Over the past 20 years, my team have pushed the frontier for the most distant known objects to higher redshifts, exploring galaxies when the Universe was young using the Hubble Space Telescope and large ground-based telescopes. As well as discovering galaxies within the first billion years (90percent of the way back in time to the Big Bang), our knowledge of the composition of the Universe has also grown - dark matter and dark energy dictate the expansion history and initial collapse of structures which ultimately form galaxies. We now know that the gas between the galaxies, initially plasma, became mostly neutral about 300,000 years after the Big Bang, but again became plasma about a billion years later. The first few generations of stars to form, with a contribution from high redshift quasars, might be responsible for this reionization, but we have yet to find the galaxies accounting for the bulk of the ionizing photons and key questions remain: what is the contribution from the faintest dwarf galaxies in the luminosity function at high redshift? what fraction of ionizing photons emitted by stars reach the intergalactic gas? is the first generation of stars forming from primordial hydrogen and helium more efficient in producing ionizing photons?
Thanks to the recent launch of the James Webb Space Telescope (JWST) at the end of 2021, we are able to start addressing these questions. JWST has unprecedented sensitivity, and works at longer wavelengths than the Hubble Space Telescope, which is crucial to explore the most distant (and highly redshifted) galaxies. I have been member of the ESA Instrument Science Team since 2005 for the near-infrared spectrograph (NIRSpec) on JWST, and much of our 900 hours of guaranteed time will be spectroscopy of high redshift galaxies. I have assembled a team under this ERC grant to measure accurate redshifts of candidate very distant galaxies, measure their stellar populations (ages and star formation rates), assess the escape fractions of ionizing photons and determine the metal enrichment. The first reporting period of this ERC grant (May 2020-October 2021) has seen the successful launch and commissioning of JWST, and in the past few months the first science data has been taken. My team analysed the Early Release Observations of some very distant galaxies (magnified by the gravitational lensing of a foreground massive cluster of galaxies), measuring the conditions of the interstellar medium at these early epochs and finding it to be significantly hotter than in galaxies at more recent times. In the past few days we have obtained spectroscopy as part of our JADES collaboration between the NIRSpec and NIRCam instrument science teams, targetting the Hubble Ultra Deep Field (the most sensitive image of the sky taken before JWST). We have spectroscopically confirmed several tens of galaxies within a billion years of the Big Bang (at redshifts beyond 6), including four selected from NIRCam images with JWST which are at record-breaking redshifts beyond 10 (within 350 million years of the Big Bang).
