The history of observational cosmology is one of studying ever-receding horizons, with scientists striving to look ever deeper into the most distant parts of the observable Universe. With all the advances being made in powerful new instruments and novel observing techniques in recent years, we’re now used to astronomers announcing new discoveries on a constant basis regarding the finding of the biggest cosmic object, the more massive, or the most distant ever, which have yielded many important insights into the workings of the early Universe. The latest such example comes from a couple of new studies that have utilised the power of NASA’s Hubble and Spitzer space telescopes as well as that of ground-based observatories, allowing astronomers to detect the telltale signs of the earliest galaxies in the Universe that were created just a few hundred million years after the Big Bang itself.
According to the leading model of cosmic evolution, the Universe has gone through several different phases in the aftermath of its creation. Following its creation, the Universe was filled with ionised hydrogen and helium which scattered light, blocking it from travelling freely and making the Cosmos opaque and unobservable. When the overall temperature of the Universe had fallen at a sufficiently low level, approximately 380,000 years after the Big Bang, it cooled enough to allow all the ionised gas to form neutral hydrogen atoms. This freed up all the photons that were previously scattered off by free electrons eventually making the Universe transparent and allowing matter to begin condensing into eventually forming the first generation of stars. The energetic ultraviolet radiation from these primordial light sources eventually re-ionised all the neutral hydrogen in the intergalactic medium during the so-called “Re-ionisation era,” which took place between 500 million to 1 billion years after the Big Bang, and signified the point during which the first galaxies in the Universe began to form.
The detailed study and characterisation of this early epoch in the history of the Universe presents one of the greatest challenges for cosmology today, due to the fact that these primordial galaxies lie at the edges of the observable Universe at distances that can exceed 13 billion light-years. Furthermore, and because of their great distance, these cosmic objects are extremely faint, making their observation with even the biggest ground-based telescopes an overwhelming task. Nevertheless, several observing campaigns in recent years utilising some of the latest state-of-the art instruments in ground- and space-based observatories have managed to push the envelope and detect cosmic structures at distances greater than 12 billion light-years away, allowing astronomers to reveal the very young Universe at a time when it was less than 10 percent its present age.
One method with which astronomers have managed to meet this goal is through the study of the diffuse extragalactic background light, or EBL. The latter is the total light emitted by all the stars and galaxies in the Universe since its creation 13.8 billion light-years ago and covers almost the entire range of wavelengths of the electromagnetic spectrum, from gamma rays to the far-infrared. But since the EBL is the total sum of light that has been emitted by every galaxy in the Universe, near and distant, how can astronomers differentiate between these objects that are relatively close to us from the ones that are located at cosmological distances at the very edges of the observable Universe? To that end, astronomers take advantage of the effects that cosmic expansion has on light. Similar to the way that the sound waves from a moving vehicle with a siren are compressed when the latter is moving closer and are stretched as it moves away, the wavelengths of light from distant cosmic objects to move toward the red end of the spectrum as they move away from the Earth at ever-greater speeds due to the Universe’s expansion. This red shifting of light means that the ultraviolet radiation that has been emitted by the first primordial galaxies in the Universe has been stretched to such longer wavelengths by the time it has reached Earth that it can now been observed in the infrared part of the spectrum. Thus, the study of the EBL in infrared wavelengths could provide important insights to the many unknowns that still remain regarding the formation of galaxies in the primordial Universe.
Such observations were the subject of a new study that was recently published at the Nature Communications journal and was undertaken by a team of U.S. astronomers led by Ketron Mitchell-Wynne, from the University of California, Irvine. The researchers analysed archival data gathered with the Hubble Space Telescope as part of the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey, or CANDELS, whose goal has been to document very high-redshift objects in the search for the Universe’s first cosmic structures. For the purposes of their study, the researchers measured the spatial fluctuations of the EBL from several deep-sky field imaging data that had been taken with the Hubble and Spitzer space telescopes, in five different wavelengths between 0.6 and 1.6 mm covering part of the optical and near-infrared part of the spectrum. Previous similar measurements of high-redshift objects at cosmological distances, with various other observational campaigns like the Cosmic Infrared Background Experiment, or CIBER, had been able to confirm the existence of intra-halo light, which is the light emitted by individual stars that have been kicked out of their host galaxies into the intergalactic medium, as a result of violent galaxy collisions. Yet Mitchell-Wynne’s team managed to observe the fluctuations of the extragalactic background light itself and in a broader range of wavelengths than what has been achieved by previous surveys like CANDELS. One important aspect of the observations by Mitchell-Wynne’s team was the fact that the fluctuations in the extragalactic background light were evident in near-infrared wavelengths but not in the visible part of the spectrum, which strongly indicated that their source were indeed very distant cosmic objects many billions of light-years away, and not just contamination from relatively nearby ones.
“CANDELS was not initiated for this cause, but it turns out that the way the data were taken was favorable for what we wanted to do,” explains Mitchell-Wynne. “From the CIBER analysis, we knew there would be a detection of intra-halo light in the infrared bands. We didn’t really know what to expect in the optical ones. With Hubble data, we saw a large drop in the amplitude of the signal between the two. With that spectra, we started to get a little more confident that we were seeing the earliest galaxies.”
In order to bring out these measurements from the Hubble data, the researchers undertook a rigorous statistical analysis so that they could further eliminate any contamination from other local light sources like faint nearby dwarf galaxies that would also appear in optical wavelengths. The results of their study showed that the extragalactic background light was coming from objects that were more than 13 billion light-years away, at a time when the Universe was less than 1 billion years old. “For this research, we had to look closely at what we call ‘empty pixels,’ the pixels between galaxies and stars,” says Dr. Asantha Cooray, a professor of astrophysics at the University of California, Irvine and lead member of Mitchell-Wynne’s team. “We can separate noise from the faint signal associated with first galaxies by looking at the variations in the intensity from one pixel to another. We pick out a statistical sig nal that says there is a population of faint objects. We do not see that signal in the optical [wavelengths], only in infrared. This is confirmation that the signal is from early times in the Universe.”
A similar key contribution to the understanding of galaxy formation and evolution in the early Universe was made by an independent team of astronomers, who used a different method of studying the light coming from primordial objects at such cosmological distances. The researchers, who were led by Dr. Adi Zitrin, a professor of astronomy and astrophysics at the University College, London, focused on EGS8p7, a distant candidate proto-galaxy that had been spotted earlier this year by the Hubble Space Telescope through the effects of gravitational lensing, as the light from EGS8p7 was bended and magnified by the mass of closer foreground object. Using the state-of-the-art Multi-Object Spectrometer for Infra-Red Exploration camera, or MOSFIRE, on the 10-m Keck I telescope in Hawaii, Zitrin’s team conducted a spectrographic analysis of EGS8p7 in order to determine its distance. The results of their study, which were published in The Astrophysical Journal Letters, confirmed that the galaxy was located at a distance of more than 13.2 billion light-years away, making it the more distant to have been detected to date and indicating that it should be no more than 600 million years old, at a time prior to the Universe’s Re-ionisation Era.
Yet, more importantly, the study by Zitrin’s team revealed that the spectrum of EGS8p7 exhibited a surprisingly strong Lyman-alpha emission line. Lyman-alpha light is the light that is emitted by ionised hydrogen when excited by a luminous, outside source. Inside galaxies, Lyman-alpha emission is a strong indicator of intense star formation, since it is the ultraviolet radiation from newly formed stars that excite the hydrogen gas in their surrounding interstellar medium. In the case of EGS8p7, however, the Lyman-alpha emission line shouldn’t be there at all. Since it was established that the galaxy had already been formed prior to the beginning of the cosmic re-ionisation era in the early Universe, that meant that it had formed in a cosmic environment that was dominated by the presence of neutral hydrogen, which absorbs Lyman-alpha light. “The surprising aspect about the present discovery is that we have detected this Lyman-alpha line in an apparently faint galaxy at a redshift of 8.68 [more than 13.2 billion light-years], corresponding to a time when the Universe should be full of absorbing hydrogen clouds,” says Dr. Richard Ellis, a professor of astronomy at the California Institute of Technology, in Pasadena and member of Zitrin’s team.
The discovery of Lyman-alpha from EGS8p7 is coming at odds with established cosmological models, which posit that such emission only became possible after the first stars and galaxies had re-ionised the neutral hydrogen gas in the intergalactic medium more than 500 million years after the Big Bang. “If you look at the galaxies in the early Universe, there is a lot of neutral hydrogen that is not transparent to this emission,” says Zitrin. “We expect that most of the radiation from this galaxy would be absorbed by the hydrogen in the intervening space. Yet still we see Lyman-alpha from this galaxy.”
Even though the reasons for this discrepancy between theory and observation is currently not well understood, the researchers speculate that it may have to do with the way cosmic re-ionisation evolved with time. Instead of happening all at once throughout all of space, it may have been more of a gradual, “patchy” process where certain parts of the Universe were re-ionised prior to others. “Evidence from several observations indicate that the re-ionization process probably is patchy,” says Zitrin. “Some objects are so bright that they form a bubble of ionized hydrogen. But the process is not coherent in all directions.”
Whatever the case, more definitive conclusions could be drawn only with the help of more detailed observations than what are available today even from the powerful Hubble Space Telescope. The next generation of space-based observatories, like the James Webb Space Telescope, which is scheduled for launch in 2018, should be able to probe even deeper into the Universe’s past and hopefully provide some answers to many of cosmology’s enduring mysteries. “This is a very exciting finding,” says Henry Ferguson, an astronomer at the Space Telescope Science Institute in Baltimore, commenting on the findings by Mitchell-Wynne, of the extragalactic background light from primordial galaxies in the early Universe. “It’s the first time that we’ve been able to convincingly measure this subtle signature of early galaxies with Hubble, giving us a firmer handle on what to look for when the James Webb Space Telescope launches a few years from now.”
Similar to the way the 1920s proved to be a revolutionary decade for cosmology and astrophysics, the 2020s also promise to be no less spectacular.