11 research outputs found
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A tale of birds and bug fixes: computational astrophysics in the Epoch of Reionisation
Deriving astrophysical insights from cosmological hydrodynamical simulations of the Epoch of Reionisation (EoR) is a many-feathered beast (if you will indulge my abuse of a phrase). Before we can derive any scientific results we must first develop robust and efficient codes to run the simulations from which we want to derive physical insights. Before we can analyse these simulations we need to ex- tract meaningful physical structures such as dark matter halos and galaxies from the simulated density field. Even then we are not finished. We can derive physical properties such as galaxy mass from these structures but more sophisticated analyses require forward modelling of fluxes including the effects of dust extinction and image creation and processing. Finally, once we have combined each of these facets we can begin to untangle the physical mechanisms governing the evolution of galaxies in our simulated universes. In this thesis, I detail my contributions to each of these ”feathers”. This work focuses on three birds, SWIFT, COLIBRE, and EAGLE. With an additional, sadly non-avian, contribution in the form of the MErger Graph Algorithm (MEGA).I will present my modifications to the SWIFT simulation code to enable efficient zoom simulations, a necessity for efficient simulations of the EoR with statistically significant galaxy samples. These modifications reduced the run time of zoom simulations by > 70% and enable the future of the First Light and Reionisation Epoch Simulations (FLARES) using the COLIBRE subgrid model. I will then present MEGA and detail its improvements in structure extraction and assembly history construction. MEGA builds on the findings of the Sussing Merger Trees comparison project, completely eliminating the pathological cases prevalent in other assembly history construction algorithms. I will then present a thorough analysis of galaxy sizes in the EoR using the first iteration of the FLARES simulations employing the EAGLE subgrid model. I first forward model the simulated galaxies to probe the observed size-luminosity relation showing that galaxies are intrinsically ultra-compact. However, after the inclusion of dust, the measured size increases, producing good agreement with observational studies. Using this observational agreement as a strong foundation, I then probe the underlying physical mechanisms governing the formation and evolution of compact galaxies. I find that the rise of compact galaxies in the EoR is driven by high centrally concentrated star formation rates. I then go on to show that the evolution of the compact high redshift Universe to the extended low redshift Universe is driven by inside-out star formation enabled by high redshift gas distributions up to 100 times larger surrounding galaxies’ stellar components.</p
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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
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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
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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
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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
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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
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Seeing sharper and deeper: JWST's first glimpse of the photometric and spectroscopic properties of galaxies in the epoch of reionization
We analyse the photometric and spectroscopic properties of four galaxies in the epoch of reionization (EoR) within the SMACS J0723.3-7327 JWST Early Release Observations field. Given the known spectroscopic redshifts of these sources, we investigated the accuracy with which photometric redshifts can be derived using NIRCam photometry alone, finding that F115W imaging is essential to distinguish between z ∼8 galaxies with high equivalent width (EW) [O iii] λ5007 emission and z ∼10 Balmer break galaxies. We find that all four sources exhibit strong (≥0.6 mag) F356W-F444W colours, which sit at the extreme end of theoretical predictions from numerical simulations. We find that these galaxies deviate (by ≈0.5 dex) from the local correlation between [O iii] λ5007/H β and [Ne iii] λ3869/[O ii], which is consistent with the predictions from simulations of high-redshift galaxies having elevated line-excitation ratios. We measure the [O iii] λ5007 rest-frame EWs both directly from the spectroscopy, and indirectly as inferred from the strong F356W-F444W colours, finding large [O iii] λ5007 EWs of 225-1740 Å. The [O iii] λ5007 and H β EWs are consistent with those seen in extreme, intensely star-forming dwarf galaxies in the local Universe. Our structural analysis indicates that these galaxies are resolved, exhibiting irregular shapes with bright clumps. In line with the predictions from the FLARES hydrodynamic simulations, such intense star formation and extreme nebular conditions are likely the norm, rather than the exception, in the EoR.</p
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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
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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
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The JWST hubble sequence: the rest-frame optical evolution of galaxy structure at 1.5< z <6.5
We present results on the morphological and structural evolution of a total of 3956 galaxies observed with JWST at 1.5 109 M ⊙ at z > 3 are not dominated by irregular and peculiar structures, either visually or quantitatively, as previously thought. We find a strong dominance of morphologically selected disk galaxies up to z = 6 in this mass range. We also find that the stellar mass and star formation rate densities are dominated by disk galaxies up to z ∼ 6, demonstrating that most stars in the Universe were likely formed in a disk galaxy. We compare our results to theory to show that the fraction of types we find is predicted by cosmological simulations, and that the Hubble Sequence was already in place as early as one billion years after the Big Bang. Additionally, we make our visual classifications public for the community.</p