The Epoch of Reionization

​THE FIRST GALAXIES LIVED IN A VERY DIFFERENT UNIVERSE THAN WE DO NOW

Over the course of the past 100 years we have built up a fantastic understanding of how the Universe itself has changed with time. What does it mean that the Universe itself has changed? Since the Universe is dark and empty, the only way to describe it is by observing light sources and developing a clever, intuitive theoretical frame work to understand them. 

As such we have identified three large-scale (over-simplified) phases of the Universe: the Big Bang, the Dark Ages, and the Galaxy Dominated Era (see the schematic to the right). The Big Bang era was dominated by the rapid initial inflation of the Universe. This extremely early Universe contained a soup of hot ionized plasma, with denser fluctuations but without gravitationally-bound bodies (stars, galaxies, etc.).  We are currently living in the galaxy dominated era where gravity has compressed gas and dark matter into large-scale structures such as stars, galaxies, and clusters of galaxies. Gravity has molded the Universe into large-scale structures of galaxies, but the gas outside of these galaxies remains hot and ionized. Between the Big Bang and the Galaxy Dominated Era is an era that is challenging to probe because there are no stars or black holes to emit light. This is called the Dark Ages. 

The transition of each of the major periods of the Universe is marked by a phase transition. Between the Big Bang and the Dark Ages is Recombination, where the Universe cooled enough for all of the cosmic hydrogen to recombine. After recombination, all gas in the Universe was neutral until the end of the Dark Ages when the first galaxies emitted enough ionizing photons to reionize the Universe. The Epoch of Reionization marks the first time in cosmic history that baryonic matter (gas and stars) shaped the entire Universe around them. 
While a crucial inflection point in the course of cosmic history, not much is known about how the Universe became reionized. There are two competing sources for the requisite ionizing photons, accretion onto black holes (AGN) and massive stars, but observations have not been able to determine the source of cosmic reionization. The observational picture will drastically change in the next 10 years as the James Webb Space Telescope and Extremely Large Telescopes open a new window onto the early Universe.

There are drastic astrophysical differences between reionization by AGN and massive stars. For one, the ionizing spectrum produced by AGN is much "harder" (i.e. energetic) than massive stars, thus reionization by AGN produces a hotter intergalactic medium. Black holes are also thought to be much more clustered in the centers of potential wells, whereas stars are more uniformly distributed in space. This means that reionization by AGNs is patchier than stars. That being said, does it really matter what reionized the Universe? Here is a quick list why it is fundamental to our understanding of the Universe:
  1. Reionization Set the Mass-Scales of Galaxies: Ionizing the hydrogen around galaxies quenches very low-mass galaxies because the galaxies are not massive enough to retain the gas. This photo-evaporation of hydrogen creates baryon deficient and dark-matter rich dwarf galaxies. This relic is still seen in the number of (quenched) Milky Way satellites, the luminosity/mass functions of low-mass galaxies and globular clusters, and the dark matter halos shapes of dwarf galaxies. The stellar mass that is quenched by the background radiation strongly depends on its Spectral Energy Distribution. Thus, the sources of Cosmic Reionzation influences the mass functions of dwarf galaxies.
  2. Reionization Altered Structure Formation: Heating the baryons provides a new pressure support against gravitational collapse that imprints itself on the subsequent large-scale structure of the subsequent galaxies. Whether reionization was concentrated (AGN) or diffuse (massive stars) will leave different imprints onto the shape of the large-scale structure that we see in the local Universe.
  3. Reionization Completes the Galaxy Evolution Paradigm: We have built a top-down structure of galaxy formation and evolution by studying the end and middle points of galaxies, but the sources of reionization are the initial conditions of galaxy formation. It's akin to writing a medical textbook by only studying the geriatrics ward at a hospital. To complete this evolutionary picture, we need to a know what the gas and stars looked like in the first galaxies (the pediatrics wing). This is the epoch of galaxy's (and human) life where explosive growth happens on short time-scales. Importantly, accretion onto galaxies latter in the Universe depend on the halo mass of the galaxy. Thus. the temperature of the intergalactic medium and the subsequent growth of galaxies was set during the Epoch of Reionization. This process sets into motion the development of gaseous reservoirs, the formation of stars, and the production of metals. These are the crucial initial conditions of galaxy evolution. 
  4. When do the First Black Holes Form? If observations determine that stars did NOT reionize the Universe, black holes must form very efficiently and rapidly in the early Universe. This would challenge the current picture of black hole formation and would suggest new important physics.​

How can we determine the source of Cosmic Reionization?

That is the major question that I have set about studying for the next few years. It keeps me up at night. I have identified two major questions that need to be answered to determine the sources of reionization.
  1. How many ionizing photons do massive stars intrinsically produce?
  2. How many ionizing photons escape the gas within galaxies and travel into the intergalactic medium? 
Follow the buttons below to discover more about how I am trying to answer these problems.