Searching for Cosmic Dawn
By Dr. Richard Ellis
Figure 1: Timeline of cosmic history (reproduced from a 2001 press release by Dr. George Djorgovski). The age of the universe is now known to greater precision at 13.7 billion years.
A giant telescope is like a time machine: we can use it to peer to great distances into the cosmos and, because of the finite velocity of light, we can see directly into the past.
Soon after the Big Bang, as the universe cooled, hydrogen atoms formed for the first time in deep space. These hydrogen gas clouds clumped together over a period of several hundred million years, causing the universe to appear opaque, a period we call the “Dark Ages” because no stars had yet formed.
Galaxies like the Milky Way are composed of stars, gas, and copious amounts of dark matter. They evolve in the sense that the dominant dark matter brings the material together under the attractive force of gravity and the gas clouds collapse and form new stars. Stars undergo their own evolution, burning hydrogen into helium and eventually fading or exploding as supernovae - ejecting material into interstellar space.
So the hydrogen clouds present during the Dark Ages must have collapsed under gravity, and the first stellar systems began to shine. Eventually the radiant energy from these primordial galaxies split apart the hydrogen atoms in space, making the universe transparent. This must have been a dramatic event in cosmic history. I believe we may witness it very soon with the Keck telescopes.
The motivation for my work is much more than simply finding the most distant galaxies. Fundamentally I want to verify that large galaxies were built up from many smaller systems and to understand at what point in cosmic history the first stellar systems “switched on.”
When I arrived at Caltech in 1999 I set out to explore the depths of the universe with the Keck Observatory telescopes. At that time the most distant galaxies known were being viewed about two or three billion years after the Big Bang. The universe is about 13.7 billion years today, so this corresponds to a time when the universe was about 20% of its present age. The most distant and earliest galaxies known in 1999 were shown to be more energetic and irregular than present-day spirals and ellipticals - suggesting that they were being observed during a formative period.
Figure 2: Illustration of the principle by which my team discovered, in 2004, the most distant known source at that time. The foreground cluster of galaxies acts as a lens and magnifies the signal from a distant galaxy. The lens is sufficiently powerful that it can refocus the light from far away, producing two or more images of the same background source seen in different places on the sky (inset).
Theoretical studies indicated that the early galaxies we observed in 1999 might be irregular because they were being viewed in the process of assembling from smaller sub-units. A present-day large galaxy, like our own, might thus have been built up from many smaller systems. Probing further back to even earlier systems would be difficult, we thought, not just because the galaxies were further away, but also because they would be smaller. On the other hand, they should be much more numerous as they had yet to merge together.
Recognizing this, my colleagues and I developed a new method to magnify the faint signals from these distant objects. Light rays from the distant universe can be bent and focused by foreground material and, under certain circumstances, this phenomenon can act like an enormous and very powerful cosmic “magnifying glass” to boost the signal from the distant past by a factor of 30-50 times. By pointing the 10-meter Keck telescope through such a cosmic lens, which typically consists of a dense cluster of nearby galaxies and its associated dark matter, we are in effect using a 60-meter telescope! Initially, we found ten such gravitational lenses, calibrated their properties carefully, and began to use the Keck telescopes to survey the highly-magnified regions that we could see through these natural lenses.
We had an early success in 2001, when we detected a very feeble source through a cosmic lens: it was seen when the universe was about a billion years old. Although not the most distant object known at that time, the galaxy we detected, boosted by a factor of 25 or so by the lens, was far less massive than any other observed galaxy at such a great distance. We had, effectively, demonstrated the method. By 2004, we had found nine additional distant sources, including a remarkable galaxy seen when the universe was only 800 million years old (Figure 3). When it was discovered, this was the most distant known source.
Figure 3: Larger view of the Target 1 object in Figure 2, at redshift 7. In 2004 this was the most distant object yet found. Now we have found the fainter sources shown in Figure 4.
Over the past two years, a Caltech student Dan Stark and I have been surveying a number of cosmic lenses for even more distant sources using an infrared spectrograph on the Keck II telescope. Focusing on the highly-magnified regions in nine of the foreground lenses, we have now located a new set of six feeble sources (see examples in Figure 2) which appear to lie much further back in time, corresponding to a time when the universe was only 500 million years old. These sources come from a period when the universe was only 3-4% of its current age. We are now in the process of verifying that each of these six sources truly emanates from a great distance away and is not a faint galaxy in the foreground we are misinterpreting.
It is conceivable that the lines we are detecting could come from very faint galaxies that are not so far away. The technique we use to locate these distant galaxies relies on evaluating an intense hydrogen emission line. In the laboratory this line is emitted in the far ultraviolet range, but the expansion of the universe over time has shifted the line into the infrared region. So we’ve developed a careful screening process to eliminate other possible explanations. This process is now about half-way complete; fortunately, all of the sources we have checked so far really are at great distances from Earth.
Figure 4: The detection signature of the most distant sources yet seen. Very faint infrared emission lines (black dots surrounded by white marker circles) have been detected by scanning the highly magnified regions of foreground lenses with the infrared spectrograph on Keck II. For the two images shown, if the line arises from hydrogen as we expect, the redshifts would be 8.60 and 10.1, corresponding to periods 590 and 470 million years after the Big Bang.
We are only scanning a tiny area of sky using our cosmic lens technique. Locating even a handful of distant sources with our survey is very exciting, and suggests that the density of early objects like the ones we are finding must be very high throughout the universe. Such a density would be sufficient to contribute a significant fraction of the energy that was necessary to break apart the hydrogen atoms in deep space to end the Dark Ages.
So have we found the earliest sources? Are they responsible for ending the Dark Ages? We believe that at least two of the sources truly are at these great distances. We are excited to be so close to answering the grand questions about the origins of our universe.
However, professional astronomers are wise to be cautious. The astronomical literature is full of false claims. Other astronomers before us have claimed to have found objects at such great distances, but they subsequently retracted their claims as either spurious detections or foreground contaminants. Every possible alternative explanation for our faint sources needs to be scrutinized, and more observations are planned to complete this task. We are pushing Keck Observatory to its limits in undertaking this work. The progress we are making is exciting and I am confident our goal is within reach.
Enjoy a podcast of Richard Ellis describing his team’s exciting hunt for galaxies in the early universe. Recorded at a public lecture at Keck Observatory on April 20, 2006.