We present spectroscopic properties of 22 Ly$\alpha$ emitters (LAEs) at $z = 5.5 - 6.6$ with Ly$\alpha$ luminosity $\mathrm{log}( L_{\mathrm{Ly}\alpha} \, [\mathrm{erg} \, \mathrm{s}^{-1}]) = 42.4 - 43.5 $, obtained using VLT/MUSE as part of the Middle Ages Galaxy Properties with Integral Field Spectroscopy (MAGPI) survey. Additionally, we incorporate broad-band photometric data from the Subaru Hyper Suprime-Cam (HSC) Wide layer for 17 LAEs in our sample. The HSC-y band magnitudes show that our LAEs are UV-bright, with rest-frame absolute UV magnitudes $ -19.74 \leq \mathrm{M}_{\mathrm{UV}} \leq -23.27$. We find that the Ly$\alpha$ line width increases with Ly$\alpha$ luminosity, and this trend becomes more prominent at $z \gt 6$ where Ly$\alpha$ lines become significantly broadened ($\gtrsim+260 \, \mathrm{km}\, \mathrm{s}^{-1}$) at luminosities $\mathrm{log}( L_{\mathrm{Ly}\alpha} \, [\mathrm{erg} \, \mathrm{s}^{-1}]) \gt 43 $. This broadening is consistent with previous studies, suggesting that these sources are located inside larger ionised bubbles. We observe a slightly elevated ionising photon production efficiency estimated for LAEs at $z \gt 6$, which indicates that younger galaxies could be producing more ionising photons per UV luminosity. A tentative anti-correlation between ionising photon production efficiency and Ly$\alpha$ rest-frame equivalent width is noticed, which could indicate a time delay between production and escape of ionising photon primarily due to supernovae activity. Furthermore, we find a positive correlation between radius of ionised regions and Ly$\alpha$ line width, which again suggests that large ionised bubbles are created around these LAEs, which are allowing them to self-shield from the scattering effects of the intergalactic medium (IGM). We also detect two very closely separated LAEs at $z = 6.046$ (projected spatial distance between the cores is 15.92 kpc). This is the LAE pair with the smallest separation ever discovered in the reionisation epoch. The size of their respective bubbles suggests that they likely sit inside a common large ionised region. Such a closely separated LAE pair increases the size of ionised bubble, potentially allowing a boosted transmission of Ly$\alpha$ through neutral IGM and also supports an accelerated reionisation scenario.

The baryon mass content (i.e. stellar and gas mass) of dark matter halos in the early Universe depends on both global factors – for example, ionising ultraviolet (UV) radiation background – and local factors – for example, star formation efficiency and assembly history. We use a lightweight semi-analytical model to investigate how both local and global factors impact the halo baryon mass content at redshifts of $z\geq 5$. Our model incorporates a time delay between when stars form and when they produce feedback of $0\leq t^d/\mathrm{Myr} \leq 30$, which can drive bursts of star formation, and a mass and redshift-dependent UV background, which captures the influence of cosmological reionisation on gas accretion onto halos. We use statistically representative halo assembly histories and assume that the cosmological gas accretion rate is proportional to the halo mass accretion rate. Delayed ($t^d$>0) feedback leads to oscillations in gas mass with cosmic time, behaviour that cannot be captured with instantaneous feedback ($t^d$=0). Highly efficient star formation drives stronger oscillations, while strong feedback impacts when oscillations occur; in contrast, inefficient star formation and weak feedback produce similar long-term behaviour to that observed in instantaneous feedback models. If the delayed feedback timescale is too long, a halo retains its gas reservoir but the feedback suppresses star formation. Our model predicts that lower mass systems (halo masses $m_\mathrm{h} \leq 10^7 \mathrm{M}_\odot$) at $z \leq 10$ should be strongly gas deficient ($m_\mathrm{g}\rightarrow 0$), whereas higher mass systems retain their gas reservoirs because they are sufficiently massive to continue accreting gas through cosmological reionisation. Interestingly, in higher mass halos, the median $m_\star/(m_\star+m_\mathrm{g}) \simeq 0.01-0.05$, but is a factor of 3–5 smaller when feedback is delayed. Our model does not include seed supermassive black hole feedback, which is necessary to explain massive quenched galaxies in the early Universe.

Epoch of Reionisation (EoR) data analysis requires unprecedented levels of accuracy in radio interferometer pipelines. We have developed an imaging power spectrum analysis to meet these requirements and generate robust 21 cm EoR measurements. In this work, we build a signal path framework to mathematically describe each step in the analysis, from data reduction in the Fast Holographic Deconvolution (FHD) package to power spectrum generation in the εppsilon package. In particular, we focus on the distinguishing characteristics of FHD/εppsilon: highly accurate spectral calibration, extensive data verification products, and end-to-end error propagation. We present our key data analysis products in detail to facilitate understanding of the prominent systematics in image-based power spectrum analyses. As a verification to our analysis, we also highlight a full-pipeline analysis simulation to demonstrate signal preservation and lack of signal loss. This careful treatment ensures that the FHD/εppsilon power spectrum pipeline can reduce radio interferometric data to produce credible 21 cm EoR measurements.

Here I discuss progress in both the theory and practice of data analysis for the Hydrogen Epoch of Reionization Array (HERA), focusing on techniques to calibrate the instrumental response and preserve the spectral smoothness that is essential to separating the cosmological 21 cm signal from foregrounds that are five orders of magnitude brighter. I explain how mis-calibration can create ruinous spectral structure and how we take advantage of HERA’s highly-redundant configuration for calibration. This proceeding draws from a talk I gave on October 3, 2017. Slides for it and all my talks are available at joshdillon.net.

Although the universe at redshifts greater than six represents only the first one billion years (< 10%) of cosmic time, the dense nature of the early universe led to vigorous galaxy formation and evolution activity which we are only now starting to piece together. Technological improvements have, over only the past decade, allowed large samples of galaxies at such high redshifts to be collected, providing a glimpse into the epoch of formation of the first stars and galaxies. A wide variety of observational techniques have led to the discovery of thousands of galaxy candidates at z > 6, with spectroscopically confirmed galaxies out to nearly z = 9. Using these large samples, we have begun to gain a physical insight into the processes inherent in galaxy evolution at early times. In this review, I will discuss (i) the selection techniques for finding distant galaxies, including a summary of previous and ongoing ground and space-based searches, and spectroscopic follow-up efforts, (ii) insights into galaxy evolution gleaned from measures such as the rest-frame ultraviolet luminosity function, the stellar mass function, and galaxy star-formation rates, and (iii) the effect of galaxies on their surrounding environment, including the chemical enrichment of the universe, and the reionisation of the intergalactic medium. Finally, I conclude with prospects for future observational study of the distant universe, using a bevy of new state-of-the-art facilities coming online over the next decade and beyond.