JADES: Balmer decrement measurements at redshifts 4 < z < 7

¹ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
² Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
³ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁴ Centro de Astrobiología (CAB), CSIC–INTA, Cra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁵ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
⁶ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200 Copenhagen, Denmark
⁷ Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
⁸ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
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 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
¹⁴ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
¹⁵ AURA for European Space Agency, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21210, USA
¹⁶ Department for Astrophysical and Planetary Science, University of Colorado, Boulder, CO 80309, USA
¹⁷ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villafranca del Castillo, Madrid, Spain
¹⁸ Department of Astronomy and Astrophysics University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 96054, USA
¹⁹ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
²⁰ NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
²¹ NRC Herzberg, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada

^⋆ Corresponding authors; lester.sandles@gmail.com, francesco.deugenio@gmail.com

Received: 7 June 2023
Accepted: 23 August 2024

Abstract

We present Balmer decrement, Hα/Hβ, measurements for a sample of 51 galaxies at redshifts z = 4 − 7 observed with the JWST/NIRSpec micro-shutter assembly (MSA), as part of the JWST Advanced Deep Extragalactic Survey (JADES). Leveraging 28-hour long exposures and the efficiency of the prism/clear configuration (but also using information from the medium-resolution gratings), we were able to directly probe the low-mass end of the galaxy population, reaching stellar masses, M_⋆, as low as 10⁷ M_⊙. We find that the correlation between the Balmer decrement and M_⋆ is already established at these high redshifts, indicating a rapid buildup of dust in moderately massive galaxies at such early epochs. The lowest-mass galaxies in our sample (M_⋆ = 1 − 3 × 10⁷ M_⊙) display a remarkably low Balmer decrement of 2.88 ± 0.08, consistent with Case B, suggesting very little dust content. However, we warn that such a low observed Balmer decrement may also partly be a consequence of an intrinsically lower Hα/Hβ resulting from the extreme conditions of the ionised gas in these primeval and unevolved systems. We further compare the Balmer decrement to continuum-derived star formation rates (SFRs), finding tentative evidence of a correlation, which likely traces the underlying connection between the SFR and the mass of cold gas. However, we note that larger samples are required to distinguish between direct and primary correlations from indirect and secondary dependences at such high redshifts.