How JWST will unlock the chemical cosmos

The James Webb Space Telescope (JWST) may steal the headlines by finding the first galaxies or detecting potential biosignatures in the atmospheres of exoplanets, but it will also take us on a giant leap forward in our understanding of cosmic chemistry.

For one UK-based astronomer, it’s also the chance to lead a science project on NASA’s new orbiting observatory, once it has safely launched and deployed. JWST is scheduled for launch at 12:20pm GMT on Christmas Day.

Patricia Schady, of the University of Bath, is leading a multinational team of scientists who will use JWST’s Near-Infrared Spectrometer (NIRSpec) to measure the abundance of heavy elements present in interstellar gas contained within galaxies over 10 billion light years away.

“One of the key questions to be addressed by JWST is the chemical enrichment of the Universe,” Schady tells Astronomy Now.

An artist’s impression of the James Webb Space Telescope with its five-layer sunshade and primary mirror fully deployed after launch atop an Ariane 5 rocket. Image: NASA

Metal galaxies

All elements heavier than hydrogen and helium are known, in astronomer-speak, as ‘metals’. These ‘metals’ are formed by stars, and as each generation of stars comes and goes, the abundance of metals in the Universe increases. Galaxies that existed 10 billion years ago therefore should have much lower metallicities than modern galaxies, but how much lower? Measuring the metallicity of distant galaxies will teach us the rate of star-formation in the early Universe, and when important molecules necessary for rocky planets or life became commonplace. It will also help us understand how our Milky Way has evolved into the galaxy that it is today.

However, measuring the metallicity of galaxies at great distances from us, which we see as they were just three or four billion years into cosmic history, is not easy. There are two ways to do this. One is to search for light emitted at specific wavelengths by various molecules in these galaxies, which is difficult because these emission lines are often very faint. The other way is to look for molecules absorbing light from a bright background object. The wavelength of light that is absorbed depends on the type of molecule doing the absorbing, making these molecules readily identifiable.

Quasars have been used as background light sources in some cases, but it’s difficult to determine the location of the gas doing the absorbing in the foreground galaxy being studied. Is the absorbing gas in the core of the galaxy, or in its outer halo? The core of a galaxy is expected to have a greater metallicity than the outer regions because more star formation will have taken place in the core. Plus, the farther back in time one looks, the fewer quasars there are to act as background light sources.

So astronomers have veered towards using the afterglows of gamma-ray bursts (GRBs), which signal the destruction of a massive star and the formation of a black hole. The afterglows are bright enough to be seen in galaxies at great distances, and have the added advantage of being located in the galaxies that are being studied. Since the absorption of their light takes place dozens, or at most hundreds, of light years from the site of the GRB, they allow astronomers to more accurately pin down the location of the gas doing the absorbing.

The complete jigsaw puzzle

However, for Schady’s project, JWST will not be measuring the absorption lines in GRB afterglows. JWST is powerful enough to directly measure the emission lines from molecules. So, instead it will be targeting ten distant galaxies that have played host to GRBs and for which the metallicity has already been measured through absorption. By measuring the metallicity from the light emitted by molecules in those galaxies, Schady’s project will provide the first detailed cross-check of the two methods in the same galaxies.

This is important for a number of reasons. Within galaxies, interstellar gas is present either as neutral gas, or as ionised gas. Whereas today about 90 per cent of the gas in the Milky Way Galaxy is ionised, in galaxies existing in the early Universe, most of the gas is present in neutral form.

“Information on the metallicity of the neutral gas can be gauged through absorption line studies,” says Schady. Meanwhile, emission-line measurements trace the presence of ionised gas. It requires both kinds of measurements to get a complete picture of the metallicity of a galaxy.

“My project will create the first sample of galaxies with both absorption- and emission-line metallicities within the same region of the galaxy,” says Schady.

The measurements will also be of broader importance, by providing a sample of galaxies against which observations of even more distant, and fainter, galaxies can be calibrated. Since GRBs tend to mostly explode in galaxies that have lower metallicity, that makes them the closest match to the first galaxies, which would also have low metallicity.

Therefore, the results of Schady’s project will be “relevant to many other studies of cosmic chemical enrichment being undertaken with JWST or other facilities,” she says.

So while the observations themselves may not make any headlines, they are going to be vital to much of the other work that astronomers will be conducting with JWST on the evolution of galaxies in the early Universe. Indeed, this will be the case for many of JWST’s projects, in that they will provide the foundations for astronomical discoveries for many years to come.

For more on the UK-based astronomers who will be using JWST, and the projects that they will be conducting, read our article in the January 2022 issue of Astronomy Now, or check out the UK JWST website.