Feedback-regulated star formation and escape of LyC photons from mini-haloes during reionization

MNRAS 466, 4826–4846 (2017) doi:10.1093/mnras/stx052 Advance Access publication 2017 January 11 Feedback-regulated star formation and escape of LyC photons from mini-haloes during reionization Taysun Kimm,1‹ Harley Katz,1 Martin Haehnelt,1 Joakim Rosdahl,2 Julien Devriendt3,4 and Adrianne Slyz3 1 Kavli Institute for Cosmology and Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, the Netherlands 3 Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK 4 Observatoire de Lyon, UMR 5574, 9 avenue Charles Andre, F-69561 Saint Genis Laval, France 2 Leiden ABSTRACT Reionization in the early Universe is likely driven by dwarf galaxies. Using cosmological radiation-hydrodynamic simulations, we study star formation and the escape of Lyman continuum (LyC) photons from mini-haloes with Mhalo  108 M . Our simulations include a new thermo-turbulent star formation model, non-equilibrium chemistry and relevant stellar feedback processes (photoionization by young massive stars, radiation pressure and mechanical supernova explosions). We find that feedback reduces star formation very efficiently in minihaloes, resulting in the stellar mass consistent with the slope and normalization reported in Kimm & Cen and the empirical stellar mass-to-halo mass relation derived in the local Universe. Because star formation is stochastic and dominated by a few gas clumps, the escape fraction in mini-haloes is generally determined by radiation feedback (heating due to photoionization), rather than supernova explosions. We also find that the photon number-weighted mean escape fraction in mini-haloes is higher (∼20–40 per cent) than that in atomic-cooling haloes, although the instantaneous fraction in individual haloes varies significantly. The escape fraction from Pop III stars is found to be significant (10 per cent) only when the mass is greater than ∼100 M . Based on simple analytic calculations, we show that LyC photons from mini-haloes are, despite their high escape fractions, of minor importance for reionization due to inefficient star formation. We confirm previous claims that stars in atomic-cooling haloes with masses 108 M  Mhalo  1011 M are likely to be the most important source of reionization. Key words: galaxies: high-redshift – dark ages, reionization, first stars – early Universe. 1 I N T RO D U C T I O N Observations of Lyman α opacities in the spectra of quasi-stellar objects (QSOs) at high redshift have shown unambiguously that the Universe becomes nearly transparent to Lyman continuum (LyC) photons (λ ≤ 912 Å) at z ∼ 6 (Becker et al. 2001; Fan et al. 2001, 2006; McGreer, Mesinger & D’Odorico 2015). Several candidates are identified as a potential source of reionization, including dwarf galaxies (e.g. Couchman & Rees 1986; Madau, Haardt & Rees 1999), active galactic nuclei (e.g. Shapiro & Giroux 1987; Haiman & Loeb 1998), accretion shock (Dopita et al. 2011), globular clusters (Ricotti 2002; Katz & Ricotti 2013, 2014) and X-rays from accreting stellar-mass black holes (e.g. Madau et al. 2004; Ricotti & Ostriker 2004; Mirabel et al. 2011). Many studies agree  E-mail: that the primary source of reionization is likely to be massive stars in dwarf galaxies (Haehnelt et al. 2001; Cowie, Barger & Trouille 2009; Fontanot et al. 2014; Madau & Fragos 2016, cf. Madau & Haardt 2015); however, the time-scale over which reionization occurred and the mass range of haloes that provided the majority of the ionizing photons are issues that remain unresolved (e.g. Bolton & Haehnelt 2007; Ahn et al. 2012; Kuhlen & FaucherGiguère 2012; Wise et al. 2014). The two critical ingredients for reionization are star formation and escaping LyC photons. The former describes how many LyC photons are available from massive stars, while the latter determines what fraction are actually used to ionize the intergalactic medium (IGM). Unsurprisingly, the prediction of both quantities is very challenging, as galaxy evolution involves highly non-linear processes, such as the interaction between the interstellar medium (ISM) and feedback from stars. For this reason, numerical studies often report discrepant results on the escape fraction. An early attempt by  C 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society Accepted 2017 January 9. Received 2016 December 20; in original form 2016 August 16 Insignificance of mini-haloes to reionization observed at lower redshift (e.g. Speagle et al. 2014), it is conceivable that the star-forming clouds are disrupted more efficiently in simulated galaxies, resulting in higher escape fractions (e.g. Kimm & Cen 2014; Cen & Kimm 2015). Moreover, since the simulated galaxies are more metal-poor than the observed bright galaxies, they are likely less affected by dust compared to observed galaxies (e.g. Izotov et al. 2016). Finally, as pointed out by Cen & Kimm (2015), individual measurements of the escape fraction may underestimate the 3D escape fraction, especially when the escape fraction is small. Unlike the observed LyC flux that conveys information about the instantaneous escape fraction, the Thompson electron optical depth (τ e ), derived from the polarization signal of cosmic microwave background (CMB) photons, provides a useful measure of how extended reionization was in the early Universe. The analysis of the nine-year Wilikinson Microwave Anisotropy Probe (WMAP9) observations suggested a high electron optical depth of τ e = 0.089 ± 0.014 (Hinshaw et al. 2013), indicating that ionized hydrogen (H II) bubbles are likely to have grown relatively early. However, the observed number density of bright galaxies in the ultraviolet (UV, MUV  −17) is unable to explain such a high τ e (e.g. Bunker et al. 2010; Finkelstein et al. 2010; Bouwens et al. 2012). By taking a parametric form of the UV luminosity density, motivated by observations of the Hubble Ultra Deep Field, Robertson et al. (2013) showed that the inclusion of small dwarf galaxies with −17 ≤ MUV ≤ −13 can increase τ e to a higher value of 0.07, provided that 20 per cent of LyC photons escape from the dark matter haloes. Wise et al. (2014) claim that mini-haloes of mass Mhalo ≤ 108 M , corresponding to MUV  −13, may be able to provide a large number of LyC photons to the IGM as LyC photons escape freely from their host halo. Because the mini-haloes emerge first and they are abundant in the early Universe (z ≥ 15), the authors find that the resulting τ e ≈ 0.09 can easily accommodate the WMAP9 analysis, demonstrating the potential importance of minihaloes to reionization of the Universe (see also Ahn et al. 2012). However, a more accurate modelling of dust emission in our Galaxy (Planck Collaboration XV 2014) and the use of the low frequency instrument on the Planck Satellite lead to a decrease in the optical depth to τ e = 0.066 ± 0.016 (Planc (...truncated)