Image Courtesy of Hannah Barsouk.
You gaze up at the sky on a clear night. The stars, too numerous to count, appear as nothing more than little specks. Together, however, they keep the universe lit up like fireflies in the dark. For astronomers, these stars and galaxies serve as lampposts throughout the universe’s history and evolution. But about thirteen billion years in the past, there was a “dark age” without visible galaxies, stars, or any kind of light, which has puzzled astronomers. Scientists aren’t sure how reionization—the epoch of high-energy radiation that ended this cosmic dark age—occurred. Recently however, two astronomers, professors James Rhoads and Sangeeta Malhotra of NASA’s Goddard Space Flight Center, presented findings that could advance our understanding of this dark era.
Before we can talk about reionization, however, we must take a few steps back to the Big Bang, when the universe began, then stretched and continued to grow as large as it is right now. Immediately following the Big Bang was an era of pure brightness: a hot soup of electrons, quarks, and photons. When the universe expanded enough for this soup to cool down, hydrogen atoms formed from the protons and electrons in an era known as the recombination epoch. This thick, dense fog of neutral hydrogen continuously absorbed light within it, thus ushering in the universe’s dark age.
Then, something amazing happened. “One might naively expect for the neutral hydrogen to just sit there, but sometime in the late 60s [we discovered] that the gas between galaxies is ionized today, and has been for at least the last 10 billion years,” Rhoads said. The neutral hydrogen did not stay neutral forever: it reionized. The question is how, when, and by what?
Cosmic objects—such as galaxies, stars, or clouds—send out a spectrum of wavelengths, from infrared light, to visible light that brightens up the night sky, to ionizing ultraviolet and X-rays. These spectra are largely determined by the objects’ chemical compositions and their corresponding emission lines. As such, these spectra are a “signature” of these cosmic objects, allowing them to be classified based on their light’s properties. Add a bit of redshift—wherein these photons’ wavelengths are stretched by the source’s speed away from us or by the universe’s expansion—and you also get information about the object’s age. In particular, because of the universe’s expansion, the further away an object is from us, the faster it seems to be moving away from us. The faster it appears to be moving away from us, the redder its spectrum. Because light takes time to travel, redshifted galaxies must be older.
To figure out what kind of galaxies drove the harsh radiation of reionization, we must find two qualities of a galaxy: the redshift and the spectrum. The problem? These galaxies are faint—extremely faint. It wasn’t until recently, with the launch of the James Webb Space Telescope (JWST), that astronomers were finally able to see these faint high-redshift galaxies at a higher resolution.
But there’s one more elephant in the room: what drove the sudden reionization? Neutral hydrogen gas left alone in a tank cannot suddenly ionize; there must be some driver for the process to occur. Now, armed with a powerful enough telescope, astronomers could finally answer their question about reionization. Cue Rhoads and Malhotra, who noticed something peculiar about the JWST data they were analyzing.
Six months after its launch, JWST sent back images that contained the answer to this puzzle. Focused on a galaxy cluster named SMACS 0723, Rhoads and Malhotra noticed that three of the galaxies closely resembled some local galaxies—galaxies that were billions of years separated from the trio. These were the Green Pea galaxies, aptly named for their greenish hue and minuscule size. They were found by citizen scientists working on Galaxy Zoo, and presented first in a paper led by astronomer Carolin Cardamone.“We [saw] that they were small, their galaxy population was young [at their redshift]… what you see in these galaxies is that their spectra are dominated by these huge [emission] lines,” Malhotra said.
Malhotra and Rhoads found that these emission lines were from glowing gas created by very young energetic stars. Galaxies with very young stars produce harsh ultraviolet radiation. These properties are unexpected for local galaxies, but it made sense that these galaxies would appear at the epoch of reionization. “We’d have expected that these Green Peas were analogs of these high-redshift galaxies, but we hadn’t tested that. So we were so excited when it actually happened!” Malhotra said. Astronomers had expected that the Green Pea galaxies, which are easier to observe since they are nearby, would be common at the epoch of reionization, so it came with great excitement that their predictions were accurate.
There is little doubt that more groundbreaking headway will be made. As more discoveries are made with JWST data, it is likely that we will see more objects like the galaxies analyzed in Rhoads’ and Malhotra’s study, and we may finally find the key to the end of the universe’s dark age. “I think the JWST is going to revolutionize [our understanding of reionization],” Malhotra said.
As for Rhoads and Malhotra, their plans going forward remain similar. They have worked in this field for many years and expect to continue working towards uncovering more about the high-redshift universe. Working with both ground-based telescopes and JWST, the two astronomers expect to build a more quantitative evidence base surrounding reionization-era galaxies. And since distant galaxies are harder to study, Rhoads and Malhotra plan to study local Green Pea analogs as well as higher redshift galaxies.
The journey to understand the end of the universe’s dark age has been an arduous one, but also highly rewarding. Just as our universe exited its dark age, our knowledge about it has as well. “It’s been quite a fun thing to do! We’re looking at images, finding new things, making new discoveries, and forming a community. Many people, including citizen scientists, have paved the way for these exciting discoveries,” Malhotra said. “This definitely represents the fun part of science.”