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

Le résumé fourni par la source

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.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.