2026/07/22 by Matteo Sapori, Sirio Belli, Letizia Bugiani +14
#astro-ph.GA
Recent JWST/NIRSpec observations have shown that Na I D absorption tracing neutral gas outflows is widespread in massive galaxies at Cosmic Noon (z∼2-3), but their prevalence at higher redshift remains largely unexplored. Here we investigate whether similar outflows are already in place during the first 2 Gyr of cosmic history (z>3), using a sample of 811 galaxies at 3<z<7 from the public DAWN JWST Archive and the Mirage or Miracle survey, with secure spectroscopic redshifts and continuum SNR>5 in their JWST/NIRSpec PRISM low-resolution spectra (R∼100). We derived physical properties and star formation histories via Prospector SED fitting and isolated the Na I D feature in each spectrum by subtracting the best-fit stellar continuum. We detect an excess of Na I D absorption in 20 galaxies, almost all at 3<z<5 with stellar mass >1010 M_\odot, an overall detection fraction of ∼2.5%. The detection fraction rises steeply with stellar mass and quiescence, reaching ∼11% among the most massive galaxies and ∼43% among massive quenched systems. The Na I D equivalent widths are large, spanning 4 to 16 Angstrom, and are highest in dusty star-forming galaxies. In previous medium-resolution observations, EWs above 5 Angstrom were mostly found in outflowing gas; we thus interpret our detections as mostly tracing neutral outflows, despite the lack of kinematic information at PRISM resolution. A stacking analysis confirms these trends and provides average Na I D EWs for different subsamples. Under conservative assumptions, we estimate mass outflow rates of 4 to 12 M_\odot yr-1, exceeding the current star formation rate for about 30% of the detected galaxies. We conclude that neutral outflows are likely already present and important in massive galaxies during the first 2 Gyr of cosmic history.