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

ABSTRACT 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 $\log (M_{*}/{\rm M}_{\odot })=8.46$ and a median star-formation rate of $\log (\mathrm{SFR}/{\rm M}_{\odot }\, \mathrm{yr^{-1}})=0.30$, placing them close to the star-formation main sequence. We find that the stacked spectrum of all our 564 sources has relatively low $[\mathrm{C/O}]=-0.70$, moderate $[\mathrm{Ne/O}]=-0.09$, and low $[\mathrm{Ar/O}]=-0.28$ values at a low gas-phase metallicity of $12+\log (\mathrm{O/H})=7.71$ ($Z\sim 0.1~Z_\odot$), 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 $[\mathrm{Si/O}]=-0.63$, 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 $z \,\gt\, 6$ 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 $\beta _{\mathrm{UV}}$, we identify [Fe iii] line detections in the high-sSFR bin and the blue-$\beta _{\mathrm{UV}}$ 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.