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

We present gas-phase abundances of carbon (C), alpha-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-circle dot)=8.46 and a median star-formation rate of log (SFR}/M-circle dot, 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 similar to 0.1 Z(circle dot)), 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 on to 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-*, star-formation rate (SFR), specific SFR (sSFR), and ultraviolet continuum slope beta(UV), we identify [Fe iii] line detections in the high-sSFR bin and the blue- beta(UV) 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.