On Monday, NASA — in tandem with the White House — released an incredible deep-field image taken by the James Webb Space Telescope (JWST), which demonstrates how this telescope will be able to look back at some of the earliest galaxies which lit up the cosmic dawn.
It’s a sort of follow-up to one of the Hubble Space Telescope’s greatest contributions to astronomy: the Hubble Deep Fields, a series of images that show distant galaxies in incredible detail. But Webb is just getting started when it comes to surveying the night sky in depth.
The COSMOS-Webb program will use JWST’s NIRCam and MIRI instruments to collect images of extremely distant galaxies in the near-infrared and mid-infrared ranges. It will cover some of the same area of the sky previously imaged by Hubble in its own COSMOS program, but at a different wavelength and higher resolution.
“It’s so much more sensitive than what we’ve been able to do before,” Jeyhan Kartaltepe, co-leader of the COSMOS-Webb program, tells Inverse. That means Webb can effectively look back in time to when the universe was in its infancy.
How far back in time can JWST look?
Soon after the Big Bang, the universe was extremely hot and much of the material in it was ionized — meaning that atoms were gaining or losing electrons and had either a positive or negative charge. But as the universe cooled, it settled into a state of mostly neutral atoms of hydrogen and helium.
This neutral state meant light couldn’t travel far because it made the universe dark and foggy. “The protons and electrons were bound together,” Kartaltepe says. “Because of that, a photon can’t travel very efficiently through that medium as it gets absorbed right away. That’s why it’s opaque — because if there is a light source, any photons from it aren’t going to make it very far.” This early stage of the universe is often called the dark age for that reason.
Light would soon arrive, however, in the form of the Epoch of Reionization. Eventually, some sources started giving off photons at a high enough energy that it ionized some of those atoms, breaking apart the protons and electrons. “Once regions have enough of the medium ionized, then photons can now travel more freely. That’s why it’s referred to as first light,” Kartaltepe says.
The photon sources which switched on the lights of the universe were the earliest stars, which were rather different beasts from the stars we see today. “At that point, basically everything was hydrogen, with a little bit of helium,” Kartaltepe says, and these two elements were the building blocks of the earliest stars. Today, stars also contain other heavier elements like oxygen, carbon, neon, silicon, magnesium, and iron — all synthesized in subsequent generations of stars via supernovae.
What were the first stars like?
The different composition of this first generation of stars would make them behave differently to stars born today as well. “They can become much more massive and much more extreme, and have very short lives. It’s a very special population of stars,” Kartaltepe says.
These first stars formed the first galaxies, but there was more gas and fewer stars around compared to today, so the early galaxies looked very different too. Studying these very early galaxies is one of the key aims of the Webb science program overall, and it’s what COSMOS-Webb is aiming to achieve.
“With Webb, we’re going to be able to identify thousands of galaxies from this period of time,” Kartaltepe says. “So we’ll be able to detect the ones which are really bright, and the ones which are fairly rare.”
The key to spotting these earliest galaxies is a phenomenon called redshift. If you’ve noticed that sirens sound different as they approach and move away from you, that’s due to something called the Doppler effect in which the frequency of sound waves change based on the movements of the source and the observer. Something similar happens with light, which means that because the universe is expanding and galaxies are moving away from us, the light they give off is shifted to longer wavelengths, so it appears shifted toward the red end of the spectrum.
This redshift is more pronounced the further away the object is. Sometimes the light from distant galaxies is shifted out of the visible light spectrum and into the infrared. As light takes time to travel to us, the galaxies we observe with the most pronounced redshift are the very oldest, and the ones which we’re interested in finding.
Webb vs. Hubble surveys
These galaxies will be very bright in the infrared wavelengths that JWST observes in, but very faint or invisible in the visible light wavelengths that Hubble typically looks in. So when researchers compare data on the particular patch of the sky observed with COSMOS and see a galaxy shining brightly in Webb data but not in Hubble data, they’ll have an indication that they’re looking at a very distant, and therefore very old, galaxy.
While tools like Hubble have been used to identify a handful of these early galaxies before, what Webb can offer is the ability to see many of them and look at how they are distributed. “We’ll not only find them and characterize their properties, but we’re going to be able to see how they’re spatially distributed: How clustered they are together, or how spread out, and how that ties into this process of reionization,” Kartaltepe says.
The data from COSMOS will help researchers answer questions like when exactly the process of reionization began, and whether this process was a quick one which spread throughout the galaxy almost immediately, or whether it happened more gradually in bubbles of ionization among the fog.
Learning about this stage in the universe can give us a glimpse back in time, but it can also help us understand many issues about how our universe is today. “There are a lot of mysteries about how galaxies form and why they look like they do that we don’t really understand, so being able to probe these early times will really help us,” Kartaltepe says.
COSMOS-Webb will begin making observations in December this year, so look out for the first images from the program to be released early next year.
