Gas outflows in two recently quenched galaxies at z = 4 and 7

¹ Cosmic Dawn Center (DAWN), Denmark
² DTU Space, Technical University of Denmark, Elektrovej 327, DK-2800 Kgs. Lyngby, Denmark
³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁴ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
⁵ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁶ Department of Astronomy, School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁷ Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
⁸ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
⁹ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
¹⁰ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
¹¹ Department of Astronomy/Steward Observatory, University of Arizona, 933 N Cherry Ave., Tucson, AZ 85721-0009, USA
¹² School of Earth and Space Exploration, Arizona State University, PO Box 876004, Tempe, AZ 85287-6004, USA
¹³ Departamento de Fisica Teorica, Modulo 8, Facultad de Ciencias, Universidad Autonoma de Madrid, 28049 Madrid, Spain
¹⁴ CIAFF, Facultad de Ciencias, Universidad Autonoma de Madrid, 28049 Madrid, Spain
¹⁵ Caltech/IPAC, MS 314-6, 1200 E. California Blvd. Pasadena, CA 91125, USA
¹⁶ Institute of Physics, Laboratory for Galaxy Evolution, EPFL, Observatoire de Sauverny, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁷ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
¹⁸ National Astronomical Observatory of Japan, Los Abedules 3085 Oficina 701, Vitacura 763 0414, Santiago, Chile
¹⁹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
²⁰ Universidad Diego Portales, Av. Ejercito Santiago de Chile, Chile
²¹ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
²² Department of Astronomical Science, The Graduate University for Advanced Studies, SOKENDAI, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²³ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²⁴ School of Science, Kwansei Gakuin University, 2-1, Gakuen, Sanda, Hyogo 669-1337, Japan
²⁵ Astronomical Institute, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
²⁶ International Centre for Radio Astronomy Research (ICRAR), M468, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
²⁷ Department of Physics & Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver BC V6T 1Z1, Canada
²⁸ Subaru Telescope, National Astronomical Observatory of Japan, National Institutes of Natural Sciences (NINS), 650 North A’ohoku Place, Hilo, HI 96720, USA
²⁹ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD, Groningen, The Netherlands
³⁰ Waseda Institute for Advanced Study (WIAS), Waseda University, 1-21-1 Nishi-Waseda, Shinjuku, Tokyo 169-0051, Japan
³¹ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Ave, Evanston, IL 60201, USA
³² Graduate Institute of Astrophysics, National Taiwan University, Taipei 10617, Taiwan

^⋆ Corresponding author.

Received: 27 January 2025
Accepted: 19 May 2025

Abstract

Outflows are a key element in the baryon cycle of galaxies, impacting their evolution by extracting gas, momentum, and energy and then injecting them into the surrounding medium. The properties of gas outflows provide a fundamental test for our models of how galaxies transition from a phase of active star formation to quiescence. Here we report the detection of outflowing gas signatures in two recently quenched, massive (M_⋆∼10^10.2 M_⊙) galaxies at z = 4.106 (NS_274) and z = 7.276 (RUBIES-UDS-QG-z7) observed at rest-frame ultraviolet to near-infrared wavelengths with JWST/NIRSpec. The outflows are traced by blueshifted magnesium (MgII) absorption lines, and in the case of the z = 4.1 system, also by iron (FeII) and sodium (NaI) features. Together, these transitions broadly trace the chemically enriched neutral phase of the gaseous medium. The rest-frame optical spectra of the two sources are similar to those of local post-starburst galaxies, showing deep Balmer stellar features, a relatively low D_n4000 index, and minimal ongoing star formation on 10 Myr timescales, as traced by the lack of bright nebular and recombination emission lines. This also suggests the absence of strong and radiatively efficient active galactic nucleus activity. The galaxies’ star formation histories are consistent with a recent and abrupt quenching, prior to which the average star formation rate was ∼15 M_⊙ yr⁻¹ over the last 100 Myr of their lives. In the case of NS_274, dedicated millimeter observations allowed us to also strongly constrain the dust obscured star formation rate to <12 M_⊙ yr⁻¹, unambiguously confirming its quiescence. Under simple geometrical assumptions, we derive mass loading factors η=Ṁ_out/SFR_100 Myr≲1 and >10 for the z = 4.1 and z = 7.3 systems, respectively, and a similarly pronounced difference in the energy carried by the outflows. Supernova feedback could account for the mass and energy of the outflow in NS_274. However, the low mass loading factor and average gas velocity (∼180 km s⁻¹, which is lower than the stellar velocity dispersion) suggest that the observed outflow is likely not the primary factor behind the quenching of NS_274, but it might represent a relic of the star formation process winding down. Star-formation-related processes seem to also be insufficient to explain the extreme mass outflow rate of RUBIES-UDS-QG-z7, which would require an additional ejective mechanism such as an undetected active galactic nucleus. Finally, the average outflow velocities per unit stellar mass, star formation rate, and surface density of star formation rate are consistent with those of lower-redshift post-starburst galaxies, suggesting that outflows in rapidly quenched galaxies might occur similarly across cosmic time. Our findings hint at the existence of a rich tapestry of galaxy quenching pathways at high redshifts, and they highlight the importance of using large spectroscopic samples that map different spectral features to account for the different timescales on which different mechanisms contribute to this process.