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Massive quiescent galaxies (MQGs) are thought to have formed stars rapidly at early times followed by a long period of quiescence. The recent discovery of a MQG, ZF-COSMOS-20115 at $z\sim4$, only 1.5 Gyr after the big bang, places new constraints on galaxy growth and the role of feedback in early star formation. Spectroscopic follow-up confirmed ZF-COSMOS-20115 as a MQG at $z=3.717$ with an estimated stellar mass of ${\sim}10^{11}\mathrm{M}_\odot$, showing no evidence of recent star formation. We use the Meraxes semi-analytic model to investigate how ZF-COSMOS-20115 analogues build stellar mass, and why they become quiescent. We identify three analogue galaxies with similar properties to ZF-COSMOS-20115. We find that ZF-COSMOS-20115 is likely hosted by a massive halo with virial mass of ${\sim}10^{13}\mathrm{M}_\odot$, having been through significant mergers at early times. These merger events drove intense growth of the nucleus, which later prevented cooling and quenched star formation. ZF-COSMOS-20115 likely remained quiescent at $z, 11 pages, 7 figures; Updated to match the published version. All results and conclusions remain unchanged

Using a series of high-resolution hydrodynamical simulations we show that during the rapid growth of high-redshift (z > 5) galaxies, reserves of molecular gas are consumed over a time-scale of 300Myr, almost independent of feedback scheme. We find that there exists no such simple relation for the total gas fractions of these galaxies, with little correlation between gas fractions and specific star formation rates. The bottleneck or limiting factor in the growth of early galaxies is in converting infalling gas to cold star-forming gas. Thus, we find that the majority of high redshift dwarf galaxies are effectively in recession, with demand (of star formation) never rising to meet supply (of gas), irrespective of the baryonic feedback physics modelled. We conclude that the basic assumption of self-regulation in galaxies - that they can adjust total gas consumption within a Hubble time - does not apply for the dwarf galaxies thought to be responsible for providing most UV photons to reionize the high redshift Universe. We demonstrate how this rapid molecular time-scale improves agreement between semi-analytic model predictions of the early Universe and observed stellar mass functions., 17 pages, 27 figures, accepted for publication in MNRAS, minor updates to align with final published version

Intergalactic medium temperature is a powerful probe of the epoch of reionisation, as information is retained long after reionisation itself. However, mean temperatures are highly degenerate with the timing of reionisation, with the amount heat injected during the epoch, and with the subsequent cooling rates. We post-process a suite of semi-analytic galaxy formation models to characterise how different thermal statistics of the intergalactic medium can be used to constrain reionisation. Temperature is highly correlated with redshift of reionisation for a period of time after the gas is heated. However as the gas cools, thermal memory of reionisation is lost, and a power-law temperature-density relation is formed, $T = T_0(1+\delta)^{1-\gamma}$ with $\gamma \approx 1.5$. Constraining our model against observations of electron optical depth and temperature at mean density, we find that reionisation likely finished at $z_{\rm{reion}} = 6.8 ^{+ 0.5} _{-0.8}$ with a soft spectral slope of $\alpha = 2.8 ^{+ 1.2} _{-1.0}$. By restricting spectral slope to the range $[0.5,2.5]$ motivated by population II synthesis models, reionisation timing is further constrained to $z_{\rm{reion}} = 6.9 ^{+ 0.4} _{-0.5}$. We find that, in the future, the degeneracies between reionisation timing and background spectrum can be broken using the scatter in temperatures and integrated thermal history., Comment: 17 pages, 17 figures, Accepted for publication in MNRAS

We use high resolution N-Body simulations to study the concentration and spin parameters of dark matter haloes in the mass range $10^8\, {\rm M}_{\odot}\, h^{-1} < {\rm M} < 10^{11}\, {\rm M}_{\odot}\, h^{-1}$ and redshifts $5{}5$ concentration$-$mass ($c(M)$) relation that is almost flat and well described by a simple power-law for both NFW and Einasto fits. The intrinsic scatter around the mean relation is $\Delta c_{\rm{vir}}{\sim1}$ (or 20 per cent) at $z=5$. We also find that the analytic model proposed by Ludlow et al. reproduces the mass and redshift-dependence of halo concentrations. Our best-fit Einasto shape parameter, $\alpha$, depends on peak height, $\nu$, in a manner that is accurately described by $\alpha {=}0.0070\nu^2{+}0.1839$. The distribution of the spin parameter, $\lambda$, has a weak dependence on equilibrium state; $\lambda$ peaks at roughly ${\sim}0.033$ for our relaxed sample, and at ${\sim}0.04$ for the full population. The spin--virial mass relation has a mild negative correlation at high redshift.

We investigate the clustering properties of Lyman-break galaxies (LBGs) at $z\sim6$ - $8$. Using the semi-analytical model {\scshape Meraxes} constructed as part of the Dark-ages Reionization And Galaxy-formation Observables from Numerical Simulation (DRAGONS) project, we predict the angular correlation function (ACF) of LBGs at $z\sim6$ - $8$. Overall, we find that the predicted ACFs are in good agreement with recent measurements at $z\sim 6$ and $z\sim 7.2$ from observations consisting of the Hubble eXtreme Deep Field (XDF), the Hubble Ultra-Deep Field (HUDF) and Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) field. We confirm the dependence of clustering on luminosity, with more massive dark matter haloes hosting brighter galaxies, remains valid at high redshift. The predicted galaxy bias at fixed luminosity is found to increase with redshift, in agreement with observations. We find that LBGs of magnitude $M_{\rm AB(1600)} < -19.4$ at $6\lesssim z \lesssim 8$ reside in dark matter haloes of mean mass $\sim 10^{11.0}$- $10^{11.5} M_{\rm \odot}$, and this dark matter halo mass does not evolve significantly during reionisation., 14 pages, 10 figures, published in MNRAS

Motivated by recent measurements of the number density of faint AGN at high redshift, we investigate the contribution of quasars to reionization by tracking the growth of central supermassive black holes in an update of the Meraxes semi-analytic model. The model is calibrated against the observed stellar mass function at $z\sim0.6-7$, the black hole mass function at $z\lesssim0.5$, the global ionizing emissivity at $z\sim2-5$ and the Thomson scattering optical depth. The model reproduces a Magorrian relation in agreement with observations at $z, 21 pages, 12 figures; Updated to match the published version. All results and conclusions remain unchanged

Direct detection of regions of ionized hydrogen (HII) has been suggested as a promising probe of cosmic reionization. Observing the redshifted 21-cm signal of hydrogen from the epoch of reionization (EoR) is a key scientific driver behind new-generation, low-frequency radio interferometers. We investigate the feasibility of combining low-frequency observations with the Square Kilometre Array and near infra-red survey data of the Wide-Field Infrared Survey Telescope to detect cosmic reionization by imaging HII bubbles surrounding massive galaxies during the cosmic dawn. While individual bubbles will be too small to be detected, we find that by stacking redshifted 21-cm spectra centred on known galaxies, it will be possible to directly detect the EoR at $z \sim 9-12$, and to place qualitative constraints on the evolution of the spin temperature of the intergalactic medium (IGM) at $z \geq 9$. In particular, given a detection of ionized bubbles using this technique, it is possible to determine if the IGM surrounding them is typically in absorption or emission. Determining the globally-averaged neutral fraction of the IGM using this method will prove more difficult due to degeneracy with the average size of HII regions., Comment: 14 pages, 11 figures, 2 tables, published in MNRAS. Updated to match published version. Additional results and comments added from previous version. All other results and conclusions remain unchanged

We investigate high-redshift galaxy sizes using a semi-analytic model constructed for the Dark-ages Reionization And Galaxy-formation Observables from Numerical Simulation project. Our fiducial model, including strong feedback from supernovae and photoionization background, accurately reproduces the evolution of the stellar mass function and UV luminosity function. Using this model, we study the size--luminosity relation of galaxies and find that the effective radius scales with UV luminosity as $R_\mathrm{e}\propto L^{0.25}$ at $z{\sim}5$--$9$. We show that recently discovered very luminous galaxies at $z{\sim}7$ (Bowler et al. 2016) and $z{\sim}11$ (Oesch et al. 2016) lie on our predicted size--luminosity relations. We find that a significant fraction of galaxies at $z>8$ will not be resolved by JWST, but GMT will have the ability to resolve all galaxies in haloes above the atomic cooling limit. We show that our fiducial model successfully reproduces the redshift evolution of average galaxy sizes at $z>5$. We also explore galaxy sizes in models without supernova feedback. The no-supernova feedback models produce galaxy sizes that are smaller than observations. We therefore confirm that supernova feedback plays an important role in determining the size--luminosity relation of galaxies and its redshift evolution during reionization., 10 pages, 4 figures, Accepted for publication in MNRAS

We use the Dark-ages, Reionization And Galaxy-formation Observables from Numerical Simulations (DRAGONS) framework to investigate the effect of galaxy-formation physics on the morphology and statistics of ionized hydrogen (HII) regions during the Epoch of Reioinization (EoR). DRAGONS self-consistently couples a semi-analytic galaxy-formation model with the inhomogeneous ionizing UV background, and can therefore be used to study the dependence of morphology and statistics of reionization on feedback phenomena of the ionizing source galaxy population. Changes in galaxy-formation physics modify the sizes of HII regions and the amplitude and shape of 21-cm power spectra. Of the galaxy physics investigated, we find that supernova feedback plays the most important role in reionization, with HII regions up to $\approx 20$ per cent smaller and a fractional difference in the amplitude of power spectra of up to $\approx 17$ per cent at fixed ionized fraction in the absence of this feedback. We compare our galaxy-formation-based reionization models with past calculations that assume constant stellar-to-halo mass ratios and find that with the correct choice of minimum halo mass, such models can mimic the predicted reionization morphology. Reionization morphology at fixed neutral fraction is therefore not uniquely determined by the details of galaxy formation, but is sensitive to the mass of the haloes hosting the bulk of the ionizing sources. Simple EoR parametrizations are therefore accurate predictors of reionization statistics. However, a complete understanding of reionization using future 21-cm observations will require interpretation with realistic galaxy-formation models, in combination with other observations., Comment: 14 pages, 12 figures, published in MNRAS

We introduce Meraxes, a new, purpose-built semi-analytic galaxy formation model designed for studying galaxy growth during reionization. Meraxes is the first model of its type to include a temporally and spatially coupled treatment of reionization and is built upon a custom (100 Mpc)$^3$ N-body simulation with high temporal and mass resolution, allowing us to resolve the galaxy and star formation physics relevant to early galaxy formation. Our fiducial model with supernova feedback reproduces the observed optical depth to electron scattering and evolution of the galaxy stellar mass function between $z$=5 and 7, predicting that a broad range of halo masses contribute to reionization. Using a constant escape fraction and global recombination rate, our model is unable to simultaneously match the observed ionizing emissivity at $z{\lesssim}6$. However, the use of an evolving escape fraction of 0.05-0.1 at $z{\sim}6$, increasing towards higher redshift, is able to satisfy these three constraints. We also demonstrate that photoionization suppression of low mass galaxy formation during reionization has only a small effect on the ionization history of the inter-galactic medium. This lack of `self-regulation' arises due to the already efficient quenching of star formation by supernova feedback. It is only in models with gas supply-limited star formation that reionization feedback is effective at regulating galaxy growth. We similarly find that reionization has only a small effect on the stellar mass function, with no observationally detectable imprint at $M_{\rm *}{>}10^{7.5}\,{\rm M_{\odot}}$. However, patchy reionization has significant effects on individual galaxy masses, with variations of factors of 2-3 at $z$=5 that correlate with environment., 30 pages, 18 figures; published in MNRAS; Updated to match published version. Additional results and fitting formulae added from previous version. All other results and conclusions remain unchanged

We present the Dark-ages Reionization and Galaxy-formation Observables from Numerical Simulations (DRAGONS) program and Tiamat, the collisionless N-body simulation program upon which DRAGONS is built. The primary trait distinguishing Tiamat from other large simulation programs is its density of outputs at high redshift (100 from z=35 to z=5; roughly one every 10 Myr) enabling the construction of very accurate merger trees at an epoch when galaxy formation is rapid and mergers extremely frequent. We find that the friends-of-friends halo mass function agrees well with the prediction of Watson et al. at high masses, but deviates at low masses, perhaps due to our use of a different halo finder or perhaps indicating a break from "universal" behaviour. We then analyse the dynamical evolution of galaxies during the Epoch of Reionization finding that only a small fraction (~20%) of galactic halos are relaxed. We illustrate this using standard relaxation metrics to establish two dynamical recovery time-scales: i) halos need ~1.5 dynamical times following formation, and ii) ~2 dynamical times following a major (3:1) or minor (10:1) merger to be relaxed. This is remarkably consistent across a wide mass range. Lastly, we use a phase-space halo finder to illustrate that major mergers drive long-lived massive phase-space structures which take many dynamical times to dissipate. This can yield significant differences in the inferred mass build-up of galactic halos and we suggest that care must be taken to ensure a physically meaningful match between the galaxy-formation physics of semi-analytic models and the halo finders supplying their input., Comment: 15 pages, 10 figures. Accepted for publication by MNRAS

In this paper we present calculations of the UV luminosity function from the Dark-ages Reionization And Galaxy-formation Observables from Numerical Simulations (DRAGONS) project, which combines N-body, semi-analytic and semi-numerical modelling designed to study galaxy formation during the Epoch of Reionization. Using galaxy formation physics including supernova feedback, the model naturally reproduces the UV LFs for high-redshift star-forming galaxies from $z{\sim}5$ through to $z{\sim}10$. We investigate the luminosity--star formation rate (SFR) relation, finding that variable SFR histories of galaxies result in a scatter around the median relation of $0.1$--$0.3$ dex depending on UV luminosity. We find close agreement between the model and observationally derived SFR functions. We use our calculated luminosities to investigate the luminosity function below current detection limits, and the ionizing photon budget for reionization. We predict that the slope of the UV LF remains steep below current detection limits and becomes flat at $M_\mathrm{UV}{\gtrsim}{-14}$. We find that $48$ ($17$) per cent of the total UV flux at $z{\sim}6$ ($10$) has been detected above an observational limit of $M_\mathrm{UV}{\sim}{-17}$, and that galaxies fainter than $M_\mathrm{UV}{\sim}{-17}$ are the main source of ionizing photons for reionization. We investigate the luminosity--stellar mass relation, and find a correlation for galaxies with $M_\mathrm{UV}{, Comment: 17 pages, 14 figures, Accepted for publication in MNRAS

We study dwarf galaxy formation at high redshift ($z\ge5$) using a suite of high- resolution, cosmological hydrodynamic simulations and a semi-analytic model (SAM). We focus on gas accretion, cooling and star formation in this work by isolating the relevant process from reionization and supernova feedback, which will be further discussed in a companion paper. We apply the SAM to halo merger trees constructed from a collisionless N-body simulation sharing identical initial conditions to the hydrodynamic suite, and calibrate the free parameters against the stellar mass function predicted by the hydrodynamic simulations at z = 5. By making comparisons of the star formation history and gas components calculated by the two modelling techniques, we find that semi-analytic prescriptions that are commonly adopted in the literature of low-redshift galaxy formation do not accurately represent dwarf galaxy properties in the hydrodynamic simulation at earlier times. We propose 3 modifications to SAMs that will provide more accurate high-redshift simulations. These include 1) the halo mass and baryon fraction which are overestimated by collisionless N-body simulations; 2) the star formation efficiency which follows a different cosmic evolutionary path from the hydrodynamic simulation; and 3) the cooling rate which is not well defined for dwarf galaxies at high redshift. Accurate semi-analytic modelling of dwarf galaxy formation informed by detailed hydrodynamical modelling will facilitate reliable semi-analytic predictions over the large volumes needed for the study of reionization., Comment: 20 pages, 10 figures; Updated to match the published version. All results and conclusions remain unchanged

We investigate how the hydrostatic suppression of baryonic accretion affects the growth rate of dark matter halos during the Epoch of Reionization. By comparing halo properties in a simplistic hydrodynamic simulation in which gas only cools adiabatically, with its collisionless equivalent, we find that halo growth is slowed as hydrostatic forces prevent gas from collapsing. In our simulations, at the high redshifts relevant for reionization (between ${\sim}6$ and ${\sim}11$), halos that host dwarf galaxies ($\lesssim 10^{9} \mathrm{M_\odot}$) can be reduced by up to a factor of 2 in mass due to the hydrostatic pressure of baryons. Consequently, the inclusion of baryonic effects reduces the amplitude of the low mass tail of the halo mass function by factors of 2 to 4. In addition, we find that the fraction of baryons in dark matter halos hosting dwarf galaxies at high redshift never exceeds ${\sim}90\%$ of the cosmic baryon fraction. When implementing baryonic processes, including cooling, star formation, supernova feedback and reionization, the suppression effects become more significant with further reductions of ${\sim}30\%$ to 60\%. Although convergence tests suggest that the suppression may become weaker in higher resolution simulations, this suppressed growth will be important for semi-analytic models of galaxy formation, in which the halo mass inherited from an underlying N-body simulation directly determines galaxy properties. Based on the adiabatic simulation, we provide tables to account for these effects in N-body simulations, and present a modification of the halo mass function along with explanatory analytic calculations., Comment: 17 pages, 11 figures; Updated to match the published version. Two changes in Figures 1 and 3 in order to 1) correct bin sizes of the 10^8 and 10^8.5 Msol bins for NOSN_NOZCOOL_NoRe (was 0.5, should be 0.25); 2) include stellar mass in baryon fraction (was missed in Fig. 3). Quantitative description of Fig. 3 changed slightly in Section 2.2. All other results and conclusions remain unchanged