Under Pressure: Decoding the Effect of High Densities on Derived Nebular Properties

Abstract Recent JWST observations have uncovered a population of compact, high-redshift ( z > 6) galaxies exhibiting extreme nebular conditions and enhanced nitrogen abundances that challenge standard chemical evolution paradigms. We present a joint UV and optical abundance analysis using a new suite of Cloudy photoionization models covering a wide density range ( n e = 10 2 –10 9 cm −3 ), combined with Hubble Space Telescope and JWST spectroscopy for a sample of star-forming galaxies across 0.0 ≲ z ≲ 10.6. We find that assuming uniform, low-density conditions ( n e ∼ 10 2 cm −3 ) in high-density environments ( n e ∼ 10 5 cm −3 ) can bias nebular diagnostics by overestimating T e (up to 1800 K), overpredicting log U (by >1 dex), and underestimating O/H (up to 0.67 dex), while only modestly inflating N/O. Therefore, robust abundance determinations at high z require a multiphase density model. Using this model, we recalculate O/H and N/O abundances for our sample and present the first log U diagnostics and ionization correction factors for high-ionization UV N lines. We find that the UV tracers systematically overestimate N/O by ∼0.3–0.4 dex relative to the optical benchmark. We find that N/O increases with redshift, correlating with both n e and star formation rate surface density (Σ SFR ), suggesting that N/O is temporarily enhanced in compact, high-pressure environments. However, the n e evolution with z is more gradual than the (1 + z ) 3 scaling of virial halo densities, suggesting that n e evolution is shaped by both cosmological structure growth and baryonic processes. These trends point to prompt N/O enrichment potentially driven by very massive stars, with key implications for interpreting UV emission and determining reliable chemical abundances from JWST observations of the early Universe.