First light and reionization epoch simulations (Flares) X: environmental galaxy bias and survey variance at high redshift

Upcoming deep galaxy surveys with JWST will probe galaxy evolution during the epoch of reionization (EoR, 5 ≤ z ≤ 10) over relatively compact areas (e.g. ∼300 arcmin2 for the JADES GTO survey). It is therefore imperative that we understand the degree of survey variance to evaluate how representative the galaxy populations in these studies will be. We use the First Light And Reionization Epoch Simulations (Flares) to measure the galaxy bias of various tracers over an unprecedentedly large range in overdensity for a hydrodynamic simulation, and use these relations to assess the impact of bias and clustering on survey variance in the EoR. Star formation is highly biased relative to the underlying dark matter distribution, with the mean ratio of the stellar to dark matter density varying by a factor of 100 between regions of low and high matter overdensity (smoothed on a scale of 14 h−1 cMpc). This is reflected in the galaxy distribution – the most massive galaxies are found solely in regions of high overdensity. As a consequence of the above, galaxies in the EoR are highly clustered, which can lead to a large variance in survey number counts. For mean number counts N ≲ 100 (1000), in a unit redshift slice of angular area 300 arcmin2 (1.4 deg2), the 2σ range in N is roughly a factor of four (two). We present relations between the expected variance and survey area for different survey geometries; these relations will be of use to observers wishing to understand the impact of survey variance on their results

First Light And Reionisation Epoch Simulations (FLARES) - XII: The consequences of star-dust geometry on galaxies in the EoR

Using the First Light And Reionisation Epoch Simulations, a suite of hydrodynamical simulations, we explore the consequences of a realistic model for star-dust geometry on the observed properties of galaxies. We find that the ultraviolet (UV) attenuation declines rapidly from the central regions of galaxies, and bright galaxies have spatially extended star formation that suffers less obscuration than their fainter counterparts, demonstrating a non-linear relationship between the UV luminosity and the UV attenuation, giving a double power-law shape to the UVLF. Spatially distinct stellar populations within galaxies experience a wide range of dust attenuation due to variations in the dust optical depth along their line of sight, which can range from completely dust obscured to being fully unobscured. The overall attenuation curve of a galaxy is then a complex combination of various lines of sight within the galaxy. We explore the manifestation of this effect to study the reliability of line ratios to infer galaxy properties, in particular, the Balmer decrement and the Baldwin, Phillips, and Terlevich (BPT) diagram. We find the Balmer decrement predicted Balmer-line attenuation to be higher (factor of 1 to 10) than expected from commonly used attenuation curves. The observed BPT line ratios deviate from their intrinsic values [median difference of 0.08 (0.02) and standard deviation of 0.2 (0.05) for log10([NHα) (log10([O iii]λ5008/Hβ)]. Finally, we explore the variation in observed properties (UV attenuation, UV slope, and Balmer decrement) with viewing angle, finding average differences of ∼0.3 mag in the UV attenuation.</p

First light and reionization epoch simulations (FLARES) IX: the physical mechanisms driving compact galaxy formation and evolution

In the First Light And Reionization Epoch Simulations (FLARES) suite of hydrodynamical simulations, we find the high-redshift (z > 5) intrinsic size–luminosity relation is, surprisingly, negatively sloped. However, after including the effects of dust attenuation, we find a positively sloped UV observed size–luminosity relation in good agreement with other simulated and observational studies. In this work, we extend this analysis to probe the underlying physical mechanisms driving the formation and evolution of the compact galaxies driving the negative size–mass/size–luminosity relation. We find the majority of compact galaxies (R1/2, * </p

First light and reionisation epoch simulations (FLARES) - VIII. The emergence of passive galaxies at z ≥ 5

Passive galaxies are ubiquitous in the local universe, and various physical channels have been proposed that lead to this passivity. To date, robust passive galaxy candidates have been detected up to z ≤ 5, but it is still unknown if they exist at higher redshifts, what their relative abundances are, and what causes them to stop forming stars. We present predictions from the first light and reionisation epoch simulations (flares), a series of zoom simulations of a range of overdensities using the eagle code. Passive galaxies occur naturally in the eagle model at high redshift, and are in good agreement with number density estimates from Hubble Space Telescope (HST) and early JWST results at 3 ≤ z ≤ 5. Due to the unique flares approach, we extend these predictions to higher redshifts, finding passive galaxy populations up to z ∼8. Feedback from supermassive black holes is the main driver of passivity, leading to reduced gas fractions and star forming gas reservoirs. We find that passive galaxies at z ≥ 5 are not identified in the typical UVJ selection space due to their still relatively young stellar populations, and present new rest-frame selection regions. We also produce mock NIRCam and MIRI fluxes, and find that significant numbers of passive galaxies at z ≥ 5 should be detectable in upcoming wide surveys with JWST. Finally, we present JWST colour distributions, with new selection regions in the observer-frame for identifying these early passive populations.</p

First light and reionization epoch simulations (flares) - XIV. The Balmer/4000 Å breaks of distant galaxies

With the successful launch and commissioning of JWST we are now able to routinely spectroscopically probe the rest-frame optical emission of galaxies at z > 6 for the first time. Among the most useful spectral diagnostics used in the optical is the Balmer/4000 Å break; this is, in principle, a diagnostic of the mean ages of composite stellar populations. However, the Balmer break is also sensitive to the shape of the star formation history, the stellar (and gas) metallicity, the presence of nebular continuum emission, and dust attenuation. In this work, we explore the origin of the Balmer/4000 Å break using the synthesizer synthetic observations package. We then make predictions of the Balmer/4000 Å break using the First Light and Reionization Epoch Simulations at 5 </p

First light and reionization epoch simulations (FLARES) V: the redshift frontier

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 Reionization Epoch simulations (flares) 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 ß, though the observational uncertainties are large. We note tension with recent constraints z ~ 13 from Harikane et al. (2021) – 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 reionization epoch simulations (FLARES) XI: [O III] emitting galaxies at 5 < z < 10

JWST has now made it possible to probe the rest-frame optical line emission of high-redshift galaxies extending to z ≈ 9, and potentially beyond. To aid in the interpretation of these emerging constraints, in this work we explore predictions for [O III]λλ4960, 5008 Å emission in high-redshift galaxies using the First Light and Reionization Epoch Simulations (FLARES). We produce predictions for the [O III] luminosity function, its correlation with the UV luminosity, and the distribution of equivalent widths (EWs). We also explore how the [O III] EW correlates with physical properties including specific star formation rate, metallicity, and dust attenuation. Our predictions are largely consistent with recent observational constraints on the luminosity function, average EWs, and line ratios. However, they fail to reproduce the observed tail of high-EW sources and the number density of extreme line emitters. Possibilities to explain these discrepancies include an additional source of ionizing photons and/or greater stochasticity in star formation in the model or photometric scatter and/or bias in the observations. With JWST now rapidly building larger samples and a wider range of emission lines the answer to this remaining discrepancy should be available imminently.</p

First light and reionization epoch simulations (FLARES) X iii: the lyman-continuum emission of high-redshift galaxies

The history of reionization is highly dependent on the ionizing properties of high-redshift galaxies. It is therefore important to have a solid understanding of how the ionizing properties of galaxies are linked to physical and observable quantities. In this paper, we use the First Light and Reionization Epoch Simulations (Flares) to study the Lyman-continuum (LyC, i.e. hydrogen-ionizing) emission of massive () galaxies at redshifts z = 5 - 10. We find that the specific ionizing emissivity (i.e. intrinsic ionizing emissivity per unit stellar mass) decreases as stellar mass increases, due to the combined effects of increasing age and metallicity. Flares predicts a median ionizing photon production efficiency (i.e. intrinsic ionizing emissivity per unit intrinsic far-UV luminosity) of, with values spanning the range. This is within the range of many observational estimates, but below some of the extremes observed. We compare the production efficiency with observable properties, and find a weak negative correlation with the UV-continuum slope, and a positive correlation with the [O iii] equivalent width. We also consider the dust-attenuated production efficiency (i.e. intrinsic ionizing emissivity per unit dust-attenuated far-UV luminosity), and find a median of. Within our sample of galaxies, it is the stellar populations in low mass galaxies that contribute the most to the total ionizing emissivity. Active galactic nuclei (AGN) emission accounts for 10 - 20 per cent of the total emissivity at a given redshift, and extends the LyC luminosity function by ∼0.5 dex.</p

Efficient NIRCam Selection of Quiescent Galaxies at 3 < z < 6 in CEERS

Substantial populations of massive quiescent galaxies at z ≥ 3 challenge our understanding of rapid galaxy growth and quenching over short timescales. In order to piece together this evolutionary puzzle, more statistical samples of these objects are required. Established techniques for identifying massive quiescent galaxies are increasingly inefficient and unconstrained at z > 3. As a result, studies report that as much as 70% of quiescent galaxies at z > 3 may be missed from existing surveys. In this work, we propose a new empirical color selection technique designed to select massive quiescent galaxies at 3 ≲ z ≲ 6 using JWST NIRCam imaging data. We use empirically constrained galaxy spectral energy distribution (SED) templates to define a region in the F277W − F444W versus F150W − F277W color plane that captures quiescent galaxies at z > 3. We apply these color selection criteria to the Cosmic Evolution Early Release Science (CEERS) Survey and use SED fitting on sources in the region to identify 44 candidate z ≳ 3 quiescent galaxies. Over half of these sources are newly discovered and, on average, exhibit specific star formation rates of poststarburst galaxies. Most of these sources would not be discovered using canonical UVJ diagrams. We derive volume density estimates of n ∼ 1–4 × 10−5 Mpc−3 at 3 </p