Nearly 10 billion years ago, the Universe was forming stars at its peak rate. Galaxies were assembling, growing and being transformed at a pace we no longer see today. Hidden in that era is a rare gem: a massive group of galaxies at redshift 1.85, caught in the act of building itself around a starbursting central galaxy.
By a stroke of luck, the light emitted by hydrogen atoms in this group (the Halpha line, a direct tracer of newborn stars) is shifted into exactly the right wavelength for one of JWST's narrow-band filters. Our accepted JWST program uses this to produce a panoramic, sharply resolved map of where stars are forming across the whole group. We expect to detect around 100 star-forming galaxies, including dwarfs about a hundred times less massive than the Milky Way, at a time when such objects are essentially unexplored.
The PhD student will analyse these brand-new data and tackle three open questions:
1) How many small galaxies does a group hold? Simulations of cold dark matter predict many more small satellites than we observe nearby. Counting them in a distant group offers a new test of this "Missing Satellite Problem" and of alternative dark-matter models.
2) How do galaxies build themselves? JWST can resolve giant star-forming clumps inside galaxies, making it possible to measure how often they form and how they shape galaxy growth.
3) How do galaxies get switched off? The student will search for signs of ram-pressure stripping, the removal of a galaxy's gas as it plunges through the hot intragroup medium, at the highest redshift ever probed.
The student will work with JWST imaging and spectroscopy, learn state-of-the-art techniques in astronomical data analysis, and confront the results with cosmological simulations. No prior JWST experience is needed. Curiosity, rigor, programming skills (Python is ideal) and a taste for big questions matter most.
The student will join a dynamic research team in Paris, one of Europe's leading centres for extragalactic astrophysics, with opportunities to collaborate internationally, attend conferences, and present their own discoveries.