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

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 10^{7 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.}}