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Early Universe Revealed by Young Galaxies

by Matt Schenker

Directly after the big bang, the universe consisted of a soup of very hot protons, electrons, and high energy photons. As the universe rapidly expanded, it began to cool as well. Some 370,000 years or so later, the temperature had dropped enough for neutral hydrogen to form, and the unverse entered a period astronomers refer to as the 'Dark Ages'. During this era, the gas and dark matter building blocks of galaxies began to slowly collapse, but had not yet grown dense enough to form stars. When the first stars and galaxies finally were able to ignite and begin producing the ultraviolet photons needed to break apart the hydrogen atoms, the Dark Ages had come to an end, and the process of reionizing the universe had begun.

Probing the epoch of reionization remains one of the most challenging and interesting problems in modern cosmology today. The transition from a largely neutral intergalactic medium (IGM) to the ionized IGM that persists today was extremely compliated and, initially, localized. When the first stars within galaxies ignited, they began to ionize the neutral hydrogen only within the area directly surrounding them. As these galaxies grew more numerous and more massive, these ionized bubbles began to expand and overlap, eventually leading to the fully ionized universe we have today. Detailed studies of this era of cosmic history can provide insight into the physical processes that shaped the precursors of the galaxies we see today.

We are able to discover these galaxies using either the Hubble Space Telescope, which excells at faint observations thanks to its privledged position above Earth's atmosphere. Since the speed of light is finite, we identify these first galaxies by searching for those that are most distant in our images. As the universe expands, the light from these galaxies is redshifted thanks to the doppler effect, so we tailor our searches to detect the reddest objects. Within the past two years, progress in these searches has rapidly progressed thanks to the installation of the new Wide Field Camera 3 on Hubble, and we now have discovered hundreds of galaxy candidates born within the first billion years of the universe.

However, to understand fully how reionization took place and study the build up of these galaxies requires more than just identifying them. A key spectral signature often seen in these young galaxies actively forming stars is strong line emission at a wavelength of 121.6 nm. This line, known as Lyman alpha, arises from the electronic transition from the n=2 to n=1 energy level of hydrogen. Since both Hydrogen and high energy, ionizing photons are plentiful in the gas clouds surrounding hot, massive, and short-lived stars, large amounts of Lyman alpha photons are emitted from these regions.

The Lyman alpha transition happens to be extremely resonant, which can be both an advantage and a hinderance for our research. Since galaxies themselves are full of neutral Hydrogen, a typical Lyman alpha photon will bounce along on a random walk between Hydrogen atoms until it is either absorbed by interstellar dust, or fortuitious enough to make its way out of the galaxy. Because of this, even a small amount of dust is capable of completely extinguishing a galaxy's Lyman alpha emission line, even if it still has the requisite hot, young stars. Work done by my advisor here at Caltech, Richard Ellis, and his former graduate students has shown that as one probes galaxies closer to the beginning of the universe, the dust content decreases on average, and Lyman alpha emission is more readily observed.

My research with Professor Ellis has focused on extending these observations to galaxies closer to the beginning of the the universe than ever before, using the Keck telescopes in Hawaii. Contrary to expectations, we found that as one probes galaxies that exsited less than about 800 million years after the beginning of the universe, the observation rate of Lyman alpha emission begins to decrease. Although this could be due to other factors, the most likely explanation is that Lyman alpha is being resonantly scattered by an IGM that is becoming increasingly neutral at these young ages of the universe.

These results are very exciting - we have probed far enough back in cosmic time to some of the earliest galaxies, as well as witnessed some of the first evidence of a dramatic shift in the state of the universe. Already, two independent groups have released work which also provides evidence for a more neutral IGM at this point in cosmic history. Within the year, a new multi-object spectrograph, MOSFIRE, is scheduled to come online at one of the Keck telescopes, which will help to dramatically improve our efficiency in surveying these galaxies. With this, we have excellent prospects for continuing our research of this important transition period of the universe.