MNRAS 000,1{ ?? (2019) Preprint 27 March 2020 Compiled using MNRAS LATEX style file v3.0 Nebular Line Emission During the Epoch of Reionization Stephen M. Wilkins,1? Christopher C. Lovell,1 Ciaran Fairhurst,1 Yu Feng,2 Tiziana Di Matteo,3;4 Rupert Croft,3;4 Jussi Kuusisto,1 Aswin P. Vijayan,1 Peter Thomas,1 1 Astronomy Centre, Department of Physics and Astronomy, University of Sussex, Brighton, BN1 9QH, UK 3 Berkeley Center for Cosmological Physics, University of California, Berkeley, Berkeley CA, 94720, USA 2 McWilliams Center for Cosmology, Carnegie Mellon University, Pittsburgh PA, 15213, USA 4 School of Physics, University of Melbourne, VIC 3010, Australia. Accepted XXX. Received YYY; in original form ZZZ ABSTRACT Nebular emission lines associated with galactic Hii regions carry information about both physical properties of the ionised gas and the source of ionising photons as well as providing the opportunity of measuring accurate redshifts and thus distances once a cosmological model is assumed. While nebular line emission has been extensively studied at lower redshift there are currently only few constraints within the epoch of reionisation (EoR, z > 6), chiefly due to the lack of sensitive near-IR spectrographs. However, this will soon change with the arrival of the Webb Telescope providing sensi- tive near-IR spectroscopy covering the rest-frame UV and optical emission of galaxies in the EoR. In anticipation of Webb we combine the large cosmological hydrodynamical simulation BlueTides with photoionisation modelling to predict the nebular emission line properties of galaxies at z = 8 ! 13. We find good agreement with the, albeit limited, existing direct and indirect observational constraints on equivalent widths though poorer agreement with luminosity function constraints. Key words: galaxies: high-redshift { galaxies: photometry { methods: numerical { galaxies: luminosity function, mass function 1 INTRODUCTION is predominantly due to lack of sensitive near-IR spectro- graphs, particularly at > 2µm where the rest-frame optical Massive stars and active galactic nuclei (AGN) are often in- lines lie at z > 4, and the comparative lack of strong lines, tense sources of Lyman-continuum (LyC, or hydrogen ionis- other than Lyman-α, in the rest-frame UV. The small num- ing) photons resulting in the formation of regions of ionised ber of detections at high-redshift come overwhelmingly from gas in their surroundings (e.g. H regions). The emission ii Lyman-α (e.g. Stark et al. 2010; Pentericci et al. 2011; Stark from these regions carries information about the physical et al. 2011; Caruana et al. 2012; Stark et al. 2013; Finkelstein conditions in the interstellar medium (ISM) as well as the et al. 2013; Caruana et al. 2014; Stark et al. 2017) though source of the ionising photons themselves. Key properties there has now been a handful of detections of the [Civ]λ1548 arXiv:1904.07504v2 [astro-ph.GA] 26 Mar 2020 that can be constrained include the star formation rate (e.g. and [Ciii],Ciii]λ1907; 1090 lines (Stark et al. 2015a,b, 2017). Kennicutt & Evans 2012; Wilkins et al. 2019), gas metal- The presence of extremely strong optical lines can also be licity (e.g. Tremonti et al. 2004), temperature, density, dust indirectly inferred from their impact on broadband photom- content (e.g. Reddy et al. 2015), and the presence of an AGN etry (e.g. Schaerer & de Barros 2010; Stark et al. 2013; (e.g. Baldwin et al. 1981). Nebular line emission also enables Wilkins et al. 2013; Smit et al. 2014; Wilkins et al. 2016b; the accurate measurement of redshifts, and thus distances De Barros et al. 2019) yielding constraints now available up once a cosmological model is assumed. to z ≈ 8 (De Barros et al. 2019). While there has been extensive progress in observing line emission at low (e.g. Brinchmann et al. 2004) and in- While existing observational constraints in the EoR termediate (e.g. Steidel et al. 1996; Shapley et al. 2003) red- are limited this will soon change with the arrival of the shifts there are few direct constraints at high-redshift. This Webb Telescope. Webb's NIRSpec instrument will provide deep near-IR single slit, multi-object, and integral field spec- troscopy from ∼ 0:7 − 5µm, while the NIRISS and NIRCam ? E-mail: [email protected] instruments will, together, provide wide field slitless spec- c 2019 The Authors 2 Stephen M. Wilkins et al. troscopy over a similar range. This is sufficient to encompass this work we adopt a more conservative approach only fo- 8 all the strong optical lines to z ∼ 7 with [Oii] potentially ac- cussing on galaxies with stellar masses > 10 M which con- cessible to z ∼ 12. Webb's mid-infrared instrument (MIRI) tain at least approximately 100 star particles in order to star will provide mid-IR single slit, and slitless spectroscopy at formation history samplings effects (see AppendixB for ex- λ > 5µm, albeit at much lower sensitivity and thus will ploration of sampling effects). The properties of galaxies in likely only detect line emission for the brightest sources in the simulation are extensively described in Feng et al.(2015, the EoR. 2016); Wilkins et al.(2016a,b); Waters et al.(2016a,b); Di The existing direct and indirect constraints and the Matteo et al.(2017); Wilkins et al.(2017, 2018). While nearing prospect of Webb motivates us to produce predic- BlueTides contains super-massive black holes, and even a tions for the nebular emission line properties of galaxies in small number of AGN dominated sources at z = 8, in this the EoR. In this paper, we combine the large BlueTides hy- work we focus on the line emission arising solely from gas drodynamical simulation with photoionisation modelling to excited by stellar sources. predict the nebular line properties of galaxies in the EoR, specifically (z = 8 ! 13). As part of this paper we also ex- plore some of the photon-ionisation modelling assumptions 2.1.1 Ages and Metallicities of Galaxies in BLUETIDES including the choice of stellar population synthesis (SPS) model and initial mass function (IMF). These predictions As emission line luminosities and equivalent widths are pre- can be used to optimise the design of Webb surveys prior to dominantly driven by galaxy star formation and metal en- launch, targeting emission lines in the EoR. The observation richment histories it is useful to explore the average ages and of these lines will also provide a powerful constraint on the metallicities predicted by BlueTides. The mean stellar age physics incorporated into galaxy formation models. and mean metallicity of young (< 10 Myr) stars are shown This work builds upon other recent efforts to model neb- as a function of stellar mass for a range of redshifts in Fig- ular lines using both simple analytical models (e.g. Charlot ure1. These correlations were previously discussed in more & Longhetti 2001; Schaerer & de Barros 2009; Inoue et al. detail in Wilkins et al.(2017) while a more detailed analy- 2014; Gutkin et al. 2016; Feltre et al. 2016; Nakajima et al. sis of the joint star formation and metal enrichment history 2018) and the modelling in semi-analytical galaxy formation is presented in Fairhurst et al. in-prep. In short, the mean models (e.g. Orsi et al. 2014) and hydrodynamical simula- stellar age appears to show little dependence on mass but tions (e.g. Shimizu et al. 2016; Hirschmann et al. 2017, 2019). evolves strongly with redshift while the mean metallicity of 0:4 This article is structured as follows: in Section2 we de- young stars shows a power-law dependence (Z / M ) on scribe the BlueTides simulation and our methodology for stellar mass but little evolution with redshift. calculating spectral energy distribution including nebular emission (x2.2). In Section3 we present our predictions. In this section we also explore the impact of some modelling as- 2.2 Spectral Energy Distribution Modelling sumptions (x3.2), including the choice of stellar population We model the spectral energy distributions (SEDs) of galax- synthesis (SPS) model (x3.2.1), initial mass function (IMF, ies in BlueTides as the sum of the SEDs of each star particle x3.2.2), photoionisation modelling parameters, including the identified as belonging to each galaxy. impact of dust (x3.2.3). In Section 3.4 we compare our pre- We begin by associating each star particle with a pure dictions to existing observational constraints. In Section4 stellar SED according to its age and metallicity. To obtain we present our conclusions. this SED we interpolate publicly available grids produced by stellar population synthesis (SPS) models. By default we make the following modelling choices: we assume the v2.2.1 SPS model (Stanway & Eldridge 2018; Eldridge 2 MODELLING NEBULAR EMISSION IN bpass et al. 2017)2 and a modified version of the Salpeter IMF BLUETIDES containing a flattened (α = −1:3) power-law at low-masses 2.1 The BlueTides Simulation (m < 0:5 M ). This IMF produces very similar (< 0:05 dex) 1 results to the assumption of a Chabrier(2003) IMF but per- The BlueTides simulation (see Feng et al. 2015, 2016, for mits a fairer comparison with the alternative IMFs available description of the simulation physics) is an extremely large for bpass. In x3.2 we explore the impact of these, and other cosmological hydrodynamical simulation designed to study assumptions. the early phase of galaxy formation and evolution with a specific focus on the formation of the massive galaxies. Blue- Tides phase 1 evolved a (400=h ≈ 577)3 cMpc3 cube to z = 8 2.2.1 Nebular emission using 2 × 70403 particles assuming the cosmological param- eters from the Wilkinson Microwave Anisotropy Probe ninth Using the intrinsic stellar SED, and assuming no escape of year data release (Hinshaw et al.