Zapped then napped? A rapidly quenched remnant leaker candidate with a steep spectroscopic β_UV slope at z = 8.5

¹ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 155A, DK-2200 Copenhagen, Denmark
² Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 OHA, UK
³ Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge, CB3 OHE, UK
⁴ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁵ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK
⁶ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
⁷ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200, Copenhagen, Denmark
⁸ Centro de Astrobiología (CAB), CSIC-INTA, Cra. de Ajalvir Km. 4, 28850 - Torrejón de Ardoz, Madrid, Spain
⁹ Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
¹⁰ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
¹¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
¹² Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK
¹³ AURA for European Space Agency, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21210, USA
¹⁴ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹⁵ Department of Astronomy and Astrophysics, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA
¹⁶ NSF National Optical-Infrared Astronomy Research Laboratory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
¹⁷ NRC Herzberg, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada

^⋆ Corresponding author: william.baker@nbi.ku.dk

Received: 15 January 2025
Accepted: 6 March 2025

Abstract

We used NIRSpec MSA spectroscopy and NIRCam photometry to explore the properties of JADES-GS8-RL-1, a rapidly quenched z = 8.5 galaxy with a stellar mass of 10^8.9 M_⊙, a steep blue UV slope, a Balmer break, and no sign of strong emission lines. With a β_UV = −2.8±0.2, as measured from the NIRSpec spectrum, JADES-GS8-RL-1 is consistent with having a negligible dust attenuation and little to no contribution from the nebular continuum alongside a probable high escape fraction. The β_UV slope measured from photometry varies from −3.0 in the central regions to −2.2 at the outskirts, suggesting possible regional differences in the escape fraction. There are no high-ionisation emission lines, only a tentative 2.9σ detection of [OII]λλ3726, 29. Using photometry, this emission appears to be extended, possibly corresponding to weakly ionised gas expelled during or after the quenching process. JADES-GS8-RL-1 is spatially resolved with a half-light radius of 240 pc and has an exponential, disc-like morphology. It appears to have formed all of its stars in a short burst within the past 100 Myr with a formation time of ≈70 Myr and a quenching time of ≈30 Myr. This quenching would have occurred rapidly, making it a more distant example of the kind of low-mass ‘mini-quenched’ galaxies previously observed at high-z. Due to the extremely blue β_UV slope, our best-fit model predicts a high value for f_esc of >10%, consistent with the value derived from the β_UV slope, which when combined with our extraordinarily low O32 upper limit suggests JADES-GS8-RL-1 is a fascinating example of a high-z ‘remnant leaker’ in one of its earliest phases deep in the epoch of reionisation.