Using Galaxy Clusters as Cosmic Telescopes to Probe the Epoch of Reionization

Using Galaxy Clusters as Cosmic Telescopes to Probe the Epoch of Reionization
By:Austin Thomas Hoag
Published on 2018 by

The Epoch of Reionization (hereafter |cosmic reionization| or simply |reionization|) is one of the most significant remaining puzzles in cosmic history. Occurring less than one billion years after the Big Bang, reionization likely took place shortly after the formation of the first baryonic structures such as stars, galaxies and active galactic nuclei (AGN). Reionization is therefore intimately linked to several core astronomical fields such as: stellar formation and evolution, galaxy formation and evolution, large scale structure, and the cosmic microwave background, to name a few. In the last decade, significant investments have been made to observe galaxies during reionization, for example in the optical and near infrared with the Hubble Ultra-Deep Field, the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey, and the Hubble Frontier Fields. Experiments at radio frequencies have recently been undertaken to detect the global 21 cm signal expected from reionization, for example the Experiment to Detect the Global Epoch of reionization Signature, as well as to measure the power spectrum of the 21 cm radiation, such as the Murchison Wide Field Array and the Low-Frequency Array. While reionization has been a major focus in astronomy for over a decade, many key properties of the process are still uncertain. The main difficulty in constraining reionization is the faintness of the sources likely driving it, i.e. the first galaxies and AGN. Even with the most powerful current telescopes such as Keck and the Hubble Space Telescope (HST), observing all but the brightest of these sources is too expensive. Using a galaxy cluster as a gravitational lens, this observational challenge can be somewhat eased. In order to take advantage of cluster lensing to study faint galaxies, a gravitational lens model is required. In this dissertation, I present gravitational lens models I developed for 11 galaxy clusters in three large cluster surveys undertaken with the Hubble Space Telescope and the Spitzer Space Telescope. I selected redshift (z) 7 to 8 galaxies behind all of these clusters and followed them up with the Multi-Object Spectrometer For InfraRed Exploration (MOSFIRE) on the Keck I telescope. During this spectroscopic campaign I discovered a Lyman-alpha emitting galaxy at z=7.64 magnified by a factor of approximately 10. Its intrinsic luminosity is an order of magnitude fainter than the handful of other known Lyman-alpha emitters (LAEs) at z\u003e7.5, all of which are bright, rare sources. While exhibiting weaker Lyman-alpha than the UV-brighter LAEs at z\u003e7.5, the underlying mechanism which allows Lyman-alpha to escape all of these galaxies may be the same. I also obtained the tightest spectroscopic constraints on the redshift of one of the highest redshift gravitationally lensed galaxies (z~9) using ultra-deep grism spectroscopy from the Hubble Space Telescope. Using a sub-sample consisting of 8 out of 11 of these clusters for which the photometric analysis is complete, I constrained the fraction of Lyman-break galaxies showing Lyman-alpha, often just called the |Lyman-alpha fraction test,| at z=8, making the first definitive measurement of a declining Lyman-alpha fraction (and hence increasing neutral hydrogen fraction) over the interval z=7-8 for UV-fainter galaxies. This is the first hint that faint galaxies suffer a similarly large decline in Lyman-alpha fraction as bright galaxies over this interval, suggesting that the overlap phase of reionization may still be underway at z=8.

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