Description
Observations with JWST have opened a new window into the physical conditions, properties, and chemical abundance patterns of galaxies at high redshift. We present the chemical abundances of C, N, and O of two star-forming galaxies (SFGs) at z~5 from the Early eXtragalactic Continuum and Emission Line Science (EXCELS) survey. We compare the C and N in concert with O for these, finding elevated N/O ratios with respect to local SFGs but C/O consistent with the predictions from core-collapse supernovae yields. To understand the evolution of C/N versus O/H, we consider chemical evolutions models with variations in the initial mass function (IMF). We find that a standard or steeper IMF can explain the observed abundance ratio at z ~ 5, implying that intermediate-mass stars contribute significantly to the N production in these galaxies. Our two EXCELS galaxies pave the way for larger samples of high redshift galaxies to fully understand the chemical evolution of C/N in the early Universe.
