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Lyman Α Radiation Hydrodynamics of Galactic Winds Before Cosmic Reionization

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Lyman Α Radiation Hydrodynamics of Galactic Winds Before Cosmic Reionization Lyman α Radiation Hydrodynamics of Galactic Winds Before Cosmic Reionization The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Smith, Aaron, Volker Bromm, and Abraham Loeb. 2016. Lyman α Radiation Hydrodynamics of Galactic Winds Before Cosmic Reionization. Monthly Notices of the Royal Astronomical Society 464, no. 3 (October 10): 2963–2978. Published Version doi:10.1093/mnras/stw2591 Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:42668793 Terms of Use This article was downloaded from Harvard University’s DASH repository, WARNING: This file should NOT have been available for downloading from Harvard University’s DASH repository.;This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Open Access Policy Articles, as set forth at http://nrs.harvard.edu/ urn-3:HUL.InstRepos:dash.current.terms-of-use#OAP MNRAS 000,1–16 (2016) Preprint 27 October 2016 Compiled using MNRAS LATEX style file v3.0 Lyman α radiation hydrodynamics of galactic winds before cosmic reionization Aaron Smith,1? Volker Bromm1 and Abraham Loeb2 1Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA 2Department of Astronomy, Harvard University, 60 Garden Street, Cambridge, MA 02138, USA Accepted XXX. Received YYY; in original form ZZZ ABSTRACT The dynamical impact of Lyman α (Lyα) radiation pressure on galaxy formation depends on the rate and duration of momentum transfer between Lyα photons and neutral hydrogen gas. Although photon trapping has the potential to multiply the effective force, ionizing ra- diation from stellar sources may relieve the Lyα pressure before appreciably affecting the kinematics of the host galaxy or efficiently coupling Lyα photons to the outflow. We present self-consistent Lyα radiation-hydrodynamics simulations of high-z galaxy environments by coupling the Cosmic Lyα Transfer code (colt) with spherically symmetric Lagrangian frame hydrodynamics. The accurate but computationally expensive Monte-Carlo radiative transfer calculations are feasible under the one-dimensional approximation. The initial starburst drives an expanding shell of gas from the centre and in certain cases Lyα feedback significantly en- hances the shell velocity. Radiative feedback alone is capable of ejecting baryons into the 8 intergalactic medium (IGM) for protogalaxies with a virial mass of Mvir . 10 M . We com- pare the Lyα signatures of Population III stars with 105 K blackbody emission to that of direct collapse black holes with a nonthermal Compton-thick spectrum and find substantial differ- ences if the Lyα spectra are shaped by gas pushed by Lyα radiation-driven winds. For both sources, the flux emerging from the galaxy is reprocessed by the IGM such that the observed Lyα luminosity is reduced significantly and the time-averaged velocity offset of the Lyα peak is shifted redward. Key words: galaxies: formation – galaxies: high-redshift – cosmology: theory. 1 INTRODUCTION tral hydrogen is opaque to the Lyα line, photon trapping effec- tively acts as a force multiplier applied to gas surrounding H ii re- Radiation from the first stars and galaxies initiated a dramatic trans- gions. Indeed, the role of Lyα radiation pressure throughout the formation throughout the Universe, marking the end of the cos- galactic assembly process has been discussed extensively, particu- mic dark ages (Bromm & Yoshida 2011; Loeb & Furlanetto 2013). larly in the context of other feedback mechanisms (e.g. Cox 1985; The observational frontier for high-redshift galaxies has been ex- Haehnelt 1995; Oh & Haiman 2002; McKee & Tan 2008). Such tended into the epoch of reionization (Bouwens et al. 2011; Finkel- discussions tend to focus on order of magnitude estimates based stein et al. 2013; Oesch et al. 2015; Stark et al. 2015; Zitrin et al. on idealized radiative transfer calculations (Wise et al. 2012; Dijk- 2015; Oesch et al. 2016). Furthermore, next-generation observato- stra & Loeb 2008; Milosavljevic´ et al. 2009). Up to this point, ac- ries such as the James Webb Space Telescope (JWST; Gardner et al. arXiv:1607.07166v2 [astro-ph.GA] 25 Oct 2016 curate radiation-hydrodynamics (RHD) simulations incorporating 2006) will probe even deeper into the past and provide essential de- Lyα feedback have not been performed, either because the effects tails about cosmic history. The Lyα transition of neutral hydrogen are considered sub-dominant or the perceived computational costs (H i) plays a prominent role in spectral observations of high-z ob- prohibited such a treatment. However, the nature of this question jects. However, due to the high opacity of pre-reionized gas, direct requires full consideration of the dynamical coupling between mat- detection is challenging. Still, it may be possible to observe the in- ter and radiation. direct signatures of radiatively-driven outflows, including from Lyα radiation pressure which is more prominent in these conditions. The first galaxies were likely atomic cooling haloes whose 4 Within the first galaxies, up to two-thirds of the ionizing pho- virial temperatures activate Lyα line cooling, i.e. Tvir & 10 K tons from massive stars are reprocessed into Lyα radiation (Par- (Bromm & Yoshida 2011). In this framework, the first galaxies tridge & Peebles 1967; Dijkstra 2014). However, because neu- greatly impacted their surroundings as the initial drivers of reion- ization. Furthermore, radiative feedback from Population III or II stars dramatically altered the gas within these relatively low-mass ? E-mail: [email protected] systems. Feedback physics is crucial for understanding the multi- c 2016 The Authors 2 A. Smith et al. scale connections of astrophysical phenomena and their observa- Much of the uncertainty regarding Lyα dynamics is related tional signatures. So far simulations have focused on thermally- to difficulties in numerical modeling. Monte-Carlo radiative trans- driven supernova (SN) feedback while the impact of Lyα radiation fer (MCRT) has emerged as the prevalent method for accurate Lyα pressure is still relatively unexplored. calculations (Ahn et al. 2002; Zheng & Miralda-Escudé 2002; Di- The physical processes that couple Lyα radiation to gas dy- jkstra et al. 2006). In many cases Lyα transfer codes are used to namics are continuum absorption by dust and momentum transfer post-process realistic hydrodynamical simulations (e.g. Tasitsiomi via multiple scattering with neutral hydrogen. These two mecha- 2006; Laursen et al. 2009; Verhamme et al. 2012; Smith et al. nisms are roughly independent of each other in the sense that high 2015). However, idealized models described by a few basic param- dust content reduces the Lyα escape fraction while an absence eters have also been widely used to study Lyα spectra from mod- of dust results in a pure scattering scenario. Wise et al. (2012) erate redshift galaxies (Ahn 2004; Verhamme et al. 2006; Gronke argue for the existence of a metallicity upper limit such that if et al. 2015; Gronke & Dijkstra 2016). Along these lines, Dijkstra Z & 0:05 Z then Lyα radiation pressure may be ignored because & Loeb (2008) performed the first, direct MCRT calculations of of the increasing impact of dust opacity (see also Henney & Arthur Lyα radiation pressure for various spherically symmetric models 1998). However, it is uncertain whether metallicity thresholds are representing different stages of galaxy formation. The authors de- universally applicable considering the nontrivial nature of Lyα ra- scribe scenarios for supersonic Lyα-driven outflows for ∼ 106 M diative transfer in inhomogeneous, dusty media. Throughout this minihalo environments. They argue that Lyα pressure is too weak paper we approximate first galaxies as metal-free environments so to affect larger (& 109 M ) high-redshift star-forming galaxies, ex- dust effects are not discussed in detail. Nonetheless, even without cept in the case of . 1 kpc galactic supershells in the interstellar absorption, Lyα photon trapping only affects the residual H i within medium as explored in greater detail by Dijkstra & Loeb (2009). ionized regions. Therefore, unless the gas remains neutral for a long We emphasize that each of the above estimates are based on non- enough duration even relatively strong sources are kinematically in- dynamical simulations. Still, we must consider the possibility that consequential. Furthermore, geometric effects such as gas clump- Lyα radiation can drive galactic winds and affect regions with neu- ing, rotation, and filamentary structure often lead to anisotropic es- tral gas throughout the galaxy formation process. In contrast to the cape, photon leakage, or otherwise altered dynamical impact. MCRT method, Latif et al. (2011) use the stiffened equation of state In regions dominated by Lyα radiation pressure we expect proposed by Spaans & Silk (2006) to mimic Lyα pressure effects expansion to set in and eventually limit the impact of subsequent in a cosmological simulation. However, it is unclear whether such feedback. Historically, this was recognized to be important in the an approach faithfully reproduces the complex physics involved context of planetary nebulae as far back as Ambarzumian (1932), with Lyα radiative transfer. The qualitative result is similar to other Zanstra (1934), Struve (1942), and Chandrasekhar (1945). The au- forms of radiation pressure, which collectively regulate star forma- thors found that expansion could lower the Lyα opacity
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