The chemical composition of galaxies has always been full of unknowns. James Webb has taken a huge step to solve it
The James Webb Space Telescope sees where others can’t: its infrared vision pierces clouds of cosmic dust and reaches galaxies so far away that it took billions of years for its light to reach us. Looking far into space is, in that sense, looking back in time. However, what James Webb has seen in these galaxies differs from what was expected: these early galaxies seem to have too much nitrogen, much more than expected. Among the exotic possible explanations of science, hypotheses such as gigantic stars never seen before, black holes functioning as catalysts for galactic chemistry or large quantities of stars have passed. In fact, that was the topic of conversation in the middle of a phone call while Mexican astrophysicist José Eduardo Méndez-Delgado waited in line for the doctor. On the other end of the line, his colleague Karla Arellano-Córdova, who was in Edinburgh. In that informal talk they decided to change the prism: perhaps the problem was not the galaxies, but how we measure them. The discovery. The proposal from this international team is to analyze three light signals from the same oxygen ion to calculate temperature and density at the same time, without starting from one to calculate the other (the original source of error). The result: the gas was a hundred or a thousand times denser than was assumed in those galaxies. With that correction, the galaxies turned out to be richer in metals than they appeared and the excess nitrogen was drastically reduced. Why it is important. First, because the metallicity of a galaxy is directly related to its history: the more metals there are in its composition, the more stars have been born and died within it. Until now we were underestimating this figure, which made those early galaxies appear very different from our own and suggested a sharp and discontinuous evolution. Now they look more like what we know. But the elements essential for life, such as carbon, oxygen or nitrogen, did not exist when the universe was born: they were manufactured by the stars inside and expanded when they died. Hence the interest in knowing the chemistry of galaxies: it helps to understand when the universe had the necessary ingredients for life. With the wrong measurements, we don’t know if those ingredients were there earlier and in more places than we thought. Context. The standard method to know the composition of a distant galaxy is to analyze the spectral lines of its light based on the density of the gas and its temperature. The problem is that in these primitive galaxies the gas is much denser than expected, so its application as a thermometer works poorly. And from here on, everything failed. The nitrogen anomalies appeared in the first scientific data from the James Webb Space Telescope, as this either this. Since the results did not fit the models, the scientific community threw itself into trying to find explanations. This paper proposes to take a step back: before interpreting stellar physics, check that the measurements are correct. Besides, the Webb now allows it: simultaneously detects oxygen lines in the ultraviolet and in the optical in such distant galaxies. How they do it. In essence, the trick is choosing the right signals. One of the oxygen light lines, visible in ultraviolet, has a special property: it does not distort even if the gas is very dense, something that happened with the lines they were using previously. By combining it with two other signals from the same atom, the research team can calculate temperature and density at the same time, as if they were solving two simultaneous and independent equations. Using statistical simulations, the team found that the results were consistent with other independent measurements of the same galaxies. Yes, but. As the team explains in the work, their method corrects the density error, but not other possible errors that are equally important: the gas of these galaxies also has internal temperature variations, and that can bias the results in ways that this study does not resolve. Furthermore, the method only works well when all three light signals from oxygen are clearly detected. In three of the six galaxies analyzed this was not possible, and the results are less precise. Nitrogen remains a problem. The overabundances come almost entirely from a particular ion whose emission is extraordinarily sensitive to temperature: a variation of just ten percent in that parameter would reduce the calculated nitrogen by half. No one has yet measured that temperature directly. However, it points out a path to follow before looking for “exotic” explanations: verify that the measurement tools are up to par. In Xataka | For a time it was one of the asteroids most watched by astronomers: the Webb has just resolved a key doubt In Xataka | James Webb has been detecting red dots in the universe for years: the only problem is that we don’t know what they are Cover | Oleg Moroz
