Cosmic Reionization: Galaxy-IGM Physics in the Early Universe

The dramatic images of galaxies taken with the Hubble Space Telescope have become familiar symbols of what it "out there" beyond our own Milky Way. What the pictures do not show, however, is that galaxies are only a small part of what fills the Universe. Stretching between galaxies is a vast and invisible network, a "cosmic web" that contains most of the matter in the Universe. This network formed as gravity amplified tiny ripples in the matter created at the Big Bang, and it provides the raw material out of which galaxies form.

Galaxies have now been observed back to within one billion years after the Big Bang, when the Universe was less than 7% of its current age. But how and when did the very first galaxies form? Because of the vast distances, most galaxies from this early time are too faint to be observed directly. To learn about the first galaxies, therefore, we must study them through their impact on the cosmic web. We know, for example, that the gas in deep space is highly ionized -- electrons have been stripped from their atoms. We believe that the energy to ionize the gas came from the ultraviolet light emitted by the first stars and galaxies. If we can determine when the gas became ionized, therefore, we will learn when the first galaxies formed.

The goal of this proposal is to study the cosmic web far back in time in order to learn about the first galaxies and stars. To do this, I will analyze the way in which the web absorbs light from very luminous objects known as quasars. The gas between a quasar and the Earth absorbs portions of the quasar light in patterns that reveal where the gas is located, its chemical composition, temperature, and ionization state. These quantities, in turn, reflect how the gas has been affected by the galaxies embedded within it.

I will first determine when the gas in deep space was ionized by measuring how smoothly it is distributed. As the gas was ionized it would also have been heated to more than 10,000 degrees. The resulting pressure would have caused the gas to expand, smoothing out the smallest bumps in the web. Earlier heating would have produced more smoothing, and so by measuring the smoothness I will be able to determine when the ionization and heating occurred.

I will then search for signs of elements made by stars, such as carbon and oxygen. The quantity of these elements in the early Universe will reflect how vigorously stars formed in the first galaxies, and the relative mix of different elements will reveal the nature of the stars themselves.

Next, I will determine how efficiently early galaxies produced ionizing photons. By measuring how completely galaxies ionized the gas in the web from one to three billion years after the Big Bang I will will determine how many ultraviolet photons were emitted by all galaxies during this period. A key question will be whether galaxies produced UV photons more efficiently at earlier times, as required if galaxies did indeed drive reionization.

Finally, I will investigate the nature of dark matter by analyzing the small-scale structure of the web. This project will determine how "warm" dark matter is, and whether rapidly moving dark matter particles may have inhibited the formation of low-mass galaxies.

The emergence of the first galaxies from the cosmic web was a key event in the process that gave rise to galaxies such as our own. Studying the early Universe, therefore, allows us investigate our origins on a grand scale. This field combines cutting-edge technology -- large telescopes and advanced computing -- with basic physics to assemble a picture of the cosmos at the most distant frontier. Ultimately, this research will help us to understand how the early Universe took shape, as well as to explore more of what is "out there."