JADES: the chemical enrichment pattern of distant galaxies – α enhancement, silicon depletion, and iron enhancement

Abstract We present gas-phase abundances of carbon (C), α-elements (O, Ne, Si, and Ar) and iron (Fe) obtained from stacked spectra of high-z star-forming galaxies with the deep Near Infrared Spectrograph medium-resolution data from the James Webb Space Telescope Advanced Deep Extragalactic Survey. Our 564 sources at z = 4–7 have a median stellar mass of log (M*/M⊙) = 8.46 and a median star-formation rate of log (SFR/M⊙ yr−1) = 0.30, placing them close to the star-formation main sequence. We find that the stacked spectrum of all our 564 sources has relatively low C/O = −0.70, moderate Ne/O = −0.09, and low Ar/O = −0.28 values at a low gas-phase metallicity of 12 + log (O/H) = 7.71 (Z ∼ 0.1 Z⊙), suggesting dominant yields of core-collapse supernovae evolved from massive stars. The detection of a weak Si iii] emission line in our stacked spectrum provides a silicon-to-oxygen abundance ratio of Si/O = −0.63, which is lower than that of stars in the Milky Way disc and lower than expected by chemical evolution models, suggesting silicon depletion onto dust grains. Likewise, this Si/O value is lower than that we newly derive for two individual z > 6 galaxies (GN-z11 and RXCJ2248) with negligible dust attenuation. By performing spectral stacking in bins of M*, SFR, specific SFR (sSFR), and ultra-violet (UV) continuum slope βUV, we identify Fe iii line detections in the high-sSFR bin and the blue-βUV bin, both of which exhibit supersolar Fe/O ratios, while their C/O, Ar/O, and Si/O ratios are comparable to those of the all-sources stack. Our findings support a chemically young gas composition with rapid dust depletion in the general population of high-z star-forming galaxies, while raising the possibility of anomalous, selective Fe/O enhancement at the very early epoch of star formation.