. 2008; Wise & Cen 2009; Razoumov & Sommer-Larsen 2010; Yajima et al. 2011; Ferrara & Loeb 2013; Paardekooper et al. 2013; Kimm & Cen 2014; Wise et al. 2014; Ma et al. 2015).”
Stellar population effects on the inferred photon density at reionization (2015, background): “High resolution simulations of galaxy properties, rather than reionization itself, struggle to produce high values, finding a typical fesc ∼ 5 per cent (Ma et al. 2015).”
Fluctuating feedback-regulated escape fraction of ionizing radiation in low-mass, high-redshift galaxies (2017, methodology): “studies (e.g. Kimm & Cen 2014; Wise et al. 2014; Ma et al. 2015), we must tread carefully in that direction, and not take this number at face value: this is the direct result of our subgrid recipe for supernovae, which explode 10 Myr after the birth of the star particle.”
Feedback-regulated star formation and escape of LyC photons from mini-haloes during reionization (2016, methodology): “…often predict very high escape fractions of fesc >∼ 50% in atomic cooling haloes, but these conclusions may be subject to numerical resolution (Ma et al. 2015; Paardekooper et al. 2015) and how the escape fraction is measured (i.e. whether or not it is photon number-weighted (Kimm & Cen…”
Detection of high Lyman continuum leakage from four low-redshift compact star-forming galaxies (2016, background): “…et al. 2011; Wise et al. 2014), while other studies find relatively low escape fractions, fesc(LyC) ≈ 5 %, with no strong dependence on galaxy mass and cosmic time (cf. Yajima et al. 2014; Ma et al. 2015), or a declining escape fraction with decreasing galaxy mass (Gnedin, Kravtsov & Chen 2008).”