Detecting molecular gas at cosmic dawn
Karin Cescon (Leiden University)
Tuesday 19/05/2026 @ 14:00, Sala Antonio Sollima (IV piano Battiferro)
Massive galaxies were already known to exist at early cosmic times, but recent JWST discoveries have revealed them in far greater numbers, in some cases with surprisingly evolved stellar populations. Understanding how such systems assembled so rapidly requires direct constraints on their molecular gas reservoirs—the fuel for star formation. In this talk, I will present new deep VLA observations of REBELS-25 (z = 7.31), a massive star-forming galaxy and the highest-redshift dynamically cold disk (V_rot,max/? ? 11) confirmed to date. Using ~40 hours of VLA data, we detect CO(3–2) emission, providing the first direct measurement of cool molecular gas in a star-forming galaxy at z > 7. The CMB-corrected line flux implies a massive gas reservoir already ~700 Myr after the Big Bang. Combining this unique detection with ALMA CO(7–6) observations and radiative transfer modeling that self-consistently treats dust, CO emission, and CMB effects, we constrain the ISM conditions and confirm the presence of a large molecular gas reservoir. We also assess alternative cold gas tracers in the early Universe, including [CII] and dust continuum emission, by comparing CO-based gas masses with estimates from ancillary data. I will conclude by discussing prospects for studying cold gas at cosmic dawn with next-generation facilities, and how combining multiple tracers is key to building a complete picture of the ISM and probing the rapid assembly of massive galaxies in the first billion years of cosmic history.