The birth of a galaxy – III. Propelling reionization with the faintest galaxies

MNRAS 442, 2560–2579 (2014) doi:10.1093/mnras/stu979 The birth of a galaxy – III. Propelling reionization with the faintest galaxies John H. Wise, 1‹ Vasiliy G. Demchenko, 1 Martin T. Halicek, 1 Michael L. Norman, 2 Matthew J. Turk, 3 Tom Abel 4 and Britton D. Smith 5 1 Center for Relativistic Astrophysics, Georgia Institute of Technology, 837 State Street, Atlanta, GA 30332, USA for Astrophysics and Space Sciences, University of California at San Diego, La Jolla, CA 92093, USA 3 Department of Astronomy, Columbia University, 538 West 120th Street, New York, NY 10027, USA 4 Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Menlo Park, CA 94025, USA 5 Institute of Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK 2 Center Accepted 2014 May 14. Received 2014 May 12; in original form 2014 March 24 Starlight from galaxies plays a pivotal role throughout the process of cosmic reionization. We present the statistics of dwarf galaxy properties at z > 7 in haloes with masses up to 10 9 M , using a cosmological radiation hydrodynamics simulation that follows their buildup starting with their Population III progenitors. We find that metal-enriched star formation is not restricted to atomic cooling (T vir ≥ 10 4 K) haloes, but can occur in haloes down to masses ∼10 6 M , especially in neutral regions. Even though these smallest galaxies only host up to 10 4 M of stars, they provide nearly 30 per cent of the ionizing photon budget. We find that the galaxy luminosity function flattens above M UV ∼ −12 with a number density that is unchanged at z 10. The fraction of ionizing radiation escaping into the intergalactic medium is inversely dependent on halo mass, decreasing from 50 to 5 per cent in the mass range log M/M = 7.0−8.5. Using our galaxy statistics in a semi-analytic reionization model, we find a Thomson scattering optical depth consistent with the latest Planck results, while still being consistent with the UV emissivity constraints provided by Lyα forest observations at z = 4–6. Key words: radiative transfer – methods: numerical – galaxies: dwarf – galaxies: formation – galaxies: high-redshift – dark ages, reionization, first stars. 1 I N T RO D U C T I O N Cosmic reionization is an extended process as individual H II regions grow around ionizing sources that gradually coalesce, culminating in a fully ionized Universe by z ∼ 6 (e.g. Gnedin & Ostriker 1997; Razoumov et al. 2002; Ciardi, Ferrara & White 2003; Sokasian et al. 2003; Furlanetto, Zaldarriaga & Hernquist 2004; Iliev et al. 2006; Robertson et al. 2010; Trac & Gnedin 2011; Zahn et al. 2011; So et al. 2014). However, there is still some tension between observational constraints on the timing and duration of reioniza- tion. First, the transmission fraction of z ∼ 6 quasar light blueward of Lyα through the intergalactic medium (IGM) indicates that the Universe was mostly ionized by this epoch (e.g. Gunn & Peterson 1965; Fan et al. 2002, 2006; Willott et al. 2007; Mortlock et al. 2011). Secondly, observations of the cosmic microwave background (CMB) from the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck have measured the optical depth to Thomson scat- tering τ e = 0.089 +0.012 −0.014 , which corresponds to the Universe being E-mail: jwise@physics.gatech. edu ∼50 per cent ionized at z = 11.1 ± 1.1 (Planck Collaboration 2013). But the ionizing emissivity measured at z = 4–6 through Lyα forest observations cannot account for this measured τ e , indi- cating that the end of reionization must be photon starved (Bolton & Haehnelt 2007) and that the emissivity must have been higher during reionization. Third, the duration 1 of reionization has been constrained to occur within z < 7.9 by measuring the kinetic Sunyaev–Zel’dovich effect with the South Pole Telescope (Zahn et al. 2012). These observations suggest that reionization was an extended process, mainly occurring at 6 z 15. What population of ionizing sources drives this global and ex- tended transition? It is clear that quasars and the very brightest galaxies, both of which are too rare, do not significantly contribute to the overall ionizing photon budget of reionization (e.g. Shapiro 1986; Dijkstra et al. 2004; Willott et al. 2010; Grissom, Ballantyne & Wise 2014). Starlight from galaxies is thought to provide the vast majority of the ionizing photon budget from extrapolating the 1 Zahn et al. (2012) define z as the redshift elapsed between 20 and 99 per cent ionized. C 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society Downloaded from http://mnras.oxfordjournals. org/ by guest on September 12, 2016 ABSTRACT