Maisie's Galaxy, candidate for the most distant known galaxy

Fanny Trifilieff: How does the JWST differ from previous telescopes?

Denis Burgarella: The James Webb Space Telescope (JWST) is the successor to the Hubble Space Telescope (HST). As with the HST, the JWSR is a collaboration between NASA and ESA. With its 6.5m-diameter mirror, it is the largest telescope to have been sent into space since ESA's Herschel telescope. This advantage enables it to detect more light, which increases its sensitivity and gives it better spatial resolution to distinguish finer details. It also provides access to a new wavelength range: the infrared (IR), which is not very accessible from the ground. IR allows us to see through the clouds of dust and gas that surround nebulae, giving us access to new information, but it also enables us to discover the most distant galaxies in the Universe by measuring their redshift. Redshift is the increase in the wavelength of light caused by the distance of the light source observed, due to the expansion of the Universe. The higher the redshift (denoted z), the greater the distance to the galaxy emitting the light.

F.T: Tell us about Maisie's galaxy, which could be one of the oldest galaxies ever observed...

D.B: At present, the highest confirmed redshift for a galaxy is z = 11.09. Thanks to the capabilities of the JWST, our CEERS team has identified a candidate galaxy at redshift z = 12 when the Universe was less than 300 million years after the Big Bang (compared with 13.8 billion years today). Named Maisie, after the daughter of the article's first author, it is therefore a candidate for being the most distant object ever detected. Its precise distance has yet to be confirmed, like other candidates, but assuming it is, it would tell us a great deal about the formation and evolution of galaxies and the Universe.

To achieve this result, our team ran a code called CIGALE (Code Investigating GALaxy Emission), developed in Marseille, with the JWST data released on July 14, 2022. This has made it possible to study the mass of the galaxy, its age, the amount of dust, as well as the amount of star formation, all in an attempt to understand the formation of Maisie.

F.T: What are the implications of such a discovery?

D.B: Models exist to predict the formation and evolution of galaxies. Until now, they didn't predict that such galaxies would exist so early in the life of the Universe. The field of view analyzed is very small, so just finding a single galaxy was statistically unexpected. If, in addition, other candidates are confirmed, the model will have to be called into question. Maisie's galaxy is also very massive: 300 million times the mass of the Sun! We used to think that galaxies got off to a slow start, but in fact it has formed a huge number of stars in a very short space of time, at a rate of between 2 and 8 times the mass of the Sun per year. Yet it contains very little dust. The presence of dust is synonymous with galaxy ageing, with several generations of stars containing silicate, carbon etc., while the first generations of stars contain only hydrogen and helium. The absence of dust therefore means that it's not out of the question for this galaxy to contain the first generations of stars to appear in the Universe: astronomers' Holy Grail!