Coming Out of the Dark Ages

Until about 400,000 years after the Big Bang,  the Universe was mostly full of electrons and protons, zipping in random directions. It was only when the Universe cooled down enough, because of expansion, that electrons and protons had a chance to combine to form neutral hydrogen (the lightest element in the Universe) for the first time. This epoch is known as the epoch of recombination. The Universe then enters and remains in what we call the Dark Ages until the formation of the first luminous sources -- first stars, first galaxies, quasars, and so on. During this period, the Universe was full of neutral hydrogen, and thus completely opaque to any ultra-violet (UV) radiation because neutral hydrogen is very efficient at absorbing UV radiation. Intense UV ionizing photons from the first stars and first galaxies then start to ionize their surrounding, forming ionized bubbles. These bubbles grow with time, and eventually the entire Universe was filled with ionized bubbles. The epoch during which this change of phase or transition occurred i.e., the ionization of most of the neutral hydrogen to ionized hydrogen -- is called the epoch of reionization (see Figure below). This was the last major transition in the history of the Universe, and had a significant impact on the large scale structure of the Universe. Therefore, this is one of the frontier research areas in modern observational cosmology.

Time line history of the Universe from Big Bang (left) to the present day Universe (right). Before the process of reionization, the Universe was completely filled with neutral hydrogen. It is only after the formation of first sources including first stars, first galaxies, that the neutral hydrogen in the Universe started ionizing, and by about one billion years after the Big Bang, most of the neutral hydrogen in the Universe was vaporized marking the end of the epoch of reionization (Image credit: NASA, ESA, A. Fields (STScI).

Probing the Epoch of Reionization

One of the most powerful and practical tools to probe the epoch of reionization is the Lyman-alpha emission test. Lyman-alpha photons are a n=2 to n=1 transition in neutral hydrogen which emits a photon with a wavelength of lambda=1215.67 Angstroms. In the presence of neutral hydrogen, Lyman-alpha photons are scattered again and again and eventually many of the Lyman-alpha photons are  scattered away form our line of sight . As a result, we expect to see fewer and fewer galaxies with Lyman-alpha emission as we probe higher and higher redshifts (closer to the Big Bang).

To study the epoch of reionization, we did exactly this using a large sample of very distant (high-redshift) galaxy candidates selected from the Hubble Space Telescope (HST) CANDELS survey -- the largest galaxy survey ever undertaken using  HST.  To know the exact distance of a galaxy, it is critical to obtain spectroscopic observations of these galaxies. We did this using a near-infrared spectrograph, MOSFIRE, on the Keck Telescope located at 13,000 ft on top of Mauna Kea, a dormant-volcano mountain in Hawaii.

To our surprise, we discovered that most of the galaxies we observed did not show Lyman-alpha emission. The figure below shows our results combined with previous studies. This figure shows the Lyman-alpha equivalent width, the ratio of strength of Lyman-alpha emission from a galaxy to its underlying blue stellar light continuum (non Lyman-alpha light), as a function of redshift (or age of the Universe on the top axis), as we probe closer and closer to the Big Bang. As can be seen, there are fewer galaxies,  and at the same time the strength of Lyman-alpha emission also decreases as we go to higher redshifts. While this can be a result of a few different things, upon careful inspection, we think that this is likely because of the Universe becoming more neutral as we go beyond redshift ~7, and we are witnessing the epoch of reionization in-progress.

This Figure shows the evolution of strength of Lyman-alpha emission in galaxies, as we get closer and closer to the Big Bang. As can be seen, the strength of Lyman-alpha emission appears to be decreasing or in other words we are missing vetry strong Lyman-alpha emitting galaxies as we go towards higher redshifts. This is likely a consequence of increasing neutral hydrogen, as expected from theoretical studies (Image credit: Tilvi et al 2014).

Currently, Lyman-alpha emission provides the best tool to discover and confirm very distant galaxies. While there are a few other emission lines that could be used to confirm distance to a galaxy, their strengths compared to the Lyman-alpha emission is much weaker.  Despite this, we have made quite a significant progress in understanding the first billion years of the Universe.

The figure below shows the summary of progress astronomers have made over the past few years, understanding the transition of Universe from a completely neutral to an ionized phase. Below redshift of about 6, that is about 1 billion years after the Big Bang, the Universe is almost completely full of ionized hydrogen—only one part in 10,000 is neutral. At redshifts greater than 6, the Universe becomes more and more neutral. The James Webb Space Telescope (JWST) will be very instrumental in discovering galaxies within the first 600 Myrs, and will help us gain even more insight into the details of the crucial epoch.

This figure shows the evolution of neutral hydrogen fraction as a function of redshift (or age of the Universe shown on top axis). Only one part in 10,000 is neutral below redshift of about 6 which implies that the Universe is mostly ionized and the process of reionization has occurred at redshifts greater than six, where the Universe is becoming increasingly neutral (Image credit: V. Tilvi).

In Search of the First Galaxies

The quest for finding the first galaxies is motivated by at least two main goals: 1) these galaxies are likely the building blocks for present-day galaxies, and 2) they are responsible for making the Universe transparent to light, a period commonly referred to as the reionization epoch. This epoch is one of the most important periods in the history of the universe  because it was when most of the neutral hydrogen in the Universe was evaporated. This event dramatically changed the  Universe forever. Before diving into the method of searching first galaxies, here is a brief summary of what the  Universe was like during the early stage of its lifetime.

 Color composite image of the three  newly discovered galaxy candidates at redshift ~7 (at this redshift the age of the universe is merely 800 million years). These galaxies were selected using a medium-band imaging survey called the Fourstar Galaxy Evolution Survey (zFOURGE). For more details about this discovery, please see Tilvi et al 2013. 

     
Immediately after the Big Bang the Universe was too hot and chaotic for electromagnetic radiations to escape. It was only after about 400,000 years that the Universe expanded and cooled enough (to about 3000 Kelvin) for free electrons to be able to combine with protons to form neutral hydrogen. Due to this attraction of electrons to protons, enough empty was created for light to escape and travel to large distances (and eventually reaching us) without bouncing back and forth, for the first time. This is what we see today as the microwave background radiation (light emitted in the microwave part of the electromagnetic spectrum). Even by this time there were no stars and no galaxies in the Universe. The Universe went into the "Dark ages" during which it was full of neutral hydrogen and we would have to wait until about 100-500 Myrs after the Big Bang for the first stars and first galaxies to form. 

The intense radiation from the first stars and galaxies likely ionized the neutral hydrogen and the Universe changed from an opaque to a transparent Universe. This important milestone is referred to as the "reionization epoch" which most likely occurred between 100 - 1000 Myrs after the Big Bang. Searching for the first galaxies and understanding how and exactly when did this dense fog of neutral hydrogen evaporate are among the frontiers of modern observational cosmology.

Lyman-break (dropout) selection method: Due to the presence of
neutral hydrogen in the intergalactic-medium, flux shortward (blueward)
of the Lyman-break gets attenuated cause a sharp flux-drop. This
break shifts towards longer wavelengths for more and more 
distant galaxies. For redshift~7 galaxies, the flux drops in the z' 
band while this object is detected in longer wavelength bands
(Tilvi et al 2013).

In order to study the early universe, there are  a few different probes that can be used -- one of them is star-forming galaxies. These galaxies provide a unique probe of the reionization epoch because such galaxies can be observed to very large distances or at very high redshifts -- redshift being a proxy to distance. Astronomers now have some observational evidence that the reionization of the universe occurred somewhere at redshift z > 6 at which the age of the universe was less than one billion years. Identifying galaxies at such large distances (or high redshifts) has been only possible recently due to the advent of near-infrared detectors that are efficient in detecting photons that are highly redshifted due to their extremely large distances in the expanding universe. Taking advantage of redshifts and some other galaxy properties, astronomers have devised a clever way of identifying high-redshift galaxies -- in an expanding Universe, the farther a galaxy is, the more its light shifted in wavelength (towards redder or longer wavelengths).

One of the popular methods of identifying extremely distant galaxy candidates is to use a technique called Lyman-break selection method  in which galaxies are selected based on strong absorption of galaxy's blue light (light at wavelengths shortward of Lyman-break wavelength) caused due to the presence of neutral hydrogen in the vicinity of the galaxy. This break gets shifted towards longer wavelengths for more distant galaxies. The figure to the right illustrates this technique --- a combination of broad-band filters (middle panel) used to identify z~7 galaxy candidates. The dropout candidate galaxy (top panel) drops out of the z' filter and is visible in all filters that are redward of about 9000Angstrom. The bottom panel shows a filter set consisting of medium-band filters that provide a better spectral resolution compared with broad-band only filter sets.

Contaminants

T-dwarf contamination: A T-dwarf template spectrum is overlaid on 
the medium-band photometry of a candidateT-dwarf. As can be seen,
 the medium-band photometry nicely traces the absorption features 
of T-dwarfs thereby helping to minimize the contamination of high-redshift
galaxy candidates by dwarfs in the Milky Way (Tilvi et al 2013). 

While the dropout method described above has been used quite extensively to identify galaxy candidates at relatively large distances, there are other objects (e.g., brown dwarfs within the Milky Way) that can mimic the Lyman-break drop. While such brown dwarfs can normally be identified based on their compact sizes using the excellent resolution of the Wide Field Camera on the Hubble Space Telescope, using only object sizes to distinguish stars can eliminate some genuine galaxy candidates because high redshift galaxies tend to be compact. Fortunately, medium-band filters (filters that allow photons with a smaller wavelength coverage compared with the broad-band filters) provide a great way to identify features that are associated with brown dwarfs. The figure below shows an example of a T-dwarf that mimics a dropout galaxy candidate. As can be seen the absorption features (or drop in the fluxes) are nicely traced by the medium-band filters.

Artist's conception of a Tdwarf (Credit: V. Tilvi).

Signatures of the First Galaxies

Defining a 'First Galaxy' is somewhat nebulous. Compared to our own Milky Way Galaxy, we expect the first galaxies to be small, less massive, lacking large quantities of heavier elements (often called metals in astronomy), and nearly dust free because these galaxies haven't had enough time to make all this stuff. But, whether the mass of the first galaxies could be comparable to the star-cluster masses and when we start differentiating between star-clusters and first galaxies is unclear. Some theoretical models suggest that the metal content of first galaxies could be anywhere between 0.000001 -0.0001 times that of the Sun, and that first galaxies might get 'polluted' within few tens of million years due to the supernovae explosions from massive stars, thus making it not dust-free. A better way to answer this question is to first find a 'first galaxy,' which should be very different from a normal star-forming galaxy (e.g. our Milkyway)  and then we will better know what the first galaxies look like.

Currently there are several candidate galaxies beyond redshift ~7 which are possibly among the first galaxies to have formed in the Universe. One of the major hurdles for finding these candidates and then confirming their real distances using spectroscopy is their faintness due to extremely large distances. Fortunately, newly installed near-infrared camera WFC3 on the Hubble Space Telescope has allowed astronomers to find candidate galaxies even up to a redshift of about 12 (at this redshift the Universe is merely 380 Myrs old). Confirming such candidates however, requires extremely deep spectroscopy (a method to measure accurate distances) which is currently not feasible from the ground-based observatories. We will likely have to wait until James Webb Space Telescope to be launched within the next few years.