Flares

## Introduction

The FLARE simulations are a suite of high-resolution hydrodynamic simulations of galaxy formation and evolution using the EAGLE physics. The suite consists of a series of 40 'zoom' resimulations selected at $z = 5$ from a $(3.2 \, \mathrm{Gpc})^3$ parent dark matter-only volume. We select a range of overdensities in order to study the environmental effect on high-redshift galaxy evolution. We also combine these resimulations in order to produce composite distribution functions, signficantly extending the dynamic range of the model over smaller volume, periodic simulations.

These simulations provide a valuable resource for studying galaxy evolution over the first 2 billion years of the Universe's evolution, and for making predictions for upcoming observatories, such as JWST, Roman and Euclid.

If you would like further scientific information please refer to the publications. For all other queries you can contact us directly.

## Publications

First Light And Reionisation Epoch Simulations (FLARES) V: The redshift frontier arXiv:2204.09431

Stephen M. Wilkins, Aswin P. Vijayan, Christopher C. Lovell, William J. Roper, Dimitrios Irodotou, Joseph Caruana, Louise T. C. Seeyave, Jussi K. Kuusisto, Peter A. Thomas, Shedeur A. K. Parris

Abstract: The James Webb Space Telescope (JWST) is set to transform many areas of astronomy, one of the most exciting is the expansion of the redshift frontier to z>10. In its first year alone JWST should discover hundreds of galaxies, dwarfing the handful currently known. To prepare for these powerful observational constraints, we use the First Light And Reionisation Epoch (FLARES) simulations to predict the physical and observational properties of the z>10 population of galaxies accessible to JWST. This is the first time such predictions have been made using a hydrodynamical model validated at low redshift. Our predictions at z=10 are broadly in agreement with current observational constraints on the far-UV luminosity function and UV continuum slope β, though the observational uncertainties are large. We note tension with recent constraints z∼13 from Harikane et al. 2022 - compared to these constraints, FLARES predicts objects with the same space density should have an order of magnitude lower luminosity, though this is mitigated slightly if dust attenuation is negligible in these systems. Our predictions suggest that in JWST's first cycle alone, around 600 galaxies should be identified at z>10, with the first small samples available at z>13.

First Light and Reionisation Epoch Simulations (FLARES) IV: The size evolution of galaxies at z ≥ 5 arXiv:2203.12627

William J. Roper, Christopher C. Lovell, Aswin P. Vijayan, Madeline A. Marshall, Dimitrios Irodotou, Jussi K. Kuusisto, Peter A. Thomas, Stephen M. Wilkins

Abstract: We present the intrinsic and observed sizes of galaxies at z≥5 in the First Light And Reionisation Epoch Simulations (FLARES). We employ the large effective volume of FLARES to produce a sizeable sample of high redshift galaxies with intrinsic and observed luminosities and half light radii in a range of rest frame UV and visual photometric bands. This sample contains a significant number of intrinsically ultra-compact galaxies in the far-UV (1500 angstrom), leading to a negative intrinsic far-UV size-luminosity relation. However, after the inclusion of the effects of dust these same compact galaxies exhibit observed sizes that are as much as 50 times larger than those measured from the intrinsic emission, and broadly agree with a range of observational samples. This increase in size is driven by the concentration of dust in the core of galaxies, heavily attenuating the intrinsically brightest regions. At fixed luminosity we find a galaxy size redshift evolution with a slope of m=1.21−1.87 depending on the luminosity sample in question, and we demonstrate the wavelength dependence of the size-luminosity relation which will soon be probed by the Webb Space Telescope.

First Light and Reionisation Epoch Simulations (FLARES) III: The properties of massive dusty galaxies at cosmic dawn arXiv:2108.00830

Aswin P. Vijayan, Stephen M. Wilkins, Christopher C. Lovell, Peter A. Thomas, Peter Camps, Maarten Baes, James Trayford, Jussi Kuusisto, William J. Roper

Abstract: Using the First Light And Reionisation Epoch Simulations (FLARES) we explore the dust driven properties of massive high-redshift galaxies at z∈[5,10]. By post-processing the galaxy sample using the radiative transfer code SKIRT we obtain the full spectral energy distribution. We explore the resultant luminosity functions, IRX-β relations as well as the luminosity-weighted dust temperatures in the Epoch of Reionisation (EoR). We find that most of our results are in agreement with the current set of observations, but under-predict the number densities of bright IR galaxies, which are extremely biased towards the most overdense regions. We see that the FLARES IRX-β relation (for 5≤z≤8) predominantly follows the local starburst relation. The IRX shows an increase with stellar mass, plateauing at the high-mass end (∼$10^10 M_⊙$) and shows no evolution in the median normalisation with redshift. We also look at the dependence of the peak dust temperature (T_peak) on various galaxy properties including the stellar mass, IR luminosity and sSFR, finding the correlation to be strongest with sSFR. The luminosity-weighted dust temperatures increase towards higher redshifts, with the slope of the T_peak - redshift relation showing a higher slope than the lower redshift relations obtained from previous observational and theoretical works. The results from FLARES, which is able to provide a better statistical sample of high-redshift galaxies compared to other simulations, provides a distinct vantage point for the high-redshift Universe.

First Light and Reionisation Epoch Simulations (FLARES) II: The Photometric Properties of High-Redshift Galaxies arXiv:2008.06057

Aswin P. Vijayan, Christopher C. Lovell, Stephen M. Wilkins, Peter A. Thomas, David J. Barnes, Dimitrios Irodotou, Jussi Kuusisto, Will Roper

Abstract: We present the photometric properties of galaxies in the First Light and Reionisation Epoch Simulations (FLARES). The simulations trace the evolution of galaxies in a range of overdensities through the Epoch of Reionistion (EoR). With a novel weighting scheme we combine these overdensities, extending significantly the dynamic range of observed composite distribution functions compared to periodic simulation boxes. FLARES predicts a significantly larger number of intrinsically bright galaxies, which can be explained through a simple model linking dust-attenuation to the metal content of the interstellar medium, using a line-of-sight (LOS) extinction model. With this model we present the photometric properties of the FLARES galaxies for z∈[5,10]. We show that the ultraviolet (UV) luminosity function (LF) matches the observations at all redshifts. The function is fit by Schechter and double power-law forms, with the latter being favoured at these redshifts by the FLARES composite UV LF. We also present predictions for the UV continuum slope as well as the attenuation in the UV. The impact of environment on the UV LF is also explored, with the brightest galaxies forming in the densest environments. We then present the line luminosity and equivalent widths of some prominent nebular emission lines arising from the galaxies, finding rough agreement with available observations. We also look at the relative contribution of obscured and unobscured star formation, finding comparable contributions at these redshifts.

First Light and Reionisation Epoch Simulations (FLARES) I: Environmental Dependence of High-Redshift Galaxy Evolution arXiv:2004.07283

Christopher Lovell, Aswin P. Vijayan, Peter A. Thomas, Stephen M. Wilkins, Dimitrios Irodotou, Will Roper

Abstract: We introduce the First Light and Reionisation Epoch Simulations (FLARES), a suite of zoom simulations using the Eagle model. We re-simulate a range of overdensities during the Epoch of Reionisation (EoR) in order to build composite distribution functions, as well as explore the environmental dependence of galaxy formation and evolution during this critical period of galaxy assembly. The regions are selected from a large (3.2 cGpc)3 parent volume, based on their overdensity within a sphere of radius 14 cMpc/h. We then re-simulate with full hydrodynamics, and employ a novel weighting scheme that allows the construction of composite distribution functions that are representative of the full parent volume. This significantly extends the dynamic range compared to smaller volume periodic simulations. We present an analysis of the galaxy stellar mass function, the star formation rate distribution function and the star forming sequence predicted by \flares, and compare to a number of observational and model constraints. We also analyse the environmental dependence over an unprecedented range of overdensity. This increased dynamic range will allow us to make predictions for a number of large area surveys that will probe the EoR in coming years, such as WFIRST and Euclid.

## Data

Our codes are publicly available at github.com/flaresimulations. Please raise any issues or support requests there. If you require any data not detailed below please get in touch with one of the team members below.

Paper I

We provide stellar masses and (instantaneous) star formation rates for all galaxies in FLARES from $z = 5-10$ here in HDF5 format. Utilities for reading the data are available at github.com/flaresimulations.

Fits to the galaxy stellar mass function, star formation rate distribution function and star forming sequence are available at the links below. We also provide the full posterior chains obtained from fitDF.

Fits: available here

Chains: available here

Paper II

We provide stellar masses (within 30pkpc), star formation rates (averaged over star particles formed in the last 10, 30, 50, 100 and 200 Myr), the luminosity (dust attenuated and intrinsic), fluxes (Euclid, HST, JWST, Spitzer, Subaru), and the line luminosity and equivalent widths (for nebular lines) for all galaxies in FLARES from $z = 5-10$ here in HDF5 format. Utilities for reading the data are available at github.com/flaresimulations and the code to produce the plots in the paper can be found in github.com/flaresimulations/flares_photometry.

## People

Jussi Kuusisto
University of Sussex

Louise Seeyave
University of Sussex

Peter Thomas
University of Sussex